Significance of Coal Seam in Recognition of Sequence Boundaries and Systems Tract of the Jurassic in East Fukang Slope, Junggar Basin, China

1. Introduction The East Fukang Slope is located in the east of the Changji Sag and on the junction of the middle-south of the Zhangbei Fault-fold Zone and the Baijiahai Salient in the northeast of the Junggar Basin, Northwest China, with an area of about 5,000 square kilometres (Fig. 1). When the Jurassic was deposited, a huge slope with a north-south dip was formed, which consisted of the Wucaiwan Sag, front of the Kelameili Mountain piedmont, the Baijiahai Salient and the East Fuka…

Significance of Coal Seam in Recognition of Sequence Boundaries and Systems Tract of the Jurassic in East Fukang Slope, Junggar Basin, China

The Jurassic in the East Fukang Slope can be divided into six sequences based on sequence stratigraphy by combining logging, core and seismic data. The indicators of sequence boundaries include unconformity, coal seams, change of spore and pollen abundance, scour surfaces and base conglomerate, change of logging curve and sedimentary facies. How to determine the location of the first flooding surfaces and the maximum flooding surface is the key step to divide the systems tract. There occurred a topographic slope break in the East Fukang Slope when the Jurassic was deposited, and therefore we can recognize the location of the first flooding surface and establish the sequence stratigraphic framework with the slope break in the study area. Coal seams regionally distributed are correlatable and isochronic, and record the termination of a depositional event or episode. So, the regional coal seam (more than 60 percent coverage) can be used as the genetic stratigraphic sequence boundary, while locally distributed coal seam (less than 60 percent coverage) can be used as the systems tract boundary. The thick coal seams distributed regionally in the middle of the Badaowan Formation and the lower part of the Xishanyao Formation in the study area act as the sequence boundaries, while the thin and locally distributed coal seam acts as the systems tract boundary, which results in the correlation of the division of sequence stratigraphy of the Jurassic to the whole basin where coal seams are developed extensively.