关键词: 寒武系, 仙女洞组, 台缘斜坡, 碳酸盐岩重力流沉积, 发育机制

Abstract: Based on field survey, outcrop analysis and microscopic observbation, we report the basic characteristics of the carbonate
gravity flow in platform margin in the lower Cambrian Xiannvdong Formation of the Northern Sichuan Basin and address its
developmental mechanism in this paper. Results show that the Xiannvdong Formation typically developed an environment of diamictic
carbonate platform marginal slope in shallow water. The bottom of this formation is mainly composed of three rock types, i.e., silty
mudstones, bioclastic micritic limestones and algal limestones. According to the planar morphologies, two types of carbonate gravityflow deposits were classified, including nodular-like and brecciated. Based on the grain size and lithological variation of the gravity flow
sediments in outcrops, five developmental stages were divided, which show an obvious superposition relationship vertically. The grain
size of these sediments roughly changes from fine to coarse within sedimentary cycles, consistent with the regressive environment of the
Xiannvdong period in general. In the vicinity of the platform marginal slope, slide and slump events occurred due to the triggering of
geological events, e.g., earthquake and storm. The mound and shoal mass moved down along the slope, and some of the blocks migrated
and deposited here, which formed the brecciated carbonate gravity flow deposits. With the increasing injection of water, the amount of
sediment-water mixture and the decomposition of the material rose, and some of the mass were broken and flowed as similar forms of
debris flow along the slope, and then the debris transformed into carbonate gravity flow deposits with plastic rheological characteristics.
Then, these deposits might be reworked by bottom current or experienced corrosion from seawater after deposition to form two types of
nodular-like carbonate gravity flow sediments. In contrast, the brecciated carbonate gravity flow sediments were products of transportation
and short-distance slumping of mega unbroken carbonate mound mass. They are co-deposited with the nodular-like calcirudites.

Key words: Cambrian, Xiannvdong Formation, platform marginal slope, carbonate gravity flow, developmental mechanism