Downslope re-sedimentation from a short-living carbonate platform: Record from the Upper Triassic Hosselkus limestone (Northern California)

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Article

Reference

Downslope re-sedimentation from a short-living carbonate platform:

Record from the Upper Triassic Hosselkus limestone (Northern California)

FUCELLI, Andréa, GOLDING, Martyn, MARTINI, Rossana

Abstract

Despite their discontinuous occurrence and poor preservation, knowledge about Triassic carbonates from North America has increased considerably during recent years. Their characterization represents a uniqueway to better assess evolution and recovery of the biosphere after the major Permo-Triassic biological crisis in the Panthalassa Ocean. The Eastern Klamath terrane, located in Northern California, is a key terrane due to its geographic position. It is placed halfway between the terranes of the Canadian Cordillera and the Northern Mexico counterparts, both extensively studied and characterized in recent decades, leaving a gap in knowledge along the Pacific coast of the United States. A few kilometers north-east of Redding, Shasta County, California, Upper Triassic carbonates (i.e., the Hosselkus limestone) crop out as a narrow north–south belt about 20 km long, near the artificial reservoir of Lake Shasta. All the accessible localities in this region have been extensively sampled for microfacies and micropaleontological analysis, leading to new insights about the depositional condition and age of the Hosselkus [...]

FUCELLI, Andréa, GOLDING, Martyn, MARTINI, Rossana. Downslope re-sedimentation from a short-living carbonate platform: Record from the Upper Triassic Hosselkus limestone (Northern California). Sedimentary Geology , 2021, vol. 422, no. 105967

DOI : 10.1016/j.sedgeo.2021.105967

Available at:

http://archive-ouverte.unige.ch/unige:153791

Disclaimer: layout of this document may differ from the published version.

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Downslope re-sedimentation from a short-living carbonate platform:

Record from the Upper Triassic Hosselkus limestone (Northern California)

Andrea Fucelli

^a,

⁎ ^, Martyn Golding

^b

, Rossana Martini

^a

aUniversity of Geneva, Department of Earth Sciences, 13 rue des Maraîchers, 1205 Genève, Switzerland

bGeological Survey of Canada Geological Survey of Canada, Paciﬁc Division, 1500-605 Robson Street, Vancouver, BC V6B 5J3, Canada

a b s t r a c t a r t i c l e i n f o

Article history:

Received 22 April 2021

Received in revised form 14 July 2021 Accepted 19 July 2021

Available online 24 July 2021 Editor: Dr. Brian Jones

Keywords:

Upper Triassic Northern California Microfacies Conodonts Slope deposits Panthalassa

Despite their discontinuous occurrence and poor preservation, knowledge about Triassic carbonates from North America has increased considerably during recent years. Their characterization represents a unique way to better assess evolution and recovery of the biosphere after the major Permo-Triassic biological crisis in the Panthalassa Ocean. The Eastern Klamath terrane, located in Northern California, is a key terrane due to its geographic position.

It is placed halfway between the terranes of the Canadian Cordillera and the Northern Mexico counterparts, both extensively studied and characterized in recent decades, leaving a gap in knowledge along the Pacific coast of the United States. A few kilometers north-east of Redding, Shasta County, California, Upper Triassic carbonates (i.e., the Hosselkus limestone) crop out as a narrow north–south belt about 20 km long, near the artificial reservoir of Lake Shasta. All the accessible localities in this region have been extensively sampled for microfacies and micropaleontological analysis, leading to new insights about the depositional condition and age of the Hosselkus limestone. A depositional model has been proposed for thefirst time, corresponding to a steep slope system subjected to platform progradation and collapse, recording shallow water facies and associated fauna in the form of calcareous breccia. Numerous conodont specimens have dated the whole succession as Upper Carnian. Identifi- cation of shallow water organisms, associated to a reliable stratigraphic interval, allowed comparison of the Hosselkus limestone with other Upper Triassic carbonates from the Panthalassan domain. Despite the faunal af- finities, especially with buildups developed at middle-paleolatitudes, the Hosselkus limestone is among the oldest of the terrane-based carbonates in Eastern Panthalassa. Thanks to peculiar geodynamical and bathymetri- cal conditions, allowing carbonate deposition slightly earlier than in other terranes, the Hosselkus limestone probably acted like a pioneer reef and may have had a great influence in the further expansion of carbonate buildups in the eastern part of the Panthalassa Ocean.

1. Introduction

During recent decades, knowledge about Panthalassan carbonates remained far lower with respect to their Tethyan counterparts. This is mostly due to the poor preservation of the rocks and to the lack of any signiﬁcant continuity in the outcrops, after their stacking in different tectonic settings (i.e., accretionary complexes and terranes) along the Circum-Paciﬁc region (Zonneveld et al., 2007;Chablais et al., 2010a;

Peybernes et al., 2016a, 2016b;Peyrotty et al., 2020a, 2020b). Neverthe- less, carbonate rocks remain one of the most valuable tools to constrain paleogeography and paleoecology of the no longer extant Panthalassa Ocean, drawing in recent years the attention of several researchers

and providing a conspicuous amount of new insights (Stanley, 1979a, 1979b;Blodgett and Stanley, 2008;Chablais et al., 2010b; Rigaud et al., 2010;Martindale et al., 2015;Peybernes et al., 2016a, 2016b;

Heerwagen and Martini, 2018). Within the Eastern Klamath terrane, continuous outcrops of Upper Triassic limestone occur in the Lake Shasta area, offering a rare opportunity to deepen our knowledge about Panthalassan paleoenvironments in a geographically strategic area and to compare the results with other Triassic carbonates scattered along the Paciﬁc coast (Silberling and Tozer, 1971;Tozer, 1982;Blodgett and Stanley, 2008;Martindale et al., 2015). The Hosselkus limestone crops out midway between widely studied terranes of North America, becoming a linking point for the complete knowledge of the area's tectonic and paleoenvironmental evolution. Preservation of these carbonates is mediocre and studies carried out in the past mostly focused on macro-fauna description (Diller, 1906;Smith, 1927;Sanborn, 1960;

Albers and Robertson, 1961;Sbeta, 1970;McCormick, 1986). However, Sedimentary Geology 422 (2021) 105967

⁎ Corresponding author.

E-mail addresses:andrea.fucelli@unige.ch(A. Fucelli),martyn.golding@canada.ca (M. Golding),rossana.martini@unige.ch(R. Martini).

https://doi.org/10.1016/j.sedgeo.2021.105967

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despite the poor preservation, other signiﬁcant information about depositional environment, hydrodynamic conditions and micropaleontology can be obtained thanks to precise microfacies description.

This paper deals with new comprehensiveﬁeld observations of the Hosselkus limestone that, coupled with microfacies analysis, allowed the recognition of a peculiar depositional setting and the assignment of an accurate chronostratigraphic range for limestone deposition.

Theseﬁndings, together with the remarkable dataset developed by the REEFCADE project (Rossana Martini 2007–2022), as well as information from the literature, allowed a comparison between the Hosselkus limestone and other Upper Triassic carbonates formed on different terranes in North America. Thanks to peculiar regional conditions (i.e., bathymetry and the cessation of volcanism), the Hosselkus limestone could have acted as foothold for subsequent reef colonization across the eastern portion of the Panthalassa Ocean.

2. Geological setting

The Hosselkus limestone and the other formations cropping out in Lake Shasta area (Northern California) are all part of the Eastern Klam- ath terrane, one of the eight terranes forming the Klamath Mountains (Irwin, 1960) (Fig. 1). This southern portion of the belt was divided at the beginning of the 1980s into three tectonostratigraphic units (Irwin, 1981): (1) the Yreka–Callahan section, (2) the Trinity Ultramafic complex and (3) the Redding Section. Thefirst two consist respectively of Lower Paleozoic (Ordovician to Devonian) sedimentary and base- ment rocks, while the third includes volcanic and sedimentary rocks from Devonian to Middle Jurassic (Fig. 2). Later on, these three units were categorized as three distinct subterranes (Silberling et al., 1987), named the Yreka, Trinity and Redding subterranes, respectively (Fig. 1). The latter comprises a series of Paleozoic and Mesozoic eastward-dipping strata similar to a monocline, including the Hosselkus limestone (Renne, 1986) (Fig. 2). The presence of diverse volcanic rocks suggests that the region underwent several episodes of volcanism during its evolution and some, like the Dekkas andesite, are consistent with an island arc hypothesis for the formation of the Eastern Klamath terrane (Burchfiel and Davis, 1981). The structural nature and sedimentol- ogy of formations accumulated during the Permian and Triassic periods, indicate deposition on an area periodically subjected to graben faulting (Renne, 1986). Some authors defined these extensional movements as related to back-arc spreading (Burchfiel et al., 1992;Dickinson, 2004).

Some of the terranes forming the Eastern Klamath Mountains amalgam- ated together before they were stacked onto the American craton, as a result of multiple thrust zones active along these long-lived volcanic- arc systems (Irwin, 1989) (Fig. 1). However, evidence for the time of accretion of Eastern Klamath terrane onto the craton is rare due to the thick and widespread layer of Cretaceous and younger strata hiding the terrane boundaries. Paleomagnetic data suggest that most likely it occurred during the Upper Jurassic period (Irwin, 1989;Mankinen and Irwin, 1982).

3. Studied area and mode of occurrence

Field investigation focused on an area located about 40 km north- east of Redding, Shasta County, California. Nine outcrops were surveyed, described and sampled in November 2018 and June 2019 (Fig. 3). GPS coordinates of each outcrop are listed inTable 1.

The main carbonate body is located on the divide between the Pit River arm and the Squaw River arm of Shasta Lake and appears as a north–south striking crest about 8 km long, disturbed by one or more faults having approximately the same direction as the ridge. The presence of these faults is signiﬁed by a signiﬁcant dip change along the crest, being sub-horizontal from the southern margin of Gray Rocks to Devils Rock and sub-vertical on the relief north of Low Pass Creek (Fig. 4A, B). The thickness of the whole succession is around 170 m at the southern margin and 100 m at the northern one. Contact with the

underlying Pit Formation is marked by a lithologic transition from non-calcareous to calcareous dark beds and it is often hidden by dense vegetation. South-east of Gray Rocks, numerous limestone bodies emerge from the dense vegetation along the banks of Lake Shasta (Fig. 4C, D). North of these locality, sub-vertical outcrops arise sporadi- cally up to 20 km away from Gray Rocks, along the North Fork of Squaw Creek (Fig. 5C, D). They are aligned with the direction of the main crest, but with a reduced thickness of 75 m. Ten km south-east of Gray Rocks, the Hosselkus limestone crops out in two main quarries along Highway 299, named Gravel Pit and Bear Gulch quarries (Fig. 5A, B). There we can observe two limestone domes around 200 m wide, with dubious dip due to the massive nature of the carbonate, but possibly showing the contact with the overlying Brock Shales. Last, a small outcrop, occurring as a lens of a few tens of meters, crops out in Bear Mountain, on the south bank of Shasta Lake. This lens turned out to be fundamental in the interpretation of the limestone evolution, thanks to the peculiar facies and stratigraphic contacts. Here, the improved preservation of the limestone allowed a precise description of the processes involved in Hosselkus limestone deposition, such as thick breccia deposits.

4. Methods

Thin sections from 187 samples wereﬁrst scanned using a high- resolutionﬁlm scanner (Nikon CoolScan 4000 ED), then analyzed in de- tail with an optical microscope (Leica DME). Data collected were used to classify facies according toDunham (1962)andEmbry and Klovan (1971). Facies were then grouped according to assemblages, biotic content and texture in order to interpret paleoenvironment and hydrodynamic conditions of deposition.

Due to the poor preservation of the samples, foraminifera were observed only in cathodoluminescence (Cathodyne, by NewTec Scientiﬁc mounted on an OlympusBX41 microscope), leaving some doubts about the nature of the test (i.e. porcelaneous, microgranular- agglutinated or aragonitic) and inevitably about their age. Conse- quently, a great part of the material has been processed for conodont ex- traction, in order to determine the age of the Hosselkus limestone.

Carbonate samples were dissolved in 12% acetic acid and sieved with a 63μm sieve (Green, 2001), then separated from an abundant lighter compound by means of sodium polytungstate 2.85 g/cm³(Jeppsson and Anehus, 1999). After picking, specimens were mounted on a con- ductive aluminum support, coated with 10 nm of gold and imaged with a Jeol JSM 7001F Scanning Electron Microscope. All the prepara- tions, observations and analyses were performed at the Department of Earth Sciences, University of Geneva, Switzerland.

5. Results and interpretations

5.1. Microfacies analysis

5.1.1. Halobiids and radiolaria mudstone to wackestone (MF1)

5.1.1.1. Description.MF1 consists of dark gray to black micrite associated with two main skeletal grains:Halobiashells and radiolaria. Thefirst show the peculiar thin shells coupled with a low varve convexity and length ranging between a few hundred microns to centimeters, depending on the integrity of the shell and on the ontogenic state (i.e. juvenile vs. adult). The thickness of the specimens is usually around a few microns and they appear completely recrystallized (Fig. 6C). However, the thickest ones show a preserved internal layering. Radiolaria appear in polarized light as small white dots completely recrystallized in calcite (Fig. 6B, C), while few internal structures are visible in cathodoluminescence (Fig. 6D). Dimensions do not exceed 200μm and preserved forms allow the presence of both Spumellaria and Nassellaria groups to be recognized, although further identifications were not possible. The presence of radiolaria is rather constant in this facies with 30–40 specimens/cm², while the number ofHalobiashells varies significantly, creating millimetric to

A. Fucelli, M. Golding and R. Martini Sedimentary Geology 422 (2021) 105967

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Fig. 1.Terranes subdivision in the Klamath Mountains (modiﬁed afterIrwin and Wooden, 1999) as result of allochthonous oceanic terranes accretion which started during the Early Paleozoic and lasted until the Early Cretaceous. Moving from west to east, each terrane on the right is earlier accreted with respect to the one at its left, furthermore, the nowadays position derives from a 110° clockwise rotation continued for the entire accretion time (Irwin and Mankinen, 1998).

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centimetric levels exceptionally rich in these remains to the point of being the only constituents of the rock (Fig. 6A). Other occasional grains occurring in this facies are cephalopods, larger bivalve shells and crinoid fragments, mostly concentrated in thin levels. The facies is always found in dark centimetric limestone beds.

5.1.1.2. Interpretation.Both radiolaria andHalobiasuggest deposition in a rather deep and quiet environment, however, above the calcite compen- sation depth (CCD). Early recrystallization of radiolaria to calcite testifies the presence of an alkaline substrate at the bottom of the basin, supported by the presence of micrite probably derived from the nearby carbonate platform (Afanasieva and Amon, 2015). At Gray Rocks, where the most complete succession crops out, this facies is thefirst appearing above the hidden contact with the underlying Pit Formation, indication of the passage from siliciclastic to carbonate deposits. In addition to lithologic differences, the two kinds of deposits reveal distinctHalobiacontent and radiolaria preservation, respectively absent and not calcified in the Pit Formation. The emplacement of this facies suggests thefirst phase of growth of a neighboring carbonate platform, now able to spread micrite

into a deep basin formerly characterized only by siliciclastic deposits. Oc- casional deposition of larger bivalves, crinoids and any other shallower organisms could be the result of a wide spectrum of events, such as storms, oceanic currents, slope destabilization and tectonic events, all able to transport such grains far from their original living position.

5.1.2. Bioclastic wackestone to packstone (MF2)

5.1.2.1. Description.Dark micrite with abundant biotic fragments, rarely preserved as whole fossils. Principal constituents are pelagic and non- pelagic bivalves and crinoid stem remains; less abundant are trepostome bryozoan fragments and cephalopod shells. These fragments present two different modes of occurrence: structureless and ho- mogeneously mixed with the surrounding matrix (wackestone) (Fig. 6E), or in concentrated levels (packstone) (Fig. 6F), slightly graded, intercalated with a matrix similar to MF1. Volcanic material is present, with isolated grains ranging from a few microns to millimeters. The facies occurs in medium bedded limestone, situated in the lower part of the carbonate succession both at Gray Rocks and Bear Gulch quarry.

Fig. 2.Geologic map of the Redding subterrane central part (modiﬁed afterFraticelli et al., 2012). The main structure represents an east-dipping sequence spanning from the Devonian Kennett Fm to the Jurassic Potem Fm. All the formations part of this monocline underwent several minor tectonic events, often resulting in chaotic dip directions within same outcrop.

The Hosselkus limestone reﬂects the monoclinal nature of the area outcropping as a discontinuous north–south crest.

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5.1.2.2. Interpretation.Although the presence of crinoids, bryozoans and cephalopods testiﬁes to deposition on an open-shelf or slope environment (Cuffey, 1970;Skelton, 1982;Wilson, 2012;Flügel, 2013), the

high fragmentation of all these biotic compounds, suggests transportation for long distances, toward the lower part of the slope (Tucker, 1969). Intercalations with MF1 suggest a similar water depth and Fig. 3.Outcrops map with the nine sampled localities. Numbers (1 to 9) are reported inTable 1with respective GPS coordinates. Names of rivers and mountains are used within the text to guide the reader.

Table 1

Coordinates of the studied localities and collected samples (only specimens illustrated in this work). Numbers on the left column refer to the ones reported inFig. 3, while locality names aim to guide the reader through the text.

Number inFig. 3 Locality name GPS coordinates Samples

1 North Fork Squaw Creek 1 40° 59′9″N;−122° 6′2″W FA110b, FA180

2 North Fork Squaw Creek 2 40° 56′41″N;−122° 7′22″W FA183, FA184

3 Low Pass Creek 1 40° 52′40″N;−122° 6′27″W FA26

4 Low Pass Creek 2 40° 51′52″N;−122° 5′59″W –

5 Gray Rocks 40° 49′5″N;−122° 7′6″W FA15, FA17, FA196

6 Brock Creek–Lake Banks 40° 48′43″N;−122° 5′33″W FA149, FA158

7 Bear Gulch quarry 40° 46′44″N;−122° 0′10″W FA49, FA50, FA12, FA129, FA130, FA140, FA210

8 Gravel Pit quarry 40° 45′9″N;−122° 2′41″W FA44

9 Bear Mountain 40° 43′51″N;−122° 15′12″W FA113, FA117, FA118

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hydrodynamic conditions with respect to the latter, though the absence of any deep-water biota in MF2 (i.e. radiolaria andHalobia) implies an allochthonous origin of the deposit. The absence of siliciclastic material and the occurrence of volcanogenic grains strengthen the hypothesis of deposition close to an active volcanic arc system.

5.1.3. Bioclastic grainstone (MF3)

5.1.3.1. Description.Biotic compounds in this facies show strong afﬁnities with the one described in MF2, with the occasional addition of shallower grains. The latter represent a broad variety of organisms and grains often micritized, slightly visible in cathodoluminescence, such as foraminifera, fecal pellets, ooids, cortoids and different types of peloids (Fig. 7C, D). In general, the observed organisms are bigger and less damaged than in MF2, with whole cephalopod shells of various dimensions (Fig. 7A). Differently from the previously described facies, MF3 is cement-supported, principally due to syntaxial overgrowth on crinoid fragments (Fig. 7C). Grain size varies from bottom to top in coarsening upward sequences (Fig. 7B), with smaller peloids concentrated in the lower part and crinoid–cephalopod remains in the upper one. This grading slightly inﬂuences the cementation, which is stronger where crinoids dispose larger space for syntaxial overgrowth.

Centimetric bioturbations (Fig. 7B) characterize the cemented levels, al- waysﬁlled by mudstone to wackestone, resembling MF2, with some exception in the biotic content (i.e., radiolaria are absent). MF3 occurs in a regular bed of about 15–20 cm, and at Gray Rocks it is located in the central part of the succession.

5.1.3.2. Interpretation.Extensive cementation is the main feature of this microfacies, reﬂecting peculiar features in terms of grain size and grading. Physical characteristics are common in hyper-concentratedﬂows

(grainflows), driven by gravity energy along a medium to high angle slope (Mulder and Alexander, 2001). These deposits represent mass movements along the slope, with a great amount of materialflowing at the same time, differently from MF2 grains whose deposition did not necessarily occur at the same time. Cementation took place just after sedimentation during periods of prolonged non-deposition, as suggested by the cloudy nature of the syntaxial cement (Walker et al., 1990;Flügel, 2013), supporting the hypothesis of occasional mass movements. Non-deposition periods may be due to a combination of two main factors: low sedimentation rate and unfavorable morphological condition (i.e. steepness of the slope). Extensive cementation and recrystallization make recognition of already poorly preserved peloids more difficult, hiding any internal features. However, shapes present in the microfacies indicate the occurrence of Bahamite peloids (Beales, 1958;Flügel, 2013) and some undetermined fecal pellets. The presence of reworked shallow-water grains suggests a position closer to the platform, with respect to previously interpreted facies, although cephalopods and crinoid remains are the main sediment producers.Halobia shells often top the cemented layers, strengthening the hypothesis of an open pelagic environment.

5.1.4. Micro-calciturbidite (MF4)

5.1.4.1. Description. This microfacies is deﬁned by small sequences (about 3–4 cm) ofﬁning upward deposits (Fig. 7F). Despite reduced dimensions, some sub-microfacies can be recognized based on granulometry and grain origin. The bottom part, a bioclastic packstone to grainstone, comprises organisms autochthonous of a slope environment (i.e., small cephalopods and crinoids). In the central part, a bioclastic wackestone to packstone, the same organisms occur with reduced dimensions, integrated with peloids, fecal pellets and sporadic Fig. 4.Outcrops of Hosselkus limestone. A. Southern margin of Gray Rocks, the exposed section is about 60 m thick. B. Subvertical crest north of Low Pass Creek, total height in the picture is about 15 m. C. Gray Rocks outcrops seen from Pit River arm of Lake Shasta. D. Limestone outcrops along Lake Shasta banks.

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slightly-micritized ooids. At the top, more abundant micrite is still associated with smaller cephalopod shells and crinoid fragments, and in addition, thin andﬂatHalobiashells are often present. Some foraminifers can be observed only in cathodoluminescence due to the poor preservation of the samples. Nonetheless, doubt about the original composition of the test did not allow precise identiﬁcation so far and they will be treated in a separate work.

5.1.4.2. Interpretation.Most of the organisms found in these deposits are still of pelagic-slope origin, not suggesting significant changes in the overall depositional environment with respect to the previously described microfacies. What differs is the normal grading, resulting from hydraulic sorting of skeletal and non-skeletal grains. While MF1 and MF2 indicate no consistent grading and MF3 shows inverse grading, in MF4 normal grading occurs as a consequence of concentrated density flow (Mulder and Alexander, 2001). Such graded deposits reflect, although some features are missing, the typical evolution of a turbiditic sequence (Herbig and Mamet, 1994). The reduced dimensions of the cy- cles probably indicate limited availability of material both from slope margins and the shallower part of the platform (Playton et al., 2010).

Relatively good preservation allows the recognition of some shallow- water grains as ooids, cortoids and the microcoproliteFavreinasp.

(Fig. 7E), reworked in exotic environments, but possibly indicating the presence of a restricted lagoon and shoals in the micro-turbidites source area (Kennedy et al., 1969;Flügel, 2013).

5.1.5. Ooidal-bioclastic grainstone (MF5)

5.1.5.1. Description.The facies is characterized by the presence of a conspicuous number of shallow water non-skeletal grains such as ooids, microbial and mud peloids, cortoids, fecal pellets and aggregate grains.

Skeletal grains are also present and comprise gastropods, bivalve shells

and minor crinoid fragments (Fig. 8A, B, E). Apart from the latter, all the components display different grades of micritization and coating. Inter- particle space isﬁlled by blocky sparite and dogtooth cement, with occasional occurrence of syntaxial overgrowth where crinoid fragments are present (Fig. 8F). Colonial worm tubes (Filogranasp.) occur in the form of millimetric intraclasts (Fig. 8C).

5.1.5.2. Interpretation.The facies is typical of a shallow-water environment constantly affected by wave agitation, like back-barrier subtidal sediments (Hine, 1977;Scholle et al., 1983;Tucker, 1985). The cortex of ooids is radial-ﬁbrous, suggesting a relatively low-energy environment (Flügel, 2013). Extensive micritization of the inner part of ooids, as well as other grains, suggests periodical exposure to the light at the water–sediment interface indicating deposition in a cyanobacteria- rich environment and possibly a slow sedimentation rate (Kendall and Alsharhan, 2011;Reid and Macintyre, 1998). Numerous grapestones, lumps and cortoids imply the presence of a neighboring calm environment (Flügel, 2013;Swinchatt, 1969), possibly a protected lagoon where abundant crinoids and gastropods thrive. The presence of Filogranasp. supports the hypothesis of a water depth above the storm wave base (SWB) (Senowbari-Daryan and Link, 2005;

Senowbari-Daryan et al., 2007).

5.1.6. Bio-constructed facies (MF6)

5.1.6.1. Description.MF6 represents a set of bio-constructed frameworks in which 3 main groups of organisms act as main biota: coral colonies, microbialites and red algae. Even if all of them are present in almost every described sample, their abundance is rather variable, making just one of them the principal constituent of the buildup in a particular sample. For this reason, MF6 has been divided in three sub-facies described separately.

Fig. 5.Outcrops of Hosselkus limestone. A. Limestone outcrops cut by Highway 299 and Little Cow Creek. B. Bear Gulch quarry. C. & D. Subvertical strata cropping out along North Fork Squaw Creek. Scales are circled.

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5.1.7. MF6a

Colonial scleractinian corals form distinct frameworks easily recognizable in outcrops as well in hand-samples (Fig. 9A). In some cases, internal structures were partially preserved showing the presence of different types of corallites: cerioid, sharing adjacent walls (Fig. 9B);

thamnasterioid, with poorly defined walls and septo-costae of different corallites welded together (Fig. 9C, D); phaceloid, having distinct walls (Fig. 9F); and platy, with a characteristicflat shape (Fig. 9E). Low preservation did not allow taxonomic identification of these corallites at the same level: cerioid corals belong to the family Pamiroseriidae while thamnasterioid corals could be associated either to Parastreomorphasp. orAstreomorphasp. Phaceloid corals have been identified asEocomoseris minima, while no identification was possible for platy corals. Dark micritefills the space between corals, while in the external part other bioclasts like crinoid fragments, brachiopods and ostracods are present. Boring from bivalve often occurs (Fig. 9D).

All these framebuilders are associated with abundant agglutinated

worm tubes referred toTerebellasp. Real dimensions of these coral colonies are impossible to verify, since they are preserved as isolated blocks reworked along the slope, as discussed later in this work. Some were already described as metric colonies by past authors and their remarkable dimensions led them to be erroneously interpreted as in- place corals (McCormick, 1986).

5.1.8. MF6b

Two types of microbial bioconstructions are associated in this facies and are differentiated based on framework features and the nature of trapped grains and micrite. Theﬁrst is composed of clotted and peloidal micrite, with grains that remain separated by microsparite (Fig. 10A, C).

The structure is organized in heterogeneous layers whose orientation allows the recognition of build-up polarity. Other constituents trapped in growth layers are often microcoprolites, ooids and small bivalve fragments with thick micritic envelopes, together with crinoid fragments.

Microbial layers are also characterized by stromatactis-like cavities

Fig. 6.Microfacies of Hosselkus limestone. (MF1 and MF2) A.Halobiarich level, shells are the only constituent of these centimetric beds (FA110b). B. Radiolaria tests completely recrystallized in calcite, no features are observable (FA149). C. Typical occurrence of MF1, with recrystallized radiolaria (Ra) andHalobia(Ha) associated to dark micrite (FA183). D.

Radiolaria tests in cathodoluminescence, morphological features testify the presence of two groups: Nassellaria (N) and Spumellaria (S) (FA184). E. Crinoidal wackestone characteristic of MF2. Main constituent within the matrix are crinoids (Cr), Trepostome bryozoans (Br) and altered volcanic material (v) (FA44b). F. MF2 occurring as alternation of packstone levels rich in crinoid fragments (Cr) and wackestone stages with Radiolaria (Ra) andHalobia(Ha) remains. Small cephalopods shells (Ce) are also present in the packstone (FA180). Scale bars: A 1 mm; B, C 500μm; D 200μm; E, F 2 mm.

Fig. 7.Microfacies of Hosselkus limestone. (MF3 and MF4) A. Cephalopod shells (Ce) and crinoid fragments (Cr) slightly cemented in MF3 (FA196). B. Typical reverse grading occurring in MF3 associated to bioturbations (bt). Coarser upper part is characterized by the presence of cephalopod (Ce) and crinoid remains (Cr), while theﬁner lower part by mud peloids (mp), cortoids (ct), microcoprolites (cp) and other micritized grains (FA17). C. Closer view of common MF3's grains like crinoids, cortoids (ct) and microbial peloids (mip). Syntaxial overgrowth on crinoid fragments is also visible (so) (FA17). D. Preserved ooids (oo) next to syntaxial cement (so) (FA19). E. Microcoprolite (Favreinasp.) (FA15). F. Typical normal grading occurring in MF4 with crinoid fragments (Cr), mud peloids (mp), bivalve shells (Biv) and small cephalopods tests (Ce) (FA15). Scale bars: A 3 mm; B, C, F 2 mm; D 500μm; E 200μm.

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that probably originated from soft organisms' decay, like sponges and algae, or possibly related to the activity of dwellers in the microbial substrate. The second type of microbial bioconstruction shows less peloidal micrite and trapped grains, having as a major constituent aphanitic micrite (Fig. 10B). Intergranular sparite is almost absent and occurs only in a few zones. The overall framework appears almost dendritic, with higher relief and more complex forms, with reduced layer length with respect to theﬁrst type. These latter types often have a half- moon shape, reaching maximum thickness in the central part and having reduced dimensions near the lateral margin of the build-up. Within layers, it is possible to observe some Nubecularids and other small microencrusters impregnated with organic matter (Fig. 10B). Interstitial sediments are similar to the ones of described before, with ostracods and peloid-rich dark micrite. In both cases, serpulid and terebellid tubes occur in a considerable amount, often in living position, together with recrystallized sponges (Fig. 10C, D).

5.1.9. MF6c

As for other shallow-water facies described in this work, a differen- tiation must be made between outcrop and thin section observation, in terms of preserved fauna. In hand-sample, MF6c presents a considerable number of sponges, up to 15 cm in diameter, often in living position (Fig. 11A). These organisms are completely recrystallized and no morphological features are visible in the thin section. Conversely, well preserved specimens of red algae (“Parachaetetes”sp.), occurring both as

fragments and in living position were observed in some samples and described byBucur et al. (2020)(Fig. 11B, C). This microfacies is marked by dark peloidal micrite, accompanied by shell and crinoid fragments of various dimensions. Red algae exist both as fragments and in living position. Extensive early dissolution affected the rocks, creating large vugs ﬁrst bounded by isopachous dogtooth cements and thenﬁlled by dark homogeneous micrite.

5.1.9.1. Interpretation.Though scleractinian corals became the main reef builders in the Upper Jurassic, they were important carbonate producers also during the Upper Triassic period (Veron, 1995). However, in contrast with modern reef-building corals, that display speciﬁc growth forms depending on water depth and wave turbulence (James and Ginsburg, 2009), they probably had completely different behavior. In fact, most Triassic coral reefs described in the literature show characteristics different from the ones of recent coral reefs and they are usually preserved as thin and laterally restricted buildups (Stanley, 1979a, 1979b;Flügel, 1982;Senowbari-Daryan and Stanley, 1992;Veron, 1995;Roniewicz and Stanley, 2013;Peyrotty et al., 2020b). Field inves- tigations and petrographic analysis reveal that most of these buildups developed in relatively quiet-water settings, some of which may have been deeper than is usual for modern reef systems (Stanley Jr, 1981;

Stanley, 1982). In the studied area, it is consistent with the hypothesis of small patch reefs in the fore-reef to upper slope zones, though devel- opment in a mud-rich environment like a lagoon and subsequent Fig. 8.Microfacies of Hosselkus limestone. (MF5) A. Bioclastic grainstone rich in cortoids (ct), crinoid and bivalve fragments (Cr, Biv), mud peloids (mp), aggregate grains (ag) and ooids (oo) (FA113). B. Coated and micritized grains like bivalves (Biv) associated with mud peloids (mp) and Bahamite peloids (bhp) (FA26). C.Filogranasp. worm tubes (Ft) occurring together with mud peloids (mp) and crinoid fragments (Cr) (FA158). D. Radialﬁbrous ooids (FA113). E. MF5 bioclastic grainstone with abundant gastropod shells (Gr) associated with mud peloids (mp), cortoids (ct) and ooids (oo) (FA111). F. Syntaxial overgrowth around a crinoid fragment (So) (FA113). Scale bars: A, C 1 mm; B 500μm; D 100μm; E 2 mm; F 300μm.

Fig. 9.Litho- and microfacies of Hosselkus limestone. (MF6a) A. Phaceloid corals colony (Phc), arrows indicate the sharp border of the block. B. Cerioid corals (Cc) belonging to the family Pamiroseriidae (FA117). C.Thamnasteroid corals(Tc)Parastreomorphasp. orAstreomorphasp. (?) growing on microbial micrite (mm) (FA118). D.Thamnasteroid corals(Tc) Parastreomorphasp. orAstreomorphasp. bored by bivalves (bt) (FA118). E. Poorly preserved platy corals (Plc) (FA140b.) F.Eocomoseris minima(Em) with numerousTerebellasp.

worm tubes (Te) (FA49b). Scale bars: B, C 2 mm; D 3 mm; E, F 4 mm.

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transportation along the slope cannot be excluded. The lack of any continuity on outcrops represents an obstacle for a clear interpretation.

Microbialites were important reef builders during Middle and Upper Triassic and their presence spans from shallow water settings to the deeper part of the euphotic zone, associated with a broad variety of other organisms (Leinfelder and Schmid, 2000;Reid, 1987).

Occurrence of stromatolitic microbialites associated with serpulids and other types of tube-dwelling polychaetes has been already described in Upper Triassic carbonates from the Tethyan Ocean and related to low energy conditions (Iannace and Zamparelli, 1996;De Zanche et al., 2000;Gale et al., 2018).Flügel (2013)placed this faunal association in two main areas of Upper Triassic carbonate platforms:

upper ramp and platform edge.Cirilli et al. (1999)also suggested that such an association of polychaetes and microbialites points to some sort of stressful circumstances in the depositional environment, such as low oxygen conditions, anomalous salinity or eutrophic setting.

This consideration is relevant considering that this facies is one of the last in the whole Hosselkus limestone succession, appearing, although in the form of reworked clasts, a few meters below the transition to Brock Shales, probably marking the cessation of carbonate production from the platform.

Red algae, often in living position, together with sponges are a typical reefal association, located in areas subject either to moderate or

strong wave action (Flügel, 2013). The lack of any buildup continuity does not permit a detailed description of the overall depositional environment. The presence of muddy areas, with red algae and sponge fragments, may suggest a calm environment, in the upper photic zone but below the fair weather wave base (FWWB), probably not far from their source area. On the other hand, the same organisms in living position, surrounded by cemented peloids, suggest the existence of a more agitated environment, where the same fauna thrived. Extensive early dissolution may be related to platform exposure episodes and occur prior to clast reworking in deeper positions when vugs werefilled by homogeneous micrite. However, the lack of any significant microorganism in the infilling matrix does not allow the determination of its source area and age.

5.2. Biostratigraphy

5.2.1. Conodont biostratigraphy

Previous records of conodonts from the Hosselkus limestone have been presented byMosher (1968, 1973)andDu et al. (1992). The only platform species illustrated in these publications was Metapolygnathus polygnathiformis(Mosher, 1973, 1968), which is a Lower to mid-Upper Carnian species; examination of thefigured specimen inMosher (1968)suggests that this may in fact belong to the Fig. 10.Microfacies of Hosselkus limestone. (MF6b) A. Microbial bioconstruction composed of clotted and peloidal micrite, calcite infillings (c) are related to the decay of soft organisms or to boring in the microbial substrate. Layering (l) indicates the polarity of the structure (FA122). B. Microbial bioconstruction mostly composed of aphanitic micrite, significant relief indicate the growing direction. Calcite infilling (c) and numerous microencrusters (Me) are also visible (FA129). C. Closer view of a microbial bioconstruction rich in peloidal micrite. Microbial peloids (mip) are separated by microsparite.Terebellasp. and serpulids worm tubes (Te, Se) occur in great amount, together with stromatactis (s) (FA130). D. Recrystallized sponges and related stromatactis (s), associated to microbial bioconstruction (view parallel to growth) (Mb) and worm tubes (Wt) (FA49). Scale bars: A 3 mm; B 2 mm; C, D 1 mm.

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Upper Carnian speciesQuadralella carpathica. In the present study, addi- tional conodonts have been recovered from six samples of the Hosselkus limestone, and all species identified are consistent with an Upper Carnian age for this formation. The diverse fauna includes specimens of typical Upper Carnian species such asQuadralellaex gr.oertlii, Quadralella tuvalicaandQuadralella carpathica(Fig. 12F, G, H), all of which have been identified in Tethys (Rigo et al., 2018) as well as in continental margin deposits of northeastern British Columbia, Canada (Orchard, 2014). In addition to these geographically widespread species, the Hosselkus limestone also contains species previously found only in northeastern British Columbia, including Quadralella willistonenseandParapetella beattyi(Fig. 12A, I). Similarly, the species Parapetella laneiandQuadralella postlobata(Fig. 12D, E) have been re- corded from northeastern British Columbia (Orchard, 2014) and from the allochthonous Stikine terrane in the western Canadian Cordillera (Golding et al., 2017), but not from elsewhere. The samples from the Hosselkus limestone also contain a single unusual specimen that appears to belong to a new species ofQuadralella, as well as a specimen that strongly resembles the Lower Carnian speciesCarnepigondolella carnicaand may also be a new species; however, the material is not abundant enough to confirm this. IfCarnepigondolella carnicawere truly present in the fauna, then this would be the oldest species identified in this study, as it occurs in the upper part of the Lower Carnian in Tethys (Krystyn, 1980;Rigo et al., 2007, 2018). However, the remainder of the Hosselkus fauna is Upper Carnian, and so the identification of Carnepigondolella carnicain these samples remains questionable. The oldest species identified in this study is Carnepigondolella zoae (Fig. 12C), whichfirst appears in thezoaesubzone (uppersamueli Zone) of British Columbia (Orchard, 1991, 2014, 2019).Quadralella carpathicaappears slightly higher in British Columbia (uppermost samueliZone), where it ranges into the basal part of the overlying primitiaZone (sagittale-beattyisubzone) and occurs together with Quadralella tuvalica,Quadralellaex gr.oertlii, andParapetella beattyiin this subzone (Orchard, 2014, 2019). However, in Tethys, some of these speciesfirst appear earlier in the Upper Carnian;Quadralellaex gr. oertlii occurs together with Carnepigondolella zoae in the praecommunistiZone, andQuadralella tuvalicaranges from the Lower to Upper Carnian boundary until the end of the Carnian (Mazza et al., 2018;Rigo et al., 2018). The youngest species recovered from the Hosselkus limestone are Norigondolella navicula (Fig. 12B) and

Quadralella willistonense, both of whichﬁrst appear in the next highest subzone of theprimitiaZone in British Columbia, theangusta-dylani subzone, and range into the lower parts of the Norian in both North America and Tethys (Orchard, 2014;Rigo et al., 2018). Taken together, the conodont species indicate an Upper Carnian age for the Hosselkus limestone, likely the middle to upper part of the Upper Carnian.

6. Discussion

6.1. Microfacies distribution in the Hosselkus limestone

Microfacies analysis together withﬁeld observations, allows some considerations about the Hosselkus limestone's depositional setting and microfacies distribution.

In several outcrops, macro-fauna and grains with completely different ecologic and hydrodynamic needs coexist, suggesting a re- sedimentation of shallow-allochthonous material in a deeper environment. Petrographic analysis conﬁrmed this hypothesis and allowed the recognition of 3 different types of deposits, all characteristic of a foreslope environment (Cook et al., 1972;Davies, 1977;Mulder and Alexander, 2001;Playton et al., 2010;Savary and Ferry, 2004).

- Mud dominated deposits (MF1 & MF2): Consist ofﬁne-grained deposits settled in a pelagic environment probably during slope quies- cence, often associated with planktonic microfossils (i.e. radiolaria).

In theﬁeld they crop out as thin bedded layers probably with considerable length along strike and dip (Cook and Enos, 1977), omitted in the visited outcrops by structural complication and dense vegetation. Occasionally, they occur intercalated with coarser-grained surfaces, probably representing cyclic activity on the slope, and they can be either structureless or laminated.

- Grain dominated deposits (MF3 & MF4): Made up of a skeletal and non-skeletal mixture, respectively autochthonous of the platform edge and platform top shoals, then transported downslope and re- deposited. Wave action, tidal and storm currents, especially basinward tidal ebbflow and storm returnflow, drive this process (Playton et al., 2010). In the Hosselkus limestone, the grain size of these deposits does not exceed 1 mm, but they are often associated with autochthonous fauna up to 20 cm (i.e. ammonoids and nauti- loids). As for mud layers, they are usually interrupted by cemented Fig. 11.Litho- and microfacies of Hosselkus limestone. (MF6c) A. Metric block rich in sponges (arrows). B. MF6 bioclasticfloatstone, with completely recrystallized sponges (Sp) but preservedParachaetetessp. red algae (Pa). Brachiopods (Bc) and other shells also occur together with clear dissolution features (df) (FA50). C.Parachaetetessp. red algae (Pa) and sponges (Sp) in living position (FA210). Scale bars: B 3 mm; C 2 mm.

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surfaces, associated with bioturbation in several samples. These deposits form thin to medium beds and extend for great distances along strike, reﬂecting the line-source nature of contributing sediment factories (Mullins and Cook, 1986) recurrently with signiﬁcant changes in grain type along the same sampled bed.

- Debris dominated deposits (MF5, MF6): Mainly generated by gravitational collapse of lithiﬁed material, the block-size of these deposits ranges from a few centimeters to a few meters (Fig. 13A, C). In the Hosselkus limestone they occur as matrix-supported breccia, with clasts native to the fore-reef environment associated withﬁne micrite with rare pelagic microorganism (Fig. 13D). In contrast with the

previous deposits, they appear massive, with no internal structures and form discontinuous tongues able to produce signiﬁcant changes in the above topography. In most of the cases, strong surface weathering makes the recognition of this type of deposit difﬁcult, as the edge of clasts is homogenized with the matrix (Fig. 13B, C).

Within the three different types of deposit, 6 different microfacies were identiﬁed and interpreted according to grain assemblages and tex- tures. Note that facies from MF1 to MF4, although containing allochthonous skeletal and non-skeletal components, were lithiﬁed after the reworking of grains on a slope environment. Conversely, facies from Fig. 13.Outcrops' details and microfacies of Hosselkus limestone. A. Debris deposit lying directly on Pit Fm at Bear Mountain. B. Transition from layered grain dominated deposits to massive debris deposits at Gray Rocks. C. Debris deposit features (see arrows) hidden by strongly weathered surfaces. D. Thin section from weathered sample of debris deposit shows an ooidal grainstone on the left and recrystallized radiolaria (Ra) on the right (113b). Scale bars: D 1 mm.

Fig. 12.Late Carnian conodonts of Hosselkus limestone. A.Quadralella willistonense; B.Norigondolella navicula; C.Carnepigondolella zoae; D.Parapetellacf.lanei; E.Quadralella postlobata; F.

Quadralellaex gr.oertlii; G.Quadralella tuvalica; H.Quadralellaex gr.carpathica; I.Parapetella beattyi. Scale bars: 100μm.

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MF5 and MF6 underwent lithiﬁcation in the original shallow-water environment and subsequently they were reworked along the slope as extraclasts. Since no autochthonous shallow-water environment outcrops exist, extraclasts were used as a proxy for paleoecological descrip- tions of the Hosselkus limestone.

6.2. Depositional model

Field observations and microfacies analysis show that the Hosselkus limestone was deposited in a foreslope environment whose geometries were strongly inﬂuenced by the heterogeneous distribution of the previously described deposits (Fig. 14). Outcrops belonging to the same ridge crest, reasonably considered part of the same horizon due to the orientation of their beds, manifest marked differences in terms of deposit content (i.e. mud, grain and debris dominated). This is mainly due to the preferential patterns followed by reworked material along the slope. These patterns must not be considered as channels with limited width, but rather as broad aprons whose dimensions range from a few tens to hundreds of meters. The result is a complex arrangement of bedded and massive carbonates, representing respectively mud- grain and debris dominated deposits. Theﬁnest one, MF1 and MF2, occurs in all the studied localities except for Bear Mountain and often marks the transition from the underlying Pit Formation to the Hosselkus

limestone. Despite few intercalations with grain-dominated deposits (MF3 and MF4) it always appears at the bottom of every sampled sequence, indicating theﬁrst phase of limestone deposition on a zone probably located between the basinﬂoor and the lower slope. MF3 and MF4 (grain-dominated) appear in the main outcrop that spans between Low Pass Creek and Gray Rocks and discontinuously in some small outcrops along highway 299, which not by chance represent the thickest sections of the limestone. They are completely absent in Bear Mountain and on the three crests along North Fork Squaw Creek.

When present, they are always placed between mud and debris dominated deposits, in the central part of the sequences. This position allowed us to measure the thickness of the deposits, which resulted in varying thickness in different outcrops along strike, ranging between

~25 and ~60 m. Though it was almost impossible to follow single beds for more than few tens of meters, differences in their thickness are easily recognizable along strike, with changes of about 5 cm. Deposition of grain dominated deposits took place on an area situated between the lower and middle slopes, where enough gravitational energy allows basin-ward movement of reworked grains. MF5 and MF6, representing debris dominated deposits, occur in all the sampled localities with the exception of North Fork Squaw Creek crests. These deposits are consis- tently at the top of the sequence, either in contact with mud or grain deposits (Fig. 13B). Accurate estimation of the thickness is not possible as

Fig. 14.Depositional model of Hosselkus limestone. Distribution of three different types of deposit and relative facies along the slope. Ratio between these three varies at each studied locality and it is represented qualitatively by small logs around the model.

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the upper contact with the Brock Shales is often eroded or covered by dense vegetation. At Gray Rocks and Bear Gulch quarry, strongly weathered and karstiﬁed deposits have a thickness ranging between 40 and 90 m, while at Bear Mountain it does not exceed 20 m. In the last locality, this type of deposit is the only one present and it occurs in contact with the Pit Formation, with no transitional facies in between (Fig. 13A). These deposits were accumulated in an area between middle and upper slopes but, in exceptional circumstances, masses of clasts could have reached the basinﬂoor where little or no mud deposits were settled, as happened at Bear Mountain.

It is extremely important to consider that information retrieved from these deposits does not allow any precise reconstruction of the shallow water environment's geometries. MF5 and MF6 prove the presence of a shallow water setting that either does not crop out anymore, or that has been dismantled during accretion and other tectonic events.

The depositional model inFig. 14has the purpose of showing how the different deposits followed preferential patterns and their temporal evolution, but it may not reﬂect the actual geometries of the shallow water environment. However, as stated in the geological setting chap- ter, the Redding subterrane was periodically subjected to graben faulting, that could have easily led to the formation of structural highs.

With this scenario, the formation of aflat-top platform seems more reasonable, with carbonate production on the footwall of the fault and debris deposition on the hangingwall. This could have created minor distance between these two zones, explaining a reworking process that would be rather difficult along a carbonate ramp with low inclina- tion. The collected shallow water facies show an overall low energy setting, as illustrated by the radial-fibrous cortex of ooids and the presence of numerous grapestones, lumps and cortoids. This is in contrast with a ramp system where waves sweep directly onto and across the shallow seafloor, leading to a higher energy level of the shallow-water environments (Flügel, 2013). Since debris-dominated deposits prevail as the last depositional event in the Hosselkus limestone, and considering the nature of the described facies (i.e. extraclasts), it is reasonable to infer that these lithified clasts collapsed when the hosting platform was already dying or drowning. This could explain the occasional deposition on a basinfloor with a low amount of pelagic micrite, barren from the fossiliferous point of view and possibly originating during the gravitational movement along the slope, rather than on the platform itself. In every studied locality, the Hosselkus limestone appears as a single phase of slope-platform progradation, characterized by the regular transition from muddy to coarser deposits. This testifies to the presence of a short-lived platform that was probably not able to survive during some stressful conditions that arose at one point (i.e. subsidence).

6.3. Comparison with other Triassic carbonates from Panthalassa Ocean Both edges of the Pangean craton were active margins in the Late Triassic and most of the carbonate deposition in northern Panthalassa is thought to have occurred on small seamounts and microcontinents (Stanley, 1979a, 1979b;Stanley, 1982;Flügel, 2002;Martindale et al., 2015;Peybernes et al., 2020). The paleogeography of these terranes in the eastern part of the Panthalassa Ocean has been a matter of discussion for a long time, especially in terms of the proximity of the terranes to each other and with respect to the continental margin. However, during recent years several authors support an Early Mesozoic paleogeography in which several terranes are in close proximity to Pangea and each other (LaMaskin et al., 2011;Petersen et al., 2004;Unterschutz et al., 2002). The Hosselkus limestone shows a series of similarities as well as discrepancies with respect to other carbonates that developed in this enigmatic paleogeographic setting during the Late Triassic.

Starting from the age of the deposits, we can afﬁrm that the Hosselkus limestone was a real pioneer depositional event, if compared with other terrane-based carbonates, that occurred as soon as conditions in the Eastern Klamath terrane allowed the proliferation of calcareous organisms. Although carbonate production in northern Panthalassa had

already begun during Middle Triassic, it was mostly related to areas closer to the American continent, with mixed carbonate-siliciclastic sequences probably related to wide ramps setting, as observed for the Ladinian–Carnian Augusta Mountain Formation in central Nevada (Bonuso et al., 2018, 2020). Other examples of Carnian carbonates from the western margin of North America are the deep-water Ludington Formation and its shallow water equivalent, the Baldonnel Formation, both located in British Columbia, Canada (Pelletier, 1964;

Gibson, 1975;Zonneveld et al., 2007;Martindale et al., 2010) but again, they were both deposited directly on the western margin of the North American craton, rather than on active volcanic arc. Most of the terrane-related carbonates, hence the ones generated far from the coast of the continent, are slightly younger than the Hosselkus limestone. Among these wefind the upper Carnian–lower Norian Martin Bridge Formation in the northeastern part of Oregon (Wallowa terrane) (Stanley et al., 2008;Martindale et al., 2012b), the middle Norian Luning Formation in the western-central Nevada (back-arc basin of the Black Rock terrane) (Martindale et al., 2012a;Sandy and Stanley, 1993), the Norian Antimonio Formation in Sonora, Mexico (Antimonio depositional system) (Heerwagen et al., 2021;Stanley et al., 1994), the late Carnian–middle Norian Parson Bay Formation in north Vancouver Is- land, Canada (Wrangellia terrane) (Stanley, 1979a, 1979b; Nixon et al., 2000;Del Piero et al., 2020) and the upper Norian–Rhaetian Han- cock member of the Aksala Formation, in Yukon, Canada (Stikinia terrane) (Hart, 1997; Reid, 1986; Yarnell et al., 1998). All these formations represent major carbonate deposition events in the eastern Panthalassa domain and show a significant areal extent but, differently from the Hosselkus limestone (Eastern Klamath terrane), they either did not begin to form, or they continued to form, after carbonate deposition in the Redding subterrane was over. Although the cessation of carbonate deposition could be related to local factors, as indicated by the Norian shales occurring in the area, it is important to note that limestone production started in the Eastern Klamath terrane prior to in most other terranes, whose position may have been much closer to the East- ern Klamath Terrane than observed now. Paleolatitudes had a strong influence on reef ecology in eastern Panthalassa, with coral buildups dominating the lower latitudes, microbial communities and bivalves oc- cupying higher paleolatitudes, and sponge-coral reefs in the mid- latitudes (Martindale et al., 2015). Positioning of the Hosselkus limestone using this approach is tricky due to its depositional characteristics (i.e. breccia deposits), that do not allow a quantitative estimation of reef-forming fauna. Nonetheless, considering reworked extraclasts as reliable proxies of the Hosselkus reef's biology, some comparisons are still possible. The occurrence of all the main reef bioconstructors (i.e.

corals, sponges, microbial fabrics, and calcareous algae), clearly indicates that the Hosselkus limestone did not develop at either high latitudes such as the Baldonnel Formation, where microbialites represent a key component in the structural architecture of the reef, to the detri- ment of sponges and algae (Martindale et al., 2010), nor at equatorial latitudes such as the Luning Formation, with corals and sponges dominating the reef construction (Martindale et al., 2012a, 2012b). The Hosselkus limestone shows strong afﬁnities with carbonate buildups where different bioconstructors play a relevant role in the overall architecture of the reef, as in the Martin Bridge Formation at Summit Point (lower Norian). Even though rock preservation represents a clear obstacle to the comparison of several species and genera, afﬁnities between the Hosselkus limestone and Martin Bridge faunas are present. In both cases, the presence of clotted or encrusting microbialites is constant although often associated with other biocontructors such as numerous scleractinian corals and calcareous red algae (Parachaetetessp.) (Bucur et al., 2020;Martindale et al., 2012b) together with other organisms such asFavreinasp., microcoprolites, Nubecularid forams and serpulid worm tubes. Noteworthy is the almost complete lack of benthic foraminifers in the Hosselkus limestone, despite their remarkable abundance and preservation in the lagoonal facies of the Martin Bridge Formation (i.e. the Black Marble Quarry outcrop, Wallowa terrane)

Downslope re-sedimentation from a short-living carbonate platform: Record from the Upper Triassic Hosselkus limestone (Northern California)

Article

Reference

Downslope re-sedimentation from a short-living carbonate platform:

Record from the Upper Triassic Hosselkus limestone (Northern California)

FUCELLI, Andréa, GOLDING, Martyn, MARTINI, Rossana

Abstract

FUCELLI, Andréa, GOLDING, Martyn, MARTINI, Rossana. Downslope re-sedimentation from a short-living carbonate platform: Record from the Upper Triassic Hosselkus limestone (Northern California). Sedimentary Geology , 2021, vol. 422, no. 105967

DOI : 10.1016/j.sedgeo.2021.105967

Available at:

http://archive-ouverte.unige.ch/unige:153791

Downslope re-sedimentation from a short-living carbonate platform:

Record from the Upper Triassic Hosselkus limestone (Northern California)

Andrea Fucelli

⁎ , Martyn Golding

, Rossana Martini

Sedimentary Geology

⁎ ^, Martyn Golding