The marine record of the Triassic–Jurassic boundary interval has been studied extensively in shallow-marine successions deposited along the margins of Pangea, particularly its Tethyan margins. Several of these successions show a facies change from carbonate-rich to carbonate-poor strata attributed to the consequences of igneous activity in the Central Atlantic Magmatic Province (CAMP), which included a biocalcification crisis and the end-Triassic mass extinction. Evidence for a decline in calcareous and an increase in biosiliceous sedimentation across the Triassic–Jurassic boundary interval is currently limited to the continental margins of Pangea with no data from the open Panthalassan Ocean, the largest ocean basin. Here, we present a facies analysis of the McCarthy Formation (Grotto Creek, southcentral Alaska), which represents Norian to Hettangian deepwater sedimentation on Wrangellia, then an isolated oceanic plateau in the tropical eastern Panthalassan Ocean.The facies associations defined in this study represent changes in the composition and rate of biogenic sediment shedding from shallow water to the outer ramp. The uppermost Norian to lowermost Hettangian represent an ∼ 8.9-Myr-long interval of sediment starvation dominated by pelagic sedimentation. Sedimentation rates during the Rhaetian were anomalously low compared to sedimentation rates in a similar lowermost Hettangian facies. Thus, we infer the likelihood of several short hiatuses in the Rhaetian, a result of reduced input of biogenic sediment. In the Hettangian, the boundary between the lower and upper members of the McCarthy Formation represents a change in the composition of shallow-water skeletal grains shed to the outer ramp from calcareous to biosiliceous. This change also coincides with an order-of-magnitude increase in sedimentation rates and represents the transition from a siliceous carbonate-ramp to a glass ramp ∼ 400 kyr after the Triassic–Jurassic boundary. Sets of large-scale low-angle cross-stratification in the Hettangian are interpreted as a bottom current–induced sediment drift (contouritic sedimentation). The biosiliceous composition of densites (turbidites) and contourites in the Hettangian upper member reflects the Early Jurassic dominance of siliceous sponges over Late Triassic shallow-water carbonate environments. This dominance was brought about by the end-Triassic mass extinction and the collapse of the carbonate factory, as well as increased silica flux to the ocean as a response to the weathering of CAMP basalts. The presence of a glass ramp on Wrangellia supports the hypothesis that global increases in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps across the Triassic to Jurassic transition.

中文翻译：

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兰格利亚的玻璃坡道：美国阿拉斯加麦卡锡组晚三叠世至早侏罗世外坡道环境

三叠纪-侏罗纪边界区间的海洋记录已在沿盘古大陆边缘沉积的浅海层序中进行了广泛研究，特别是其特提斯边缘。其中几个演替显示了从富含碳酸盐地层到贫碳酸盐地层的相变，这归因于中大西洋岩浆省 (CAMP) 的火成活动的后果，其中包括生物钙化危机和三叠纪末的大规模灭绝。目前，关于三叠纪-侏罗纪边界区间钙质减少和生物硅质沉积增加的证据仅限于盘古大陆边缘，没有来自最大海洋盆地开放的 Panthalassan 海洋的数据。在这里，我们对 McCarthy 组（阿拉斯加中南部的 Grotto Creek）进行了相分析，它代表了 Wrangellia 上的 Norian 到 Hettangian 深水沉积，然后是热带东部 Panthalassan 海洋中的一个孤立的海洋高原。本研究中定义的相关联代表了从浅水到外坡道的生物沉积物的组成和速率的变化。最上面的诺里安到最下面的赫坦格代表了一个以中上层沉积为主的～8.9-Myr 长的沉积物匮乏区间。与类似的最低赫坦阶相中的沉积速率相比，Rhaetian 期间的沉积速率异常低。因此，我们推断，由于生物沉积物输入减少，Rhaetian 可能会出现几个短暂的中断。在赫坦格语中，McCarthy 组的下段和上段之间的边界代表了从钙质到生物硅质的浅水骨骼颗粒组成的变化。这种变化也与沉降速率的数量级增加相吻合，并代表在三叠纪 - 侏罗纪边界之后从硅质碳酸盐斜坡到玻璃斜坡的过渡 ∼ 400 kyr。Hettangian 的成套大规模低角度交叉分层被解释为底流诱导的沉积物漂移（等高线沉积）。Hettangian 上段致密岩（浊积岩）和等高岩的生物硅质组成反映了早侏罗世硅质海绵在晚三叠世浅水碳酸盐环境中的优势。这种主导地位是由三叠纪末的大规模灭绝和碳酸盐工厂的倒塌以及作为对 CAMP 玄武岩风化的反应而增加的二氧化硅通量导致的。兰格利亚上玻璃坡道的存在支持了这样一种假设，即全球海洋二氧化硅浓度的增加促进了三叠纪到侏罗纪过渡坡道上广泛的生物硅质沉积。