Article

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 [...]

Reference

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

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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 a University of Geneva, Department of Earth Sciences, 13 rue des Maraîchers, 1205 Genève, Switzerland b Geological Survey of Canada Geological Survey of Canada, Pacific Division, 1500-605 Robson Street, Vancouver, BC V6B 5J3, Canada article info abstract

Article history: Despite their discontinuous occurrence and poor preservation, knowledge about Triassic carbonates from North Received 22 April 2021 America has increased considerably during recent years. Their characterization represents a unique way to better Received in revised form 14 July 2021 assess evolution and recovery of the biosphere after the major Permo-Triassic biological crisis in the Panthalassa Accepted 19 July 2021 Ocean. The Eastern Klamath terrane, located in Northern California, is a key terrane due to its geographic position. Available online 24 July 2021 It is placed halfway between the terranes of the Canadian Cordillera and the Northern Mexico counterparts, both fi Editor: Dr. Brian Jones extensively studied and characterized in recent decades, leaving a gap in knowledge along the Paci c 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 reser- Keywords: voir of Lake Shasta. All the accessible localities in this region have been extensively sampled for microfacies and Upper Triassic micropaleontological analysis, leading to new insights about the depositional condition and age of the Hosselkus Northern California limestone. A depositional model has been proposed for the first time, corresponding to a steep slope system sub- Microfacies jected to platform progradation and collapse, recording shallow water facies and associated fauna in the form of Conodonts calcareous breccia. Numerous conodont specimens have dated the whole succession as Upper Carnian. Identifi- Slope deposits cation of shallow water organisms, associated to a reliable stratigraphic interval, allowed comparison of the Panthalassa 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. © 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction and providing a conspicuous amount of new insights (Stanley, 1979a, 1979b; Blodgett and Stanley, 2008; Chablais et al., 2010b; Rigaud During recent decades, knowledge about Panthalassan carbonates et al., 2010; Martindale et al., 2015; Peybernes et al., 2016a, 2016b; remained far lower with respect to their Tethyan counterparts. This is Heerwagen and Martini, 2018). Within the Eastern Klamath terrane, mostly due to the poor preservation of the rocks and to the lack of any continuous outcrops of Upper Triassic limestone occur in the Lake significant continuity in the outcrops, after their stacking in different Shasta area, offering a rare opportunity to deepen our knowledge tectonic settings (i.e., accretionary complexes and terranes) along the about Panthalassan paleoenvironments in a geographically strategic Circum-Pacific region (Zonneveld et al., 2007; Chablais et al., 2010a; area and to compare the results with other Triassic carbonates scattered Peybernes et al., 2016a, 2016b; Peyrotty et al., 2020a, 2020b). Neverthe- along the Pacificcoast(Silberling and Tozer, 1971; Tozer, 1982; Blodgett less, carbonate rocks remain one of the most valuable tools to constrain and Stanley, 2008; Martindale et al., 2015). The Hosselkus limestone paleogeography and paleoecology of the no longer extant Panthalassa crops out midway between widely studied terranes of North America, Ocean, drawing in recent years the attention of several researchers becoming a linking point for the complete knowledge of the area's tec- tonic and paleoenvironmental evolution. Preservation of these carbon- ates is mediocre and studies carried out in the past mostly focused on ⁎ Corresponding author. E-mail addresses: [email protected] (A. Fucelli), [email protected] macro-fauna description (Diller, 1906; Smith, 1927; Sanborn, 1960; (M. Golding), [email protected] (R. Martini). Albers and Robertson, 1961; Sbeta, 1970; McCormick, 1986). However,

https://doi.org/10.1016/j.sedgeo.2021.105967 0037-0738/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). A. Fucelli, M. Golding and R. Martini Sedimentary Geology 422 (2021) 105967 despite the poor preservation, other significant information about depo- underlying Pit Formation is marked by a lithologic transition from sitional environment, hydrodynamic conditions and micropaleontology non-calcareous to calcareous dark beds and it is often hidden by dense can be obtained thanks to precise microfacies description. vegetation. South-east of Gray Rocks, numerous limestone bodies This paper deals with new comprehensive field observations of the emerge from the dense vegetation along the banks of Lake Shasta Hosselkus limestone that, coupled with microfacies analysis, allowed (Fig. 4C, D). North of these locality, sub-vertical outcrops arise sporadi- the recognition of a peculiar depositional setting and the assignment cally up to 20 km away from Gray Rocks, along the North Fork of Squaw of an accurate chronostratigraphic range for limestone deposition. Creek (Fig. 5C, D). They are aligned with the direction of the main crest, These findings, together with the remarkable dataset developed by but with a reduced thickness of 75 m. Ten km south-east of Gray Rocks, the REEFCADE project (Rossana Martini 2007–2022), as well as infor- the Hosselkus limestone crops out in two main quarries along Highway mation from the literature, allowed a comparison between the 299, named Gravel Pit and Bear Gulch quarries (Fig. 5A, B). There we can Hosselkus limestone and other Upper Triassic carbonates formed on dif- observe two limestone domes around 200 m wide, with dubious dip ferent terranes in North America. Thanks to peculiar regional conditions due to the massive nature of the carbonate, but possibly showing the (i.e., bathymetry and the cessation of volcanism), the Hosselkus lime- contact with the overlying Brock Shales. Last, a small outcrop, occurring stone could have acted as foothold for subsequent reef colonization as a lens of a few tens of meters, crops out in Bear Mountain, on the across the eastern portion of the Panthalassa Ocean. south bank of Shasta Lake. This lens turned out to be fundamental in the interpretation of the limestone evolution, thanks to the peculiar fa- 2. Geological setting cies and stratigraphic contacts. Here, the improved preservation of the limestone allowed a precise description of the processes involved in The Hosselkus limestone and the other formations cropping out in Hosselkus limestone deposition, such as thick breccia deposits. Lake Shasta area (Northern California) are all part of the Eastern Klam- ath terrane, one of the eight terranes forming the Klamath Mountains 4. Methods (Irwin, 1960)(Fig. 1). This southern portion of the belt was divided at the beginning of the 1980s into three tectonostratigraphic units Thin sections from 187 samples were first scanned using a high- (Irwin, 1981): (1) the Yreka–Callahan section, (2) the Trinity Ultramafic resolution film scanner (Nikon CoolScan 4000 ED), then analyzed in de- complex and (3) the Redding Section. The first two consist respectively tail with an optical microscope (Leica DME). Data collected were used to of Lower Paleozoic (Ordovician to Devonian) sedimentary and base- classify facies according to Dunham (1962) and Embry and Klovan ment rocks, while the third includes volcanic and sedimentary rocks (1971). Facies were then grouped according to assemblages, biotic con- from Devonian to Middle Jurassic (Fig. 2). Later on, these three units tent and texture in order to interpret paleoenvironment and hydrody- were categorized as three distinct subterranes (Silberling et al., 1987), namic conditions of deposition. named the Yreka, Trinity and Redding subterranes, respectively Due to the poor preservation of the samples, foraminifera were ob- (Fig. 1). The latter comprises a series of Paleozoic and Mesozoic served only in cathodoluminescence (Cathodyne, by NewTec Scientific eastward-dipping strata similar to a monocline, including the Hosselkus mounted on an OlympusBX41 microscope), leaving some doubts limestone (Renne, 1986)(Fig. 2). The presence of diverse volcanic rocks about the nature of the test (i.e. porcelaneous, microgranular- suggests that the region underwent several episodes of volcanism dur- agglutinated or aragonitic) and inevitably about their age. Conse- ing its evolution and some, like the Dekkas andesite, are consistent with quently, a great part of the material has been processed for conodont ex- an island arc hypothesis for the formation of the Eastern Klamath ter- traction, in order to determine the age of the Hosselkus limestone. rane (Burchfiel and Davis, 1981). The structural nature and sedimentol- Carbonate samples were dissolved in 12% acetic acid and sieved with a ogy of formations accumulated during the Permian and Triassic periods, 63 μm sieve (Green, 2001), then separated from an abundant lighter indicate deposition on an area periodically subjected to graben faulting compound by means of sodium polytungstate 2.85 g/cm3 (Jeppsson (Renne, 1986). Some authors defined these extensional movements as and Anehus, 1999). After picking, specimens were mounted on a con- related to back-arc spreading (Burchfiel et al., 1992; Dickinson, 2004). ductive aluminum support, coated with 10 nm of gold and imaged Some of the terranes forming the Eastern Klamath Mountains amalgam- with a Jeol JSM 7001F Scanning Electron Microscope. All the prepara- ated together before they were stacked onto the American craton, as a tions, observations and analyses were performed at the Department of result of multiple thrust zones active along these long-lived volcanic- Earth Sciences, University of Geneva, Switzerland. arc systems (Irwin, 1989)(Fig. 1). However, evidence for the time of ac- cretion of Eastern Klamath terrane onto the craton is rare due to the 5. Results and interpretations thick and widespread layer of Cretaceous and younger strata hiding the terrane boundaries. Paleomagnetic data suggest that most likely it 5.1. Microfacies analysis occurred during the Upper Jurassic period (Irwin, 1989; Mankinen and Irwin, 1982). 5.1.1. Halobiids and radiolaria mudstone to wackestone (MF1)

3. Studied area and mode of occurrence 5.1.1.1. Description. MF1 consists of dark gray to black micrite associated with two main skeletal grains: Halobia shells and radiolaria. The first Field investigation focused on an area located about 40 km north- show the peculiar thin shells coupled with a low varve convexity and east of Redding, Shasta County, California. Nine outcrops were surveyed, length ranging between a few hundred microns to centimeters, depend- described and sampled in November 2018 and June 2019 (Fig. 3). GPS ing on the integrity of the shell and on the ontogenic state (i.e. juvenile coordinates of each outcrop are listed in Table 1. vs. adult). The thickness of the specimens is usually around a few microns The main carbonate body is located on the divide between the Pit and they appear completely recrystallized (Fig. 6C). However, the thickest River arm and the Squaw River arm of Shasta Lake and appears as a ones show a preserved internal layering. Radiolaria appear in polarized north–south striking crest about 8 km long, disturbed by one or more light as small white dots completely recrystallized in calcite (Fig. 6B, C), faults having approximately the same direction as the ridge. The pres- while few internal structures are visible in cathodoluminescence ence of these faults is signified by a significant dip change along the (Fig. 6D). Dimensions do not exceed 200 μm and preserved forms allow crest, being sub-horizontal from the southern margin of Gray Rocks to the presence of both Spumellaria and Nassellaria groups to be recognized, Devils Rock and sub-vertical on the relief north of Low Pass Creek although further identifications were not possible. The presence of radio- (Fig. 4A, B). The thickness of the whole succession is around 170 m at laria is rather constant in this facies with 30–40 specimens/cm2, while the the southern margin and 100 m at the northern one. Contact with the number of Halobia shells varies significantly, creating millimetric to

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Fig. 1. Terranes subdivision in the Klamath Mountains (modified after Irwin 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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Fig. 2. Geologic map of the Redding subterrane central part (modified after Fraticelli 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. centimetric levels exceptionally rich in these remains to the point of being into a deep basin formerly characterized only by siliciclastic deposits. Oc- the only constituents of the rock (Fig. 6A). Other occasional grains occur- casional deposition of larger bivalves, crinoids and any other shallower or- ring in this facies are cephalopods, larger bivalve shells and crinoid frag- ganisms could be the result of a wide spectrum of events, such as storms, ments, mostly concentrated in thin levels. The facies is always found in oceanic currents, slope destabilization and tectonic events, all able to dark centimetric limestone beds. transport such grains far from their original living position.

5.1.1.2. Interpretation. Both radiolaria and Halobia suggest deposition in a 5.1.2. Bioclastic wackestone to packstone (MF2) rather deep and quiet environment, however, above the calcite compen- sation depth (CCD). Early recrystallization of radiolaria to calcite testifies 5.1.2.1. Description. Dark micrite with abundant biotic fragments, rarely the presence of an alkaline substrate at the bottom of the basin, supported preserved as whole fossils. Principal constituents are pelagic and non- by the presence of micrite probably derived from the nearby carbonate pelagic bivalves and crinoid stem remains; less abundant are platform (Afanasieva and Amon, 2015). At Gray Rocks, where the most trepostome bryozoan fragments and cephalopod shells. These frag- complete succession crops out, this facies is the first appearing above ments present two different modes of occurrence: structureless and ho- the hidden contact with the underlying Pit Formation, indication of the mogeneously mixed with the surrounding matrix (wackestone) passage from siliciclastic to carbonate deposits. In addition to lithologic (Fig. 6E), or in concentrated levels (packstone) (Fig. 6F), slightly graded, differences, the two kinds of deposits reveal distinct Halobia content intercalated with a matrix similar to MF1. Volcanic material is present, and radiolaria preservation, respectively absent and not calcified in the with isolated grains ranging from a few microns to millimeters. The fa- Pit Formation. The emplacement of this facies suggests the first phase of cies occurs in medium bedded limestone, situated in the lower part of growth of a neighboring carbonate platform, now able to spread micrite the carbonate succession both at Gray Rocks and Bear Gulch quarry.

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Fig. 3. Outcrops map with the nine sampled localities. Numbers (1 to 9) are reported in Table 1 with respective GPS coordinates. Names of rivers and mountains are used within the text to guide the reader.

5.1.2.2. Interpretation. Although the presence of crinoids, bryozoans and high fragmentation of all these biotic compounds, suggests transporta- cephalopods testifies to deposition on an open-shelf or slope environ- tion for long distances, toward the lower part of the slope (Tucker, ment (Cuffey, 1970; Skelton, 1982; Wilson, 2012; Flügel, 2013), the 1969). Intercalations with MF1 suggest a similar water depth and

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 in Fig. 3, while locality names aim to guide the reader through the text.

Number in Fig. 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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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. hydrodynamic conditions with respect to the latter, though the absence (grain flows), driven by gravity energy along a medium to high angle of any deep-water biota in MF2 (i.e. radiolaria and Halobia) implies an slope (Mulder and Alexander, 2001). These deposits represent mass allochthonous origin of the deposit. The absence of siliciclastic material movements along the slope, with a great amount of material flowing and the occurrence of volcanogenic grains strengthen the hypothesis of at the same time, differently from MF2 grains whose deposition did deposition close to an active volcanic arc system. not necessarily occur at the same time. Cementation took place just after sedimentation during periods of prolonged non-deposition, as 5.1.3. Bioclastic grainstone (MF3) suggested by the cloudy nature of the syntaxial cement (Walker et al., 1990; Flügel, 2013), supporting the hypothesis of occasional mass 5.1.3.1. Description. Biotic compounds in this facies show strong affinities movements. Non-deposition periods may be due to a combination of with the one described in MF2, with the occasional addition of two main factors: low sedimentation rate and unfavorable morpholog- shallower grains. The latter represent a broad variety of organisms ical condition (i.e. steepness of the slope). Extensive cementation and and grains often micritized, slightly visible in cathodoluminescence, recrystallization make recognition of already poorly preserved peloids such as foraminifera, fecal pellets, ooids, cortoids and different types of more difficult, hiding any internal features. However, shapes present peloids (Fig. 7C, D). In general, the observed organisms are bigger and in the microfacies indicate the occurrence of Bahamite peloids (Beales, less damaged than in MF2, with whole cephalopod shells of various di- 1958; Flügel, 2013) and some undetermined fecal pellets. The presence mensions (Fig. 7A). Differently from the previously described facies, of reworked shallow-water grains suggests a position closer to the plat- MF3 is cement-supported, principally due to syntaxial overgrowth on form, with respect to previously interpreted facies, although cephalo- crinoid fragments (Fig. 7C). Grain size varies from bottom to top in pods and crinoid remains are the main sediment producers. Halobia coarsening upward sequences (Fig. 7B), with smaller peloids concen- shells often top the cemented layers, strengthening the hypothesis of trated in the lower part and crinoid–cephalopod remains in the upper an open pelagic environment. one. This grading slightly influences the cementation, which is stronger where crinoids dispose larger space for syntaxial overgrowth. 5.1.4. Micro-calciturbidite (MF4) Centimetric bioturbations (Fig. 7B) characterize the cemented levels, al- ways filled by mudstone to wackestone, resembling MF2, with some ex- 5.1.4.1. Description. This microfacies is defined by small sequences ception in the biotic content (i.e., radiolaria are absent). MF3 occurs in a (about 3–4cm)offining upward deposits (Fig. 7F). Despite reduced di- regular bed of about 15–20 cm, and at Gray Rocks it is located in the cen- mensions, some sub-microfacies can be recognized based on tral part of the succession. granulometry and grain origin. The bottom part, a bioclastic packstone to grainstone, comprises organisms autochthonous of a slope environ- 5.1.3.2. Interpretation. Extensive cementation is the main feature of this ment (i.e., small cephalopods and crinoids). In the central part, a microfacies, reflecting peculiar features in terms of grain size and grad- bioclastic wackestone to packstone, the same organisms occur with re- ing. Physical characteristics are common in hyper-concentrated flows duced dimensions, integrated with peloids, fecal pellets and sporadic

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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. slightly-micritized ooids. At the top, more abundant micrite is still asso- and minor crinoid fragments (Fig. 8A, B, E). Apart from the latter, all the ciated with smaller cephalopod shells and crinoid fragments, and in ad- components display different grades of micritization and coating. Inter- dition, thin and flat Halobia shells are often present. Some foraminifers particle space is filled by blocky sparite and dogtooth cement, with oc- can be observed only in cathodoluminescence due to the poor preserva- casional occurrence of syntaxial overgrowth where crinoid fragments tion of the samples. Nonetheless, doubt about the original composition are present (Fig. 8F). Colonial worm tubes (Filograna sp.) occur in the of the test did not allow precise identification so far and they will be form of millimetric intraclasts (Fig. 8C). treated in a separate work. 5.1.5.2. Interpretation. The facies is typical of a shallow-water environ- 5.1.4.2. Interpretation. Most of the organisms found in these deposits are ment constantly affected by wave agitation, like back-barrier subtidal still of pelagic-slope origin, not suggesting significant changes in the sediments (Hine, 1977; Scholle et al., 1983; Tucker, 1985). The cortex overall depositional environment with respect to the previously de- of ooids is radial-fibrous, suggesting a relatively low-energy environ- scribed microfacies. What differs is the normal grading, resulting from ment (Flügel, 2013). Extensive micritization of the inner part of ooids, hydraulic sorting of skeletal and non-skeletal grains. While MF1 and as well as other grains, suggests periodical exposure to the light at the MF2 indicate no consistent grading and MF3 shows inverse grading, in water–sediment interface indicating deposition in a cyanobacteria- MF4 normal grading occurs as a consequence of concentrated density rich environment and possibly a slow sedimentation rate (Kendall and flow (Mulder and Alexander, 2001). Such graded deposits reflect, al- Alsharhan, 2011; Reid and Macintyre, 1998). Numerous grapestones, though some features are missing, the typical evolution of a turbiditic lumps and cortoids imply the presence of a neighboring calm environ- sequence (Herbig and Mamet, 1994). The reduced dimensions of the cy- ment (Flügel, 2013; Swinchatt, 1969), possibly a protected lagoon cles probably indicate limited availability of material both from slope where abundant crinoids and gastropods thrive. The presence of margins and the shallower part of the platform (Playton et al., 2010). Filograna sp. supports the hypothesis of a water depth above the Relatively good preservation allows the recognition of some shallow- storm wave base (SWB) (Senowbari-Daryan and Link, 2005; water grains as ooids, cortoids and the microcoprolite Favreina sp. Senowbari-Daryan et al., 2007). (Fig. 7E), reworked in exotic environments, but possibly indicating the presence of a restricted lagoon and shoals in the micro-turbidites source 5.1.6. Bio-constructed facies (MF6) area (Kennedy et al., 1969; Flügel, 2013). 5.1.6.1. Description. MF6 represents a set of bio-constructed frameworks 5.1.5. Ooidal-bioclastic grainstone (MF5) 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 5.1.5.1. Description. The facies is characterized by the presence of a con- every described sample, their abundance is rather variable, making spicuous number of shallow water non-skeletal grains such as ooids, just one of them the principal constituent of the buildup in a particular microbial and mud peloids, cortoids, fecal pellets and aggregate grains. sample. For this reason, MF6 has been divided in three sub-facies de- Skeletal grains are also present and comprise gastropods, bivalve shells scribed separately.

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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 the finer 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 (Favreina sp.) (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 500 μm; E 200 μm.

5.1.7. MF6a worm tubes referred to Terebella sp. Real dimensions of these coral col- Colonial scleractinian corals form distinct frameworks easily recog- onies are impossible to verify, since they are preserved as isolated nizable in outcrops as well in hand-samples (Fig. 9A). In some cases, in- blocks reworked along the slope, as discussed later in this work. Some ternal structures were partially preserved showing the presence of were already described as metric colonies by past authors and their re- different types of corallites: cerioid, sharing adjacent walls (Fig. 9B); markable dimensions led them to be erroneously interpreted as in- thamnasterioid, with poorly defined walls and septo-costae of different place corals (McCormick, 1986). corallites welded together (Fig. 9C, D); phaceloid, having distinct walls (Fig. 9F); and platy, with a characteristic flat shape (Fig. 9E). Low pres- 5.1.8. MF6b ervation did not allow taxonomic identification of these corallites at Two types of microbial bioconstructions are associated in this facies the same level: cerioid corals belong to the family Pamiroseriidae and are differentiated based on framework features and the nature of while thamnasterioid corals could be associated either to trapped grains and micrite. The first is composed of clotted and peloidal Parastreomorpha sp. or Astreomorpha sp. Phaceloid corals have been micrite, with grains that remain separated by microsparite (Fig. 10A, C). identified as Eocomoseris minima, while no identification was possible The structure is organized in heterogeneous layers whose orientation for platy corals. Dark micrite fills the space between corals, while in allows the recognition of build-up polarity. Other constituents trapped the external part other bioclasts like crinoid fragments, brachiopods in growth layers are often microcoprolites, ooids and small bivalve frag- and ostracods are present. Boring from bivalve often occurs (Fig. 9D). ments with thick micritic envelopes, together with crinoid fragments. All these framebuilders are associated with abundant agglutinated Microbial layers are also characterized by stromatactis-like cavities

Fig. 6. Microfacies of Hosselkus limestone. (MF1 and MF2) A. Halobia rich 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) and Halobia (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) and Halobia (Ha) remains. Small cephalopods shells (Ce) are also present in the packstone (FA180). Scale bars: A 1 mm; B, C 500 μm; D 200 μm; E, F 2 mm.

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

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. Filograna sp. worm tubes (Ft) occurring together with mud peloids (mp) and crinoid fragments (Cr) (FA158). D. Radial fibrous 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 500 μm; D 100 μm; E 2 mm; F 300 μm.

that probably originated from soft organisms' decay, like sponges and fragments and in living position were observed in some samples and algae, or possibly related to the activity of dwellers in the microbial sub- described by Bucur et al. (2020) (Fig. 11B, C). This microfacies is marked strate. The second type of microbial bioconstruction shows less peloidal by dark peloidal micrite, accompanied by shell and crinoid fragments of micrite and trapped grains, having as a major constituent aphanitic various dimensions. Red algae exist both as fragments and in living po- micrite (Fig. 10B). Intergranular sparite is almost absent and occurs sition. Extensive early dissolution affected the rocks, creating large vugs only in a few zones. The overall framework appears almost dendritic, first bounded by isopachous dogtooth cements and then filled by dark with higher relief and more complex forms, with reduced layer length homogeneous micrite. with respect to the first type. These latter types often have a half- moon shape, reaching maximum thickness in the central part and hav- 5.1.9.1. Interpretation. Though scleractinian corals became the main reef ing reduced dimensions near the lateral margin of the build-up. Within builders in the Upper Jurassic, they were important carbonate producers layers, it is possible to observe some Nubecularids and other small also during the Upper Triassic period (Veron, 1995). However, in con- microencrusters impregnated with organic matter (Fig. 10B). Interstitial trast with modern reef-building corals, that display specific growth sediments are similar to the ones of described before, with ostracods forms depending on water depth and wave turbulence (James and and peloid-rich dark micrite. In both cases, serpulid and terebellid Ginsburg, 2009), they probably had completely different behavior. In tubes occur in a considerable amount, often in living position, together fact, most Triassic coral reefs described in the literature show character- with recrystallized sponges (Fig. 10C, D). istics different from the ones of recent coral reefs and they are usually preserved as thin and laterally restricted buildups (Stanley, 1979a, 5.1.9. MF6c 1979b; Flügel, 1982; Senowbari-Daryan and Stanley, 1992; Veron, As for other shallow-water facies described in this work, a differen- 1995; Roniewicz and Stanley, 2013; Peyrotty et al., 2020b). Field inves- tiation must be made between outcrop and thin section observation, in tigations and petrographic analysis reveal that most of these buildups terms of preserved fauna. In hand-sample, MF6c presents a considerable developed in relatively quiet-water settings, some of which may have number of sponges, up to 15 cm in diameter, often in living position been deeper than is usual for modern reef systems (Stanley Jr, 1981; (Fig. 11A). These organisms are completely recrystallized and no mor- Stanley, 1982). In the studied area, it is consistent with the hypothesis phological features are visible in the thin section. Conversely, well pre- of small patch reefs in the fore-reef to upper slope zones, though devel- served specimens of red algae (“Parachaetetes” sp.), occurring both as opment in a mud-rich environment like a lagoon and subsequent

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) Parastreomorpha sp. or Astreomorpha sp. (?) growing on microbial micrite (mm) (FA118). D. Thamnasteroid corals (Tc) Parastreomorpha sp. or Astreomorpha sp. bored by bivalves (bt) (FA118). E. Poorly preserved platy corals (Plc) (FA140b.) F. Eocomoseris minima (Em) with numerous Terebella sp. worm tubes (Te) (FA49b). Scale bars: B, C 2 mm; D 3 mm; E, F 4 mm.

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

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

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. Terebella sp. 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.

transportation along the slope cannot be excluded. The lack of any con- strong wave action (Flügel, 2013). The lack of any buildup continuity tinuity on outcrops represents an obstacle for a clear interpretation. does not permit a detailed description of the overall depositional envi- Microbialites were important reef builders during Middle and Upper ronment. The presence of muddy areas, with red algae and sponge frag- Triassic and their presence spans from shallow water settings to the ments, may suggest a calm environment, in the upper photic zone but deeper part of the euphotic zone, associated with a broad variety of below the fair weather wave base (FWWB), probably not far from other organisms (Leinfelder and Schmid, 2000; Reid, 1987). their source area. On the other hand, the same organisms in living posi- Occurrence of stromatolitic microbialites associated with serpulids tion, surrounded by cemented peloids, suggest the existence of a more and other types of tube-dwelling polychaetes has been already de- agitated environment, where the same fauna thrived. Extensive early scribed in Upper Triassic carbonates from the Tethyan Ocean and re- dissolution may be related to platform exposure episodes and occur lated to low energy conditions (Iannace and Zamparelli, 1996; De prior to clast reworking in deeper positions when vugs were filled by Zanche et al., 2000; Gale et al., 2018). Flügel (2013) placed this faunal homogeneous micrite. However, the lack of any significant microorgan- association in two main areas of Upper Triassic carbonate platforms: ism in the infilling matrix does not allow the determination of its source upper ramp and platform edge. Cirilli et al. (1999) also suggested that area and age. such an association of polychaetes and microbialites points to some sort of stressful circumstances in the depositional environment, such 5.2. Biostratigraphy as low oxygen conditions, anomalous salinity or eutrophic setting. This consideration is relevant considering that this facies is one of the 5.2.1. Conodont biostratigraphy last in the whole Hosselkus limestone succession, appearing, although Previous records of conodonts from the Hosselkus limestone have in the form of reworked clasts, a few meters below the transition to been presented by Mosher (1968, 1973) and Du et al. (1992).The Brock Shales, probably marking the cessation of carbonate production only platform species illustrated in these publications was from the platform. Metapolygnathus polygnathiformis (Mosher, 1973, 1968), which is a Red algae, often in living position, together with sponges are a typi- Lower to mid-Upper Carnian species; examination of the figured speci- cal reefal association, located in areas subject either to moderate or men in Mosher (1968) suggests that this may in fact belong to the

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

Fig. 11. Litho- and microfacies of Hosselkus limestone. (MF6c) A. Metric block rich in sponges (arrows). B. MF6 bioclastic floatstone, with completely recrystallized sponges (Sp) but preserved Parachaetetes sp. red algae (Pa). Brachiopods (Bc) and other shells also occur together with clear dissolution features (df) (FA50). C. Parachaetetes sp. red algae (Pa) and sponges (Sp) in living position (FA210). Scale bars: B 3 mm; C 2 mm.

Upper Carnian species Quadralella carpathica. In the present study, addi- Quadralella willistonense, both of which first appear in the next highest tional conodonts have been recovered from six samples of the subzone of the primitia Zone in British Columbia, the angusta-dylani Hosselkus limestone, and all species identified are consistent with an subzone, and range into the lower parts of the Norian in both North Upper Carnian age for this formation. The diverse fauna includes speci- America and Tethys (Orchard, 2014; Rigo et al., 2018). Taken together, mens of typical Upper Carnian species such as Quadralella ex gr. oertlii, the conodont species indicate an Upper Carnian age for the Hosselkus Quadralella tuvalica and Quadralella carpathica (Fig. 12F, G, H), all of limestone, likely the middle to upper part of the Upper Carnian. which have been identified in Tethys (Rigo et al., 2018)aswellasincon- tinental margin deposits of northeastern British Columbia, Canada 6. Discussion (Orchard, 2014). In addition to these geographically widespread spe- cies, the Hosselkus limestone also contains species previously found 6.1. Microfacies distribution in the Hosselkus limestone only in northeastern British Columbia, including Quadralella willistonense and Parapetella beattyi (Fig. 12A, I). Similarly, the species Microfacies analysis together with field observations, allows some Parapetella lanei and Quadralella postlobata (Fig. 12D, E) have been re- considerations about the Hosselkus limestone's depositional setting corded from northeastern British Columbia (Orchard, 2014) and from and microfacies distribution. the allochthonous Stikine terrane in the western Canadian Cordillera In several outcrops, macro-fauna and grains with completely differ- (Golding et al., 2017), but not from elsewhere. The samples from the ent ecologic and hydrodynamic needs coexist, suggesting a re- Hosselkus limestone also contain a single unusual specimen that ap- sedimentation of shallow-allochthonous material in a deeper environ- pears to belong to a new species of Quadralella, as well as a specimen ment. Petrographic analysis confirmed this hypothesis and allowed that strongly resembles the Lower Carnian species Carnepigondolella the recognition of 3 different types of deposits, all characteristic of a carnica and may also be a new species; however, the material is not foreslope environment (Cook et al., 1972; Davies, 1977; Mulder and abundant enough to confirm this. If Carnepigondolella carnica were Alexander, 2001; Playton et al., 2010; Savary and Ferry, 2004). truly present in the fauna, then this would be the oldest species identi- fied in this study, as it occurs in the upper part of the Lower Carnian in - Mud dominated deposits (MF1 & MF2): Consist of fine-grained de- Tethys (Krystyn, 1980; Rigo et al., 2007, 2018). However, the remainder posits settled in a pelagic environment probably during slope quies- of the Hosselkus fauna is Upper Carnian, and so the identification of cence, often associated with planktonic microfossils (i.e. radiolaria). Carnepigondolella carnica in these samples remains questionable. The In the field they crop out as thin bedded layers probably with consid- oldest species identified in this study is Carnepigondolella zoae erable length along strike and dip (Cook and Enos, 1977), omitted in (Fig. 12C), which first appears in the zoae subzone (upper samueli the visited outcrops by structural complication and dense vegeta- Zone) of British Columbia (Orchard, 1991, 2014, 2019). Quadralella tion. Occasionally, they occur intercalated with coarser-grained sur- carpathica appears slightly higher in British Columbia (uppermost faces, probably representing cyclic activity on the slope, and they can samueli Zone), where it ranges into the basal part of the overlying be either structureless or laminated. primitia Zone (sagittale-beattyi subzone) and occurs together with - Grain dominated deposits (MF3 & MF4): Made up of a skeletal and Quadralella tuvalica, Quadralella ex gr. oertlii,andParapetella beattyi in non-skeletal mixture, respectively autochthonous of the platform this subzone (Orchard, 2014, 2019). However, in Tethys, some of edge and platform top shoals, then transported downslope and re- these species first appear earlier in the Upper Carnian; Quadralella ex deposited. Wave action, tidal and storm currents, especially gr. oertlii occurs together with Carnepigondolella zoae in the basinward tidal ebb flow and storm return flow, drive this process praecommunisti Zone, and Quadralella tuvalica ranges from the Lower (Playton et al., 2010). In the Hosselkus limestone, the grain size of to Upper Carnian boundary until the end of the Carnian (Mazza et al., these deposits does not exceed 1 mm, but they are often associated 2018; Rigo et al., 2018). The youngest species recovered from the with autochthonous fauna up to 20 cm (i.e. ammonoids and nauti- Hosselkus limestone are Norigondolella navicula (Fig. 12B) and loids). As for mud layers, they are usually interrupted by cemented

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

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

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.

surfaces, associated with bioturbation in several samples. These de- previous deposits, they appear massive, with no internal structures posits form thin to medium beds and extend for great distances and form discontinuous tongues able to produce significant changes along strike, reflecting the line-source nature of contributing sedi- in the above topography. In most of the cases, strong surface ment factories (Mullins and Cook, 1986) recurrently with significant weathering makes the recognition of this type of deposit difficult, as changes in grain type along the same sampled bed. theedgeofclastsishomogenizedwiththematrix(Fig. 13B, C). - Debris dominated deposits (MF5, MF6): Mainly generated by gravita- tional collapse of lithified material, the block-size of these deposits Within the three different types of deposit, 6 different microfacies ranges from a few centimeters to a few meters (Fig. 13A, C). In the were identified and interpreted according to grain assemblages and tex- Hosselkus limestone they occur as matrix-supported breccia, with tures. Note that facies from MF1 to MF4, although containing allochtho- clasts native to the fore-reef environment associated with fine micrite nous skeletal and non-skeletal components, were lithified after the with rare pelagic microorganism (Fig. 13D). In contrast with the reworking of grains on a slope environment. Conversely, facies from

Fig. 12. Late Carnian conodonts of Hosselkus limestone. A. Quadralella willistonense;B.Norigondolella navicula;C.Carnepigondolella zoae;D.Parapetella cf. lanei;E.Quadralella postlobata;F. Quadralella ex gr. oertlii;G.Quadralella tuvalica;H.Quadralella ex gr. carpathica;I.Parapetella beattyi. Scale bars: 100 μm.

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

MF5 and MF6 underwent lithification in the original shallow-water en- limestone. Despite few intercalations with grain-dominated deposits vironment and subsequently they were reworked along the slope as (MF3 and MF4) it always appears at the bottom of every sampled se- extraclasts. Since no autochthonous shallow-water environment out- quence, indicating the first phase of limestone deposition on a zone crops exist, extraclasts were used as a proxy for paleoecological descrip- probably located between the basin floor and the lower slope. MF3 tions of the Hosselkus limestone. and MF4 (grain-dominated) appear in the main outcrop that spans be- tween Low Pass Creek and Gray Rocks and discontinuously in some 6.2. Depositional model small outcrops along highway 299, which not by chance represent the thickest sections of the limestone. They are completely absent in Bear Field observations and microfacies analysis show that the Hosselkus Mountain and on the three crests along North Fork Squaw Creek. limestone was deposited in a foreslope environment whose geometries When present, they are always placed between mud and debris domi- were strongly influenced by the heterogeneous distribution of the pre- nated deposits, in the central part of the sequences. This position viously described deposits (Fig. 14). Outcrops belonging to the same allowed us to measure the thickness of the deposits, which resulted in ridge crest, reasonably considered part of the same horizon due to the varying thickness in different outcrops along strike, ranging between orientation of their beds, manifest marked differences in terms of de- ~25 and ~60 m. Though it was almost impossible to follow single beds posit content (i.e. mud, grain and debris dominated). This is mainly for more than few tens of meters, differences in their thickness are eas- due to the preferential patterns followed by reworked material along ily recognizable along strike, with changes of about 5 cm. Deposition of the slope. These patterns must not be considered as channels with lim- grain dominated deposits took place on an area situated between the ited width, but rather as broad aprons whose dimensions range from a lower and middle slopes, where enough gravitational energy allows few tens to hundreds of meters. The result is a complex arrangement basin-ward movement of reworked grains. MF5 and MF6, representing of bedded and massive carbonates, representing respectively mud- debris dominated deposits, occur in all the sampled localities with the grain and debris dominated deposits. The finest one, MF1 and MF2, oc- exception of North Fork Squaw Creek crests. These deposits are consis- curs in all the studied localities except for Bear Mountain and often tently at the top of the sequence, either in contact with mud or grain de- marks the transition from the underlying Pit Formation to the Hosselkus posits (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.

16 A. Fucelli, M. Golding and R. Martini Sedimentary Geology 422 (2021) 105967 the upper contact with the Brock Shales is often eroded or covered by already begun during Middle Triassic, it was mostly related to areas dense vegetation. At Gray Rocks and Bear Gulch quarry, strongly weath- closer to the American continent, with mixed carbonate-siliciclastic se- ered and karstified deposits have a thickness ranging between 40 and quences probably related to wide ramps setting, as observed for the 90 m, while at Bear Mountain it does not exceed 20 m. In the last local- Ladinian–Carnian Augusta Mountain Formation in central Nevada ity, this type of deposit is the only one present and it occurs in contact (Bonuso et al., 2018, 2020). Other examples of Carnian carbonates with the Pit Formation, with no transitional facies in between from the western margin of North America are the deep-water (Fig. 13A). These deposits were accumulated in an area between middle Ludington Formation and its shallow water equivalent, the Baldonnel and upper slopes but, in exceptional circumstances, masses of clasts Formation, both located in British Columbia, Canada (Pelletier, 1964; could have reached the basin floor where little or no mud deposits Gibson, 1975; Zonneveld et al., 2007; Martindale et al., 2010) but were settled, as happened at Bear Mountain. again, they were both deposited directly on the western margin of the It is extremely important to consider that information retrieved North American craton, rather than on active volcanic arc. Most of the from these deposits does not allow any precise reconstruction of the terrane-related carbonates, hence the ones generated far from the shallow water environment's geometries. MF5 and MF6 prove the pres- coast of the continent, are slightly younger than the Hosselkus lime- ence of a shallow water setting that either does not crop out anymore, stone. Among these we find the upper Carnian–lower Norian Martin or that has been dismantled during accretion and other tectonic events. Bridge Formation in the northeastern part of Oregon (Wallowa terrane) The depositional model in Fig. 14 has the purpose of showing how the (Stanley et al., 2008; Martindale et al., 2012b), the middle Norian Luning different deposits followed preferential patterns and their temporal Formation in the western-central Nevada (back-arc basin of the Black evolution, but it may not reflect the actual geometries of the shallow Rock terrane) (Martindale et al., 2012a; Sandy and Stanley, 1993), the water environment. However, as stated in the geological setting chap- Norian Antimonio Formation in Sonora, Mexico (Antimonio deposi- ter, the Redding subterrane was periodically subjected to graben tional system) (Heerwagen et al., 2021; Stanley et al., 1994), the late faulting, that could have easily led to the formation of structural highs. Carnian–middle Norian Parson Bay Formation in north Vancouver Is- With this scenario, the formation of a flat-top platform seems more rea- land, Canada (Wrangellia terrane) (Stanley, 1979a, 1979b; Nixon sonable, with carbonate production on the footwall of the fault and de- et al., 2000; Del Piero et al., 2020) and the upper Norian–Rhaetian Han- bris deposition on the hangingwall. This could have created minor cock member of the Aksala Formation, in Yukon, Canada (Stikinia ter- distance between these two zones, explaining a reworking process rane) (Hart, 1997; Reid, 1986; Yarnell et al., 1998). All these that would be rather difficult along a carbonate ramp with low inclina- formations represent major carbonate deposition events in the eastern tion. The collected shallow water facies show an overall low energy set- Panthalassa domain and show a significant areal extent but, differently ting, as illustrated by the radial-fibrous cortex of ooids and the presence from the Hosselkus limestone (Eastern Klamath terrane), they either of numerous grapestones, lumps and cortoids. This is in contrast with a did not begin to form, or they continued to form, after carbonate depo- ramp system where waves sweep directly onto and across the shallow sition in the Redding subterrane was over. Although the cessation of sea floor, leading to a higher energy level of the shallow-water environ- carbonate deposition could be related to local factors, as indicated by ments (Flügel, 2013). Since debris-dominated deposits prevail as the the Norian shales occurring in the area, it is important to note that lime- last depositional event in the Hosselkus limestone, and considering stone production started in the Eastern Klamath terrane prior to in most the nature of the described facies (i.e. extraclasts), it is reasonable to other terranes, whose position may have been much closer to the East- infer that these lithified clasts collapsed when the hosting platform ern Klamath Terrane than observed now. Paleolatitudes had a strong in- was already dying or drowning. This could explain the occasional depo- fluence on reef ecology in eastern Panthalassa, with coral buildups sition on a basin floor with a low amount of pelagic micrite, barren from dominating the lower latitudes, microbial communities and bivalves oc- the fossiliferous point of view and possibly originating during the grav- cupying higher paleolatitudes, and sponge-coral reefs in the mid- itational movement along the slope, rather than on the platform itself. In latitudes (Martindale et al., 2015). Positioning of the Hosselkus lime- every studied locality, the Hosselkus limestone appears as a single phase stone using this approach is tricky due to its depositional characteristics of slope-platform progradation, characterized by the regular transition (i.e. breccia deposits), that do not allow a quantitative estimation of from muddy to coarser deposits. This testifies to the presence of a reef-forming fauna. Nonetheless, considering reworked extraclasts as short-lived platform that was probably not able to survive during reliable proxies of the Hosselkus reef's biology, some comparisons are some stressful conditions that arose at one point (i.e. subsidence). still possible. The occurrence of all the main reef bioconstructors (i.e. corals, sponges, microbial fabrics, and calcareous algae), clearly indi- 6.3. Comparison with other Triassic carbonates from Panthalassa Ocean cates that the Hosselkus limestone did not develop at either high lati- tudes such as the Baldonnel Formation, where microbialites represent Both edges of the Pangean craton were active margins in the Late a key component in the structural architecture of the reef, to the detri- Triassic and most of the carbonate deposition in northern Panthalassa ment of sponges and algae (Martindale et al., 2010), nor at equatorial is thought to have occurred on small seamounts and microcontinents latitudes such as the Luning Formation, with corals and sponges domi- (Stanley, 1979a, 1979b; Stanley, 1982; Flügel, 2002; Martindale et al., nating the reef construction (Martindale et al., 2012a, 2012b). The 2015; Peybernes et al., 2020). The paleogeography of these terranes in Hosselkus limestone shows strong affinities with carbonate buildups the eastern part of the Panthalassa Ocean has been a matter of discus- where different bioconstructors play a relevant role in the overall archi- sion for a long time, especially in terms of the proximity of the terranes tecture of the reef, as in the Martin Bridge Formation at Summit Point to each other and with respect to the continental margin. However, dur- (lower Norian). Even though rock preservation represents a clear obsta- ing recent years several authors support an Early Mesozoic paleogeog- cle to the comparison of several species and genera, affinities between raphy in which several terranes are in close proximity to Pangea and the Hosselkus limestone and Martin Bridge faunas are present. In both each other (LaMaskin et al., 2011; Petersen et al., 2004; Unterschutz cases, the presence of clotted or encrusting microbialites is constant al- et al., 2002). The Hosselkus limestone shows a series of similarities as though often associated with other biocontructors such as numerous well as discrepancies with respect to other carbonates that developed scleractinian corals and calcareous red algae (Parachaetetes sp.) (Bucur in this enigmatic paleogeographic setting during the Late Triassic. et al., 2020; Martindale et al., 2012b) together with other organisms Starting from the age of the deposits, we can affirm that the Hosselkus such as Favreina sp., microcoprolites, Nubecularid forams and serpulid limestone was a real pioneer depositional event, if compared with worm tubes. Noteworthy is the almost complete lack of benthic fora- other terrane-based carbonates, that occurred as soon as conditions in minifers in the Hosselkus limestone, despite their remarkable abun- the Eastern Klamath terrane allowed the proliferation of calcareous or- dance and preservation in the lagoonal facies of the Martin Bridge ganisms. Although carbonate production in northern Panthalassa had Formation (i.e. the Black Marble Quarry outcrop, Wallowa terrane)

17 A. Fucelli, M. Golding and R. Martini Sedimentary Geology 422 (2021) 105967 and of the Aksala Formation (i.e. Lime Peak outcrop, Stikinia terrane). information, depositional environment and bathymetric data can be in- This lack is attributable to two different scenarios: the absence on the tegrated into paleogeographic and geodynamic reconstructions of this original platform of depositional settings favorable to the development no longer extant ocean. of foraminifers, crucial for such highly facies-controlled organisms (Martini et al., 2009; Peyrotty et al., 2020a); or the absence of lagoonal Declaration of competing interest facies, which are the most favorable for the development of Upper Trias- sic foraminifers, in the breccia accumulations that characterize the The authors declare that they have no known competing financial Hosselkus limestone. Optical cathodoluminescence observation interests or personal relationships that could have appeared to influ- allowed the recognition of the orders Lituolida, Miliolida and ence the work reported in this paper. Spirillinida; however, the complete recrystallization of the walls pre- vents any further determination. Still, paleogeographic conditions of Acknowledgments these terranes were considerably similar and allowed, even if at differ- ent times, the development of analogous carbonate buildups. The differ- Special thanks to Sylvain Rigaud for his valuable help during the sec- ence in age between shallow-water deposits from the two terranes ond field mission. The authors acknowledge Camille Peybernes for his could be explained by the physical impossibility for any bioconstructor appreciated assistance with fossil identifications and for the critical revi- to thrive in the Wallowa area during the late Carnian, since the terrane sion of this manuscript together with Giovan Peyrotty. We thank was dominated by strong volcanism (Seven Devils Group) (Whalen, Manuel Rigo and an anonymous reviewer for providing very useful 1988; Follo et al., 1994). In this case, despite the inferred proximity of comments on the manuscript. François Gischig is appreciated for the the terranes to each other, both the inadequacy of the submarine topog- great quality of the thin sections used in this study and Agathe raphy and the seawater geochemistry were crucial in preventing the de- Martignier for her technical support with the SEM instrument. The pre- velopment of any carbonate buildups, that started just after the sented research has been fully financed by the Swiss National Science cessation of volcanism. Whatever the cause for an earlier deposition, Foundation, as a part of the REEFCADE project (Grant 200020_178908 the Hosselkus limestone played a key role in the distribution of carbon- by Rossana Martini). ate buildups across part of eastern Panthalassa, acting as a shelter as well as dispersal agent for succeeding reefal communities. Ages of all References the above-mentioned terrane-related carbonates, together with their Afanasieva, M., Amon, E., 2015. Fossilization of radiolarian skeletons. Paleontological Jour- faunal-related paleolatitudes, suggest a progressive reefal settlement nal 48, 1487–1501. moving further away from the middle latitudes, where the Hosselkus Albers, J.P., Robertson, J.F., 1961. Geology and ore deposits of East Shasta Copper-Zinc Dis- limestone was supposed to have developed. trict Shasta County, California. USGS Professional Paper. vol. 338. Beales, F.W., 1958. Ancient sediments of Bahaman type. 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