Linkage between calcirudite lithofacies and storm-generated depositional processes in the Upper Cambrian Series, Shandong Province, eastern China

摘要

Ideal carbonate tempestites consist of five units from base to top: (i) calcirudite with a scoured base and basal gutter casts; (ii) normally graded calcarenite; (iii) parallel-laminated calcarenite; (iv) hummocky cross bedded to wavy laminated calcarenite calcisiltite; and, (v) horizontal laminated mudstone. However, the calciruditic part remains relatively poorly documented in terms of its lithofacies characteristics and relationship to storm-generated depositional processes. Based on outcrop observation, we address this knowledge gap by analyzing the petrologic and sedimentary properties of over 100 intervals of calcirudite from carbonate tempestites in Upper Cambrian strata in Shandong Province, eastern China: Fair weather deposits formed in a carbonate ramp recording three episodes of relative sea-level rise and fall, corresponding to the Middle, lower Upper and upper-Upper Cambrian Series, respectively. Calcirudite tempestite intervals, which are embedded in fair weather deposits, arc classified into five types of lithofacies according to their petrologic and sedimentary structures: horizontal- to imbricate-aligned calcirudite (LF1); perpendicularly-aligned calcirudite (LF2); well-rounded sand supported massive calcirudite (LF3); poorly rounded matrix supported massive calcirudite (LF4); and, poorly-rounded tightly-stacked calcirudite (LF5). These lithofacies are interpreted, respectively, as gravel shoal deposits linked to storm waves and storm enhanced tides, gutter cast infilling deposits linked to storm vortices, high density turbidites, debrites, and collapse deposits mainly linked to storm back flow. Storm depositional successions are dominantly constructed by single beds of LF1, and to a lesser extent by scour based successions comprising LF1, LF3, LF4, or LF5 passing upward into graded gravelly calcarenite to calcarenite, capped with parallel to hummocky cross stratified calcisiltite to calcarenite. The considerable variation in the tempestite successions suggests that storm-generated depositional processes differed between the inner ramp, mid-ramp, and out ramp sub environments. Significantly, this study recognizes carbonate gravelly shoal deposits, which arc seldom documented in the literature. Moreover, the abundance of gutter casts strongly supports the importance of storm vortices as a depositional process in carbonate tempestite generation. The linkages between carbonate tempestite lithofacies and storm generated depositional processes demonstrated herein can be used to improve interpretations of palcoenvironments and paleogeography in other carbonate successions around the world and through geological time. (C) 2019 Elsevier B.V. All rights reserved.