Origin of Phosphate‐bearing Carbonate Concretions in the Upper Triassic Lacustrine Black Shales of the Southern Ordos Basin, China

Abstract

Carbonate concretions are conspicuous in organic‐rich shales and are generally related to decomposition of organic matter. The black shales from the Chang 7 Member of the Upper Triassic Yanchang Formation of the southern Ordos Basin host abundant carbonate concretions, which provide a unique record of depositional and early diagenetic conditions of the paleo‐lake sediments. However, little attention has been given to the genesis and growth processes of the concretions in these lacustrine petroleum source rocks. New petrographic observations and geochemical analysis show that the concretions are composed of calcite, phosphate fossil fragments, K‐NH4‐feldspar, quartz, bitumen, and minor Fedolomite. Phosphate minerals, mainly carbonate fluorapatite (CFA), show pervasive replacement by calcite, most of which contains phosphorus, ranging in concentration from 0.26 to 2.35 wt%. This suggests that the phosphate minerals are the precursors for concretion growth. Positive δ13C (+5.6 to +12.4 ‰ V‐PDB) signatures and the absence of pyrite indicate that microbial methanogenesis was the dominant driver for concretion growth, rather than bacterial sulfate reduction. Quartz, bitumen, and Fe‐dolomite are the last cements that occurred, at deep burial depths and high temperatures. The formation of phosphate minerals might have been induced by upwelling of phosphate‐enriched deep water in the Late Triassic paleolake, which promoted phytoplankton blooms and further enrichment of organic matter. Extremely slow sedimentation rates of fine‐grained detrital minerals, relative to dead organism accumulation, led to the high permeabilities of the organic‐rich sediments and rapid concretion growth during shallow burial. The close association of phosphate‐bearing carbonate concretions and organic‐rich shales reflects that upwelling played a critical role in the formation of the high‐quality petroleum source rocks in the Triassic paleo‐Ordos lake.

Shale Lithofacies and Sedimentary Environment of the Third Member, Shahejie Formation, Zhanhua Sag, Eastern China

“…In sequence stratigraphic architectures, shale was considered historically to be deposited by quiet water suspension sedimentation (Macquaker and Taylor, 1996;Macquaker et al, 1998;Hammes, 2009;Teng, 2018). However, recent studies have suggested that its sedimentation was also related to gravity flow or contour current deposition (Stow et al, 1990(Stow et al, , 2001Tripsanas et al, 2004;Macquaker and Bohacs, 2007;Ochoa et al, 2013;Yang et al, 2017;Peng et al, 2020a, b;Zhang et al, 2021). Flume experiments demonstrated that flocculated muds can be transported in bedload in the same mode of coarse fragment hydraulics, forming cross-bedding as high-density flow or turbid flow on the seabed (Schieber et al, 2007(Schieber et al, , 2010Schieber, 2016a, b).…”

Luminescence Chronology of Reticulated Laterites in the Humid Subtropical Mountains of South China

“…This result is consistent with the age of the adjacent Longdengshan Paleolithic site that has been formed during the Late Pleistocene and exhibits definite buried strata (Jin, Wang, et al., 2021). In previous studies (Fan et al., 2019; Jin, Wang, et al., 2021; Olsen & Miller‐Antonio, 1992; Y. Wang, 1997; X. Wang, 2013; S. Zhang, 1983), the ages of the Paleolithic sites in South China were inferred from stone tool‐making technology and the reticulate laterites developed by buried strata are still questionable, and they are most likely not the sites that originated during the Middle Pleistocene and early Late Pleistocene, but the sites that originated during the middle and late Pleistocene.…”

“…Wang, 1997;X. Wang, 2013;S. Zhang, 1983), the ages of the Paleolithic sites in South China were inferred from stone tool-making technology and the reticulate laterites developed by buried strata are still questionable, and they are most likely not the sites that originated during the Middle Pleistocene and early Late Pleistocene, but the sites that originated during the middle and late Pleistocene.…”

Genesis of Fibrous Calcite in the Chang 7 Member of the Yanchang Formation, Ordos Basin, China

“…In the Chang 7 black shale of the Ordos Basin, carbonate rocks are generally developed, which can be divided into two types: nodules and veins. Researchers have studied more about the spherical carbonatite nodules found in outcrops in the south of the Ordos Basin (Li et al, 2019;Zhang et al, 2021;Zhu et al, 2021;Yuan et al, 2023), and consider them as having deep hydrothermal origin (Liu et al, 2020;Fan et al, 2021). Zheng et al (2021) defined the bedding-parallel carbonaceous laminar sediments distributed in the black rock formation of Chang 7 reservoir as hydrothermal exhalative sedimentary rocks.…”