Ber. Inst. Erdwiss. K.-F.-Univ. Graz ISSN 1608-8166 Band 21 Graz 2015 STRATI 2015 Graz, 19 – 23 July 2015

Carbonate factory in the aftermath of the end- mass extinction: Griesbachian crinoidal limestones from Oman

BAUD, A.1, GOUDEMAND, N.2, NÜTZEL, A.3, BROSSE, M.2, FRISK, Å. M.4, MEIER, M. 2, BUCHER, H.2

1 Parc de la Rouvraie 28, CH-1018 Lausanne, Switzerland, email: [email protected] 2 Paleontological Institute and Museum, CH-8006 Zurich, Switzerland 3 Bayerische Staatssammlung für Paläontologie und Geologie, D-80333 München. Germany 4 Palaeobiology, Department of Earth Sciences, Uppsala University, 753 36, Uppsala, Sweden

In the Batain region of Oman, basal crinoid limestones occur as exotic boulders within the Guweiza Formation near Asselah. Some of these boulders contain the conodont association Hindeodus parvus - Isarcicella isarcica, indicating a Griesbachian age. Disarticulated crinoid columnal segments are the main component of these calcarenitic boulders. Additional fauna include small-sized ammonoids (Permian holdovers such as medlicottids and xenodiscids), numerous gastropods, ostracods, bivalves, brachiopods, echinoid spines, microconchids, and foraminifers. The rock is a crinoidal packstone with a dominantly micritic matrix and a great abundance of bioclasts, showing in some cases an early phase of cementation (partly coarse spar and cement fringe). The cylindrical columnals are 0.2 - 2mm in diameter and display up to 32 radial ridges. These ossicles are not well sorted and mixed with mostly unbroken molluscan shells showing no signs of abrasion or bioerosion. This preservation suggests minimal lateral transport. Thin-sections also reveal that the limestone yield a diverse assemblage of small gastropods which cannot be extracted from the rock due to strong cementation. The neritimorph gastropod sp. is abundant. Their specimens are well-preserved due to a thin, primarily calcitic outer shell layer. The size of these specimens of Naticopsis sp. can exceed 20 mm, demonstrating the presence of relatively large gastropods (‘Gullivers’) (1) as soon as during the Griesbachian, thus questioning the presumed Lilliput Effect that would have affected gastropods after the end-Permian crisis. Late Paleozoic Naticopsis species range from less than 10 mm to more than 130 mm in size. 20 mm as reported here for the present Griesbachian species from Oman is common in many Late Paleozoic Naticopsis species. Naticopsis, has been reported from various Early Triassic sites including the Wadi Wasit block of the Hawasina nappes in Oman Mountains (2) (3). However, Naticopsis occurrences from Asselah and Wadi Wasit are not conspecific. Despite the seemingly low taxonomic diversity of earliest Triassic crinoids (but see (4)), the Batain blocks show that some populations were sufficiently abundant to produce material in rock forming quantity. These neritic plateaus colonized by crinoids and various skeletal organisms obviously functioned as local and healthy carbonate factories. Such crinoidal sediments are rare during the Griesbachian: they have never been reported from the inter-tropical realm (Tethyan northern margins and South China) and are known only from mid-latitude localities such as Wadi Wasit (2) (4), Batain (this study) and Salt Range (dolomitized encrinite at the base and within the Kathwai Member) (5). Yet, it is not understood what factor(s) constrained their occurrences. According to (6), after the end-Triassic extinction, crinoidal limestones often developed on topographic highs and seamounts of the Western Tethys and corresponded to stratigraphically condensed sequences. As for the post- extinction Early Jurassic, the Griesbachian crinoidal limestones developed on hard-substrate, well- oxygenated submarine topographic highs, during the drowning and faulting of a former shallow platform. These carbonate ‘oases’ indeed imply: (a) paleotopographic areas protected from clastic input, (b) an adequate (shallow?) water depth, and (c) normal water chemistry (oxygen, pH, salinity) and temperature. They contradict the common view of global, homogeneously catastrophic conditions in the oceans in the direct aftermath of the end-Permian mass extinction.

References (1) BRAYARD et al., 2013, Earth-Sc. Rev., 146: 31-64. (2) KRYSTYN et al., 2003, P.P.P., 191: 329-344. (3) Wheeley & Twitchett, 2005. Lethaia, 38: 37–4. (4) OJI & TWITCHETT, 2015, Zool. Sc., 32/2: 211-215. (5) KUMMEL & TEICHERT, 1970, Univ. Kansas Sp. Pub., 44: 1–109. (6) JENKYNS, 1971, Geol. Rundschau, 60/2: 471-488.

31