Ramp morphology controlling the facies differentiation of a Late Ordovician reef complex at Bachu, Tarim Block, NW China

Abstract

A carbonate ramp in the shallow-marine northwestern part of the Central Tarim Uplift, Bachu, NW China, exhibits an extraordinary Late Ordovician reef complex along the Lianglitag Mountains, exposed for a distance of about 25 km. Seven localities within the 'Middle Red Limestone' of the Upper Member of the Lianglitag Formation (Katian, Late Ordovician) illustrated the changes in biofacies and lithofacies: northern, seaward-directed patch reefs are replaced towards the south by coeval grain banks. The patch reef units are dominated by microbial and calcareous algal components. The reefs at the northernmost locality are knoll-shaped, kalyptra-shaped or irregularly shaped with sizes of individual reefs increasing from about 2 m in height and diameter. Stratigraphically upward, reefs notably expand to larger structures by several mounds coalescing; they are generally about 10 m thick and tens of metres in lateral extent. The maximum thickness of the main patch reef is more than 30 m, and its diameter is around 100 m. The reefal units turn into biostromes with gentler relief southward and still further south grade into banks composed of peloids and coated grains. The southernmost locality is still a shallow-water bank, and the coastline is not documented in the study area. The present evidence indicates that the Late Ordovician palaeo-oceanography provided a number of environments for the optimal growth of carbonate build-ups; microbial-calcareous algal communities could thrive in areas where the innovative metazoan reef frameworks consisting of corals and stromatoporoids did not play a significant role. The ramp morphology, especially changes in water depth, controlled the configuration of the reef complex. □ Bachu, Late Ordovician, Lianglitag Formation, NW China, reef complex, Tarim Block.Yuanyuan Zhang [yuanyuanzh85@163.com], and Yue Li [yueli@nigpas.ac.cn], Nanjing

New insights on Lower Ordovician (Floian) reefs from the Argentine Precordillera: Biostratigraphic, sedimentologic and paleogeographic implications

“…11). Although sea-level fluctuation is an important variable that controls regional reef development, it is estimated that energy change is a first-order factor that controls reef growth (Zhang et al, 2015b(Zhang et al, , 2016. Based on this statement we interpret a probable shallower depositional environment in the Niquivil section compared to the Talacasto section.…”

Sedimentary facies, depositional environments and conceptual outcrop analogue (Dam Formation, early Miocene) Eastern Arabian Platform, Saudi Arabia: a new high-resolution approach

“…The thrombolite gives it biostromal or coalescent mound shape with a height of 1 m or more. These mounds are closely associated with stromatolitic boundstones facies (Zhang et al 2015). However, according to Alkhaldi (2009), this facies was not encountered in the core samples of shallow subsurface from the Al-Lidam area.…”

“…However, according to Alkhaldi (2009), this facies was not encountered in the core samples of shallow subsurface from the Al-Lidam area. The thrombolite mounds, although they grow similar or some time on the digitate stromatolitic facies (Pratt 1982;Zhang et al 2015), appear to lack distinctive internal fabrics that are characteristic of thrombolitic facies, i.e. thrombolitic fenestrae and clots (Armella 1994).…”

Exploring the depositional, diagenetic and carbon-oxygen isotope record of an evolving Ordovician carbonate system, Tarim Basin, China

“…2-4) forms part of a carbonate ramp that deepens over about 50 km from the Leyayilitag region into the northern basinal settings of the Dawangou region (Figs. 1, 2; Li et al, 2009;Zhang et al, 2015;Wang et al, 2017a;Li et al, 2017;Shen & Neuweiler, 2018). The Yangjikan region marks the transitional zone toward outer-ramp-to-basinal settings (Figs.…”

“…The upper part of the Yingshan Formation (Floain according to Wang et al, 2017b;Zhao et al, 2018) is an alternation of micritic limestone and dolostone. There is an unconformity between the Yingshan Formation and the Yijianfang Formation defined by erosion, karstification, and brecciation (Li et al, 2007(Li et al, , 2009Zhang et al, 2014aZhang et al, , 2015Zhang & Munnecke, 2016;Wang et al, 2017b;Zhao et al, 2018;Figs. 2-4).…”