Uppermost Triassic Halstätt-like cephalopod limestone (Lilu Facies) and Foraminifera, Timor-Leste

Abstract

Ammonoid-rich wackestone beds forming a ca 2 m section between shallow-marine limestones of the Bandeira Group (Norian to possibly Rhaetian) and a wackestone–calcareous mudstone succession belonging to the Lower Jurassic Wailuli Group are recorded from the western part of Timor Leste. They are included in the Halstätt-like Lilu Facies of the Bandeira Group. Age-diagnostic involutinid foraminiferal species, not previously recorded from Timor, include Involutina liassica, Trocholina sp. ex. gr. Trocholina turris and Lamelliconus permodiscoides, which collectively indicate a Rhaetian age, possibly ranging down to the Sevatian. The conodont Norigondolella steinbergensis, recorded low in the section, is consistent with this age determination. The Lilu Facies was deposited at a water depth of a few tens of metres, as suggested by the presence of extensive cyanobacterial borings into shell fragments and the presence of microbial laminations including the presence of stromatolites and bacinellid-like fabrics. Among macrofossils, common ammonoids are present in the section, and marine reptile bones are conspicuous in the lower beds. Other biogenic components of the rock observed in acetate peels and thin sections are abundant echinoderm debris (including pentacrinoid columnal plates), bivalve fragments and micro-gastropods, rare siliceous sponge spicules, solitary corals, and brachiopod debris. In the studied region, a major drowning of a shallow-marine carbonate platform took place probably in the Rhaetian or possibly latest Norian. Similar ‘ammonitico rosso limestone’ interbedded with thick-bedded limestones of a shallow-marine carbonate platform is present in the uppermost Triassic of Seram. More detailed biostratigraphical comparisons and correlations of similar units in Timor and other nearby islands may indicate a general marine drowning of Late Triassic carbonate platforms, at least in the northern part of the East Gondwana Interior Rift system, during the Rhaetian or possibly latest Norian.