Lower Triassic peritidal cyclic limestone: an example of anachronistic carbonate facies from the Great Bank of Guizhou, Nanpanjiang Basin, Guizhou province, South China

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 in thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone. The cyclic limestone is Olenekian in age, 164 in thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2-7.4 in thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating Lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences. The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo-Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic. (C) 2001 Elsevier Science B.V. All rights reserved.