Sequence-Stratigraphic Distribution of Coals in the Rock Springs Formation, Joining and its Influence on Seismic and Geochemical Character
BOHACS, KEVIN M., RICK T. BEAUBOUEF, and PETER P. McLAUGHLIN
Integration of outcrop, well-log, seismic, and geochemical observations
outlines the sequence stratigraphy of the Rock Springs Formation of the Washakie
Basin and illustrates the major controls on coal distribution, preservation, and
seismic character. Coaly rocks accumulated during the Campanian under a warm,
temperate climate in delta- and strand-plain mires. Coals are moderately bright
to moderately dull, high-volatile bituminous C, ranging from 1 to 4 meters (3 to
1 14 feet) thick in a zone 5 to 27 kilometers (3 to 16 miles) wide behind their
contemporary shorelines.
The occurrence and distribution of coals are strongly tied to the shoreline-stacking patterns and vary systematically through the depositional sequences. The most widespread, moderately thick and well-preserved coal zones occur just above deltaic sandstones that extend relatively far out into the basin, at the change in shoreline-parasequence stacking from aggradational to retrogradational, just beneath the major flooding surface interpreted as the top of the lowstand systems tract. Locally thick, but isolated, moderately well-preserved coals are found behind the shoreline of slightly retrogradational parasequences (basal transgressive systems tract). Strongly retrogradational shorelines are backed by thin, discontinuous coals (mid transgressive systems tract). Above them, relatively thick, but isolated coals occur behind aggradationally stacked shoreline parasequences (basal highstand systems tract). Thin, very isolated, generally dull and poorly preserved coals occur behind the strongly progradational parasequence at the top of the depositional sequences (upper highstand system tract).
Seismic velocity also varies systematically in relation to coal preservation, distribution, and setting. Increased seismic velocity in these rocks correlates with decreased coal preservation, thickness, and extent.