Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China
Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China
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DOI：
10.2110/jsr.68.311
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发表时间：
1998-03
影响因子：
2
通讯作者：
D. Lehrmann;Jiayong Wei;P. Enos
中科院分区：
文献类型：
--
作者：
D. Lehrmann;Jiayong Wei;P. Enos
贵州大浅滩是南盘江盆地一个出露异常良好的三叠系孤立台地。该平台被挖掘出来，其沉积剖面被保存下来，并被一个断层向斜解剖，暴露出一个完整而简单的横截面，提供了一个独特的机会来评估其诞生、演化和消亡的机制。晚二叠世，在南盘江盆地扩张并淹没GBG周围地区的一次加深事件中，GBG形成于扬子地台南缘附近。GBG的最初积累开始于最新的二叠纪，有小的礁土丘和开阔的海洋骨骼堆积岩。在二叠纪末灭绝之后，蓝藻界石在河岸顶部生长。早三叠世期间，GBG发育了一个低起伏的河岸剖面，边缘为移动的鲕滩，内部为浅潮下带和潮缘沉积，盆地边缘为以深海沉积、碎屑流沉积和浊积岩为主的缓坡。在中三叠世（Anisian），GBG形成了一个逐渐变陡的剖面，边缘是巨大的Tubiphytes礁。该平台顶部平坦，内部有潮坪沉积物。盆地边缘沉积以浊流沉积和碎屑流沉积为主，但随着斜坡向休止角变陡，最终转变为雪崩和落石沉积。在中三叠世（Ladinian），在边缘发育了高达1700米的侵蚀悬崖。台地边缘地层是类似于内部地层的层状充填岩，而盆地边缘的角砾岩含有珊瑚-边界岩碎屑，表明陡崖对珊瑚礁的侵蚀。内部形成一个限制性潮下泻湖，形成一个类似环礁的形态。后来，随着滩涂遍布内陆，顶部平坦的轮廓得到了恢复。在晚三叠世初期，在硅质浊积岩和页岩沉积在台地上之前，加深导致了GBG的终止。与意大利北方白云岩的著名台地相比，GBG包含丰富的泥质碳酸盐和逐渐变陡的河岸，形成礁缘和悬崖建筑。意大利的平台几乎不含泥土，具有休止角和尖顶几何形状。GBG的较大尺寸增加了泥浆产量，并保护其免受大量泥浆的扬筛，这反过来又导致相对平缓的盆地边缘斜坡上稳定的泥质沉积物的离岸脱落，这些斜坡逐渐变陡，最终导致雪崩沉积物和高浮雕侵蚀悬崖。相反，白云岩平台的泥含量较少，迫使雪崩和岩屑沉积占主导地位的整个沉积盆地边缘，这反过来又产生了他们的休止角几何形状。
ABSTRACT The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.