Controls on accumulation of anomalously thick coals: Implications for sequence stratigraphic analysis

Abstract

Coal seams preserve high-resolution records of ancient terrestrial water table (base level) fluctuations in ancient peat accumulations, but little is known about base level change in anomalously thick coal seams. Using the Early Cretaceous 91 m anomalously thick No. 6 coal (lignite) seam in the Erlian Basin (north-east China) as a case study, the origin and evolution of peat accumulation in a continental faulted basin is revealed by sedimentological, sequence stratigraphic and coal petrological analyses. The lignite is dominated by huminite, indicating oxygen-deficient and waterlogged conditions in the precursor mire. Four types of key sequence stratigraphic surfaces are recognized, including paludification, terrestrialization, accommodation-reversal and give-up transgressive surfaces. Vertically, the No. 6 coal seam consists of fourteen superimposed wetting-up and dryingup cycles separated by key sequence stratigraphic surfaces, with each of these cycles having a mean duration of about 156 to 173 kyr. In a high accommodation peat swamp system, the wetting-up cycles are generally characterized by an upward increase in mineral matter and inertodetrinite and an upward decrease in huminite with the paludification surface as their base and the give-up transgressive surface or accommodation-reversal surface as their top, representing a trend of upward-increasing accommodation. In contrast, the drying-up cycles are generally characterized by an upward decrease in mineral matter and inertodetrinite and an upward increase in huminite, with the terrestrialization surface as their base and the accommodation-reversal surface as their top, representing a trend of upward-decreasing accommodation. A multi-phase mire stacking model for accumulation of the coal seam is proposed based on high-frequency accommodation cycles and the stratigraphic relationships between coal and clastic sediments. High-frequency accommodation cycles in the coal are closely related to water table fluctuations in the precursor mires and are driven by high-frequency climate via changes in the intensity and seasonality of precipitation in a relatively stable subsidence regime. Recognition that the No. 6 coal seam is composed of multiple stacked mires has implications for 991

Petrology and hydrocarbon significance of the coaly source rocks from the Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin

“…Vitrinite reflectance is a good indicator of coal rank (Stach And Murchison, 1982; Wang et al., 2019). The vitrinite reflectance of the samples, ranging between 0.55 and 1.28%, corresponds to bituminous coals (ISO 11760, 2005), of which the West sub-Sag (B-1 and B-2) has a slightly higher coal rank (bituminous C to bituminous B) than that in West Slope Belts (A-1, A-3, and A-2) whose coal rank mostly ranges from bituminous D to bituminous C. The reason for this difference is because of a deeper burial depth and higher geothermal gradient in the western sub-Sag than those in the West Slope Belts (Tong et al., 2009).…”

Sequence stratigraphy, paleogeography, and coal accumulation regularity of major coal-accumulating periods in China

“…In the western sub-zone, to the west of the Da Hinggan Mountains, the tectonic activities were relatively weak and beneficial for the accumulation of thick coal seams. In the central sub-zone, represented by the Songliao Basin, the intensity of tectonic movements and volcanic activities were strong, therefore the coals were thinner compared to the western sub-zone (Guo et al 2018;Wang et al 2019Wang et al , 2020). The regional differences of tectonic setting between the east and west of Da Hinggan Mountains was closely related to subduction zone migration of the paleo-Pacific plate.…”

“…The sequence stratigraphy approach can not only establish the isochronous stratigraphic framework for paleogeography reconstruction, but also provides an opportunity to reconsider the mechanism of coal accumulation in term of base-level cycles and accommodation space changes in different tectonic setting (Diessel 1992;Bohacs and Suter 1997;Li et al 1996Li et al , 2001Holz et al 2002). The accumulating mechanisms of the thick coal seams can be well interpreted by the sequence stratigraphy (Shearer et al 1994;Jerrett et al 2011;Wang et al 2016;Guo et al 2018;Wang et al 2020). Coal accumulation patterns and distribution of coal-accumulating centers in sequence stratigraphic framework provide a reliable guarantee for prediction of coal resources (Shao et al 2009;Li et al 2020).…”

A study of sequence stratigraphy of the Early Cretaceous coal-bearing series in the southeastern Songliao Basin, NE China

“…Coal accumulation models and depositional models of continental basins in the sequence stratigraphic framework have been widely discussed (Bohacs and Suter 1997;Shao et al 2011;Li et al 2018b;Wang et al 2020). Olsen (1990) developed three lacustrine depositional models based on observations from the Triassic fault basins of the North American Atlantic continental margin: Richmondtype, Newark-type, and Fundy-type.…”

“…Since 1990s, a great number of coal geologists also regard sequence stratigraphy as a useful tool for analyzing the origin, form, thickness, distribution, and quality of coal seams in a sequence stratigraphic framework (Bohacs and Suter 1997;Holz et al 2002;Shao et al 2003;Gibling et al 2004;Fielding et al 2007;Jerrett et al 2011;Wang et al 2019). The thick coals could be considered as an isochronous surface in the coal-bearing series and often occur near the maximum flooding surface (Diessel 1992(Diessel , 2007Flint et al 1995;Shao et al 2011;Wang et al 2020), which depends on the balance between the rates of accommodation and peat accumulation (Bohacs and Suter 1997). In intra-continental rift coal-bearing basin, coal seams exhibit poor stability with only local areas possessing great thickness (Li et al 2018b;Wang et al 2019).…”