Upper Triassic (Carnian) mud mounds from northern Sichuan ( China)

Abstract

During the late Carnian a great number of carbonate mud mounds were constructed along the submerget northwestern margin of the Yangzi Platform in Sichuan, in the transition zone between the Songpan Garze Trough in the west and the Chuan Dian Shelf in the east. Individual mounds are up to 80 m thick and have base diameters of up to several hundred meters. Thickness ratios between mound and intermound deposits are about 5:1 with mound slopes not exceeding 15^o. The bulk of the rock volume consists of a microsparitic matrix which due to its peloidal structure and common laminated crusts is probably a cyanobacterial precipitate. Siliceous sponges contributed to mound stabilization but did not form a rigid framework. Several other invertebrate groups (mainly crinoids) inhabitated the mound surfaces and were transported into adjacent depressions. Stromatactoid cavities, which are so characteristic for Palaeozoic mud mounds, are extremely rare. Generally they are completely occluded with late diagenetic blocky spar; only a few cavities are lined with an early marine RFC cement. The latter is distinguished by relatively heavy δ¹⁸O signatures of -3.4 ‰PDB which contrast with the isotopic composition of the burial cements displaying mean values of - 9.7‰  δ¹⁸O PDB. ⁸⁷Sr/⁸⁶Sr ratios of early cements, matrix and non-luminescent brachiopod shells fit well into the curve of the isotopic evolution of the Phanerozoic seawater.