BASAL TRIASSIC OMAN OASES: GRIESBACHIAN CRINOIDAL LIMESTONE FROM THE BATAIN PLAIN, EASTERN OMAN

In Oman, Griesbachian marine faunas were first reported from an exotic block at Wadi Wasit in the Hawasina nappes (Krystyn et al. 2003). Here, we document a new exotic boulder of the same age discovered in the Batain nappes (eastern Oman). The Asselah boulder is a 1 metre-thick crinoidal limestone which yields an abundant and diversified fauna including crinoids, ammonoids, gastropods and conodonts. Typical conodont index species such as Hindeodus parvus and several Isarcicella species allow to give the entire succession a Griesbachian age (= base of Induan).

Deposition of this crinoidal limestone on off-shore sea mounts was contemporaneous with that of the “anachronistic” microbial limestones known from shelves of equatorial Cimmerian blocks, a facies that has been interpreted as devastated environments illustrating complete diversity collapses (Awramik 1990; Schubert and Bottjer 1992; Schubert and Bottjer 1995). The Asselah and Wasit blocks record basal Triassic fossil assemblages that are unknown from continental shelves during this post-extinction transgressive interval. The associated community was living in well oxygenated shallow marine waters. These neritic off shore plateaus harbored dense prairies of crinoids and various skeletal organisms and functioned as local and healthy carbonate factories. They escaped the siliclastic depositional environments that prevailed on many continental platforms, slopes and basins.

Furthermore, despite their poor preservation, the crinoid ossicles suggest the occurrence of a holocrinid with subpentagonal proximal columnals and long cylindrical cirrals with transverse articulation ridges. The distal columnals are cylindrical with multiradiate facets; culmina may bifurcate as typical for Holocrinidae. Obviously, this crinoid is conspecific with Baudicrinus krystyni Oji and Twitchett, 2015 from the Wadi Wasit block. However, the Asselah material suggests holocrinid assignment of Baudicrinus rather than dadocrinid. Holocrinids had hitherto not been documented until the Olenekian (Hagdorn 2011), and this discovery pushes back their origin into the Griesbachian.

Size analyses of the gastropod Naticopsis sp. also provide further evidence against the Lilliput effect hypothesis already questioned by Brayard et al. (2010).