Abstract: Sedimentation and Multiple Coal-Seam Correlations in Upper Cretaceous Swamp Complex, John Henry Member of Straight Cliffs Formation, Southern Kaiparowits Region, Utah

Jerry Vaninetti

A maximum of 11 coal seams greater than 4 ft (1.3 m) thick comprising 6 major coal zones has been delineated in a stratigraphic study of the John Henry Member of the Straight Cliffs Formation (Upper Cretaceous). The data base for this study is comprised of over 200 drill holes on one-half mi (0.1 km) centers and numerous complete and partly measured sections within a 70-sq mi (182 sq km) area in the southern Kaiparowits region, south-central Utah.

The depositional setting for the coal-bearing strata is interpreted as the backswamp or landward part of a strand-plain swamp complex. At least 10 different times within the subject area, swamp conditions were interrupted by an influx of clastic river-borne sediments. Once floods terminated peat-forming conditions, short-lived, highly meandering, north and northeast flowing fluvial channel systems were established over the flood sediments. After short periods of time, geologically speaking, the fluvial-channel systems either abandoned their courses to other sites in the backswamp or became inactive. At these times, swamp conditions were reestablished and coal-forming environments again prevailed. The sedimentary succession in the coal-bearing section is essentially a repeating record f the processes.

The isopach maps of the strata between coal seams as compared with paleochannel sandstone trend maps illustrates the significant effect of differential compaction around paleochannel sandstone bodies. Areas marginal to paleochannel sandstone axes compact significantly, whereas the axis areas show minimal compaction. In addition, coal seam isopach maps, in cases, show an influence of paleochannel sandstone position where coals are thinner over minimally compacted sandstone axes as compared to thicker coal in paleomarsh areas. Variations in the thickness of coal seams as well as the strata between coal seams can be understood most easily and predicted with an awareness of depositional processes and trends.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana