Microfacies and palaeoenvironments of late Cisuralian and Guadalupian (Early to Middle Permian) alatoconchid-bearing limestone in Loei fold belt, Indochina Terrane

Abstract

This study presents the first record of Early Permian alatoconchid bivalves from Thailand, the Erawan section, in the north of Loei fold belt with prolific fusulines in association with gastropods, and brachiopods. Fusuline taxa are dominated by Pseudofusulina sp. with Darvasites sp., Staffella cf. labanalensis, Staffella sp., Nankinella sp., Neofusulinella sp., Schubertella sp., Pamirina darvasica, Pseudoendothyra sp. and others, indicating an Artinskian age. Fusuline wackestone with common, well-preserved smaller foraminifers, Pseudovermiporella sp. and ostracods indicates a restricted lagoon environment with low to moderate water circulation. These fossils, along with well-preserved, articulated alatoconchids in life-position, suggest the autochthonous nature of the deposits in an intertidal environment.

In the central part of the fold belt, the Pak Chong section exhibits intervals of alatoconchid biostromes and coquinites. Carbonate build-ups of the alatoconchid biostromes with gregarious bivalves, in life-position embedded in a micritic wackestone matrix, suggest their preferred life habit and environment. The bivalve coquinites containing packed bivalve shells and fragments, however, indicate a high-energy event. This rock fabric along with common fusuline storm sheets suggests occasional storm events in a restricted, with moderate water circulation to open lagoon environment. Fusulines are predominated by staffellids and others including Staffella sphaerica, Staffella sp., Sphaerulina croatica, Pisolina subsphaerica, Nankinella sp., Neoschwagerina simplex, Presumatrina sp., Afghanella sp. and others, indicating a Wordian (Middle Permian) age.

In the southern portion of the fold belt along the Thai-Cambodian border, the Khao Taa Ngog section contains limestone with alatoconchid shells with massive rugose corals which formed local carbonate build-ups in high-energy open platform environments. Poorly-sorted bioclasts with common fusuline tests, shell fragments and coated grains suggest an open marine, lagoon, back reef environment. The occurrence of a fusuline assemblage including Neoschwagerina sp., Yabeina sp., Lepidolina sp., Codonofusiella sp., Verbeekina verbeeki and Chusenella sp. indicates the Capitanian (late Middle Permian) age.