TRIASSIC EVOLUTION OF THE YANGTZE PLATFORM IN GUIZHOU PROVINCE, PEOPLE'S REPUBLIC OF CHINA

Abstract

Deposition of shallow - water carbonates on the vast Yangtze Platform of south China spanned the late Proterozoic through Middle Triassic, accumulating as much as 4000 m during the Early and Middle Triassic. Deeper-water carbonates and silici - clastics accumulated to comparable thickness in the Nanpanjiang Basin southeast of the platform. After the demise of the platform, an additional 2500 m of mostly siliciclastics spread across the platform in Late Triassic. Deposition of platform carbonates was also widespread in the Permian of south China, but a transgression in perhaps the last 2 m.y. of the Permian combined with differential subsidence to reconfigure the Yangtze Platform in the Triassic. The margin retreated ~100 km northward in the Guiyang (eastern) sector, and a low-relief ramp developed over the flat top of thePermian platform, whereas the Early Triassic margin mimicked the Permian location in the Zhenfeng sector to the southwest. Nevertheless, deposition was essentially continuous from the Permian into the Triassic in most localities in Guizhou. Triassic deposition began with widespread terrigenous mud bearing thin-shelled bivalves or ammonoids. Renewed carbonate deposition within the Induan produced thin-bedded, laminated, dark-gray lime mudstone with planktonic biota in the basin interspersed with carbonate breccias. Thin-bedded lime mudstones with prominent burrows formed farther updip to the north and west. Thick intervals of oolite and of shallow - water lime mudstone interbedded with terrigenous clastic wedges that thicken and merge to the west mark the updip limit of deposition on a carbonate ramp. Platform - interior carbonates are more than three times as thick as their basinal equivalents. The ramp configuration evolved into a flat-topped platform with a slight rim in the Olenekian, recorded by peritidal carbonate cycles at the platform margin and subtidal lagoonal muds and ultimately evaporites in the interior. Carbonate deposition spread farther westward to cover the terrigenous siliciclastics of the Induan. A major deepening event within the Olenekian is marked by black, ammonoid-bearing, nodular limestone and fissile shale. The patchy distribution of this facies indicates differential warping of the platform rather than purely eustatic causes. The basin received a starvation diet of siliciclastic and carbonate mud with minor silty turbidites and carbonate debris flows. At the end of Early Triassic, platform-interior deposits averaged 1175 m thick and basinal equivalents only 250 m. Acidic volcanic ash spread across the Yangtze Platform at the Olenekian-Anisian transition. Anisian deposits in the Nanpanjiang Basin are dominantly siliciclastics that thicken dramatically in southwestern Guizhou and adjacent Guangxi. Biogenic frame - stones constructed by organisms of questionable origins, Tubiphytes and Plexoramea, assisted by sponges, arborescent corals, skeletal stromatolites, and copious encrusters, rimmed the Anisian platform. This rim collapsed along most of the Guiyang sector to form a basin-margin wedge 175 km long deposited by turbidity currents, debris flows, and rock fall. Collapse led to retreat of the margin by ~2.7 km; exceptionally by 10 km. In contrast the reef margin advanced slightly in parts of the Zhenfeng sector. Muddominated peritidal cycles formed in the lee of the reefs. Deposition in the platform interior was entirely subtidal with alternating episodes of normal marine water, hypersalinity, and siliciclastic influx. An increase in siliciclastic content eastward throughout the Middle Triassic signals the emergence of the Jiangnan Massif, which had been covered throughout the Permian and at least the Induan. Platform sedimentation in the Ladinian features peritidal cycles that extended far into the interior to define a flat - topped platform. High depositional energy is reflected in grainstones and packstones composed of grapestone, bioclasts, and ooids. Barriers other than sand shoals appear to have been absent. Biogenic facies are confined to small outcrops of Tubiphytes and coral boundstone, interpreted as patch reefs. Tepee structures cap many cycles or disrupt successive cycles, indicating extended subaerial exposure. At the beginning of the Ladinian, the platform margin of the Guiyang sector prograded at least 0.6 km. In the Zhenfeng sector the margin retreated slightly, further indication of more rapid subsidence of the western part of the platform. The Nanpanjiang Basin apparently starved early in the Ladinian, but filled to overflowing with siliciclastic turbidites and mud in the later Ladinian or the early Carnian. Transport direction of the very fine sand of the turbidites was from the east, pointing to the Jiangnan Massif as a continued source of sediment, both by land onto the platform and by sea into the basin. East and west sectors of the Yangtze Platform in Guizhou present stark contrasts during the Carnian and Norian. Shallow-water carbonate deposition continued into the Carnian in the Guiyang sector, but tongues of terrigenous mud and sand from the northeast reached to the platform margin and damped out carbonate deposition by the end of the Carnian. Erosion prevailed in the Norian, truncating formations toward the north down to the level of the Anisian. Shallow-water carbonate deposition ended dramatically with the beginning of the Carnian in the Zhenfeng sector. Nodular-bedded, dark-gray lime mudstone with ammonoids overlies peritidal deposits, documenting the drowning of the Yangtze Platform. A very condensed sequence of black shale with concentrations of manganese, reduced iron, and organic carbon followed. Siliciclastic flysch and shallow-water sandstone totaling 1265 m thick filled the accommodation space resulting from the drowning by the end of the Carnian. Norian deposits are cross-bedded sandstones and conglomerates that form thinning- and fining-upward cycles attributed to braided streams that encroached on coastal swamps represented by commercial coal and by mudrocks containing fresh-water, brackish, or marine fossils. The braided streams were rejuvenated and apparently reversed in the Rhaetian to form a coarse-grained clastic wedge that thins and fines toward the north and east across an erosion surface that cuts as deep as Anisian rocks in northern Guizhou. Jurassic and Lower Cretaceous rocks overlie the Rhaetian rocks concordantly, contradicting prevalent interpretations of a major orogeny (Indosinian) in the Late Triassic in Guizhou. The angular unconformity underlying Upper Cretaceous conglomerates dates major deformation in Guizhou as mid-Cretaceous. Copyright ©2006 Geological Society of America.