Type and evolution of carbonate platforms in Jixian Period Mesoproterozoic: southwestern margin of Ordos Basin

Abstract

The Ordos block experienced extension in the Mesoproterozoic, during which the Longshan-Kuanping and Haiyuan rifts developed on its southwest margin. The Jixian Group (1.6-1.4 Ga) is mainly exposed on the rimmed and ramp-type platforms at the Qishan County, southwest of the Ordos block. The deposition associated with the rimmed platform margin consists of platform facies, platform edge facies, and slope facies, which are characterized by small tidal creeks, mud cracks and scour troughs, rockfalls, respectively. The ramp-type platform is dominated by platform edge facies, shelf facies and slope facies and featured by intraclastic dolostones, tempestites and debris flow sediments respectively. The syndepositional normal faults were well developed at the early stage of the rifting. Fault-related cracks and folds present in the adjacent strata. The uplift of the footwall rimmed the platform. The drag structures determine the local sedimentary environment. In the final stage of the rifting, the fault movement stopped. The surrounding rocks were undeformed. The slope became less steep, resulting in the formation of ramp-type platform. The study area in the Jixian period, Mesoproterozoic is located at a palaeocontinental margin, which has already shown good petroleum prospectivity. A clear study on the type of carbonate platforms and the evolution of the platforms can help reduce exploration risks by improve the understanding of the presence of the source rocks.

Carbon and oxygen isotope characteristics of carbonate rocks in the Mesoproterozoic Jixian System of the Ordos Basin and their implications

“…This formula has been widely used in restoring the paleosalinity of carbonate sedimentary environments in various geological historical periods 16 , 48 , 49 . In this study, most of the samples have Z values greater than 120, except for KH-13 and KH-15 (which have values less than 120 but near it), which indicates that the carbonate rocks in the region were formed in a stable marine environment; this is consistent with the research results of Liu et al 56 and also with the sedimentary environment of that period 29 . In terms of overall results, the range of Z value variation is not significant, indicating a relatively small climate change and stable tectonic environment during the Mesoproterozoic.…”

“…The research area was located near the equator during the Mesoproterozoic era 56 , and the tidal flat environment during the deposition of carbonate rocks in the Ordos Basin had a high salinity and temperature. This may be due to its location near the equator, high temperature, relatively warm and humid climate, sustained thermal subsidence, and sea level rise under the expansion of the Qinqi Trough 29 , leading to an increase in seawater temperature. In addition, carbon isotope changes are generally closely related to biological productivity 16 .…”

New insights about the Mesoproterozoic sedimentary framework of North China Craton

“…Although a lot of study have been made about the Mesoproterozoic sedimentary and tectonic evolution history, the study areas mainly distributed at the southern or western margin of the Ordos Block (Y. Z. Chen et al, ; K. Deng, Zhang, Zhou, Liu, & Li, ; J. P. Gao, Zhou, & Luo, ; M. Lu, Chen, Wu, Peng, & Wang, ; L. J. Song, Liu, Zhao, Wang, & Zhang, ).…”

Geochemical characteristics and genesis of Middle Proterozoic Jixian System dolomite in the Southwest margin of Ordos Basin

“…1) [31][32]. The Ordos block is affected by the pull trough of the Qinjin aulacogen and the continuous rift depression within the block, and the water body is deepening (relatively small amplitude) [33][34][35]. Under this background, a set of thick Jixian carbonate rocks of Middle Proterozoic era were deposited in the southwest edge of Ordos Basin, and the sedimentary thickness gradually increased from the edge of the block (more than 225 m) to the interior of the block (more than 616 m).…”