RETHINKING THE INCISED-VALLEY FILL PARADIGM FOR CAMPANIAN BOOK CLIFFS STRATA, UTAH-COLORADO, USA: EVIDENCE FOR DISCRETE PARASEQUENCE-SCALE, SHOREFACE-INCISED CHANNEL FILLS

摘要

The conventional sequence stratigraphic model of large-scale incised-valley fills floored by third-order sequence boundaries is inappropriate for most of the Campanian Book Cliffs strata, Utah-Colorado, including the Desert Member to Lower Castlegate Sandstone stratigraphic interval. Shoreface-incised, parasequence-scale, deltaic river-dominated distributaries, tidally influenced distributaries, terminal distributaries, and/or tidal-estuarine channels are ubiquitous. These do not coalesce into sequence-scale incised-valley fills. There are no basinward shifts of facies across regionally mappable sequence boundaries, no sediment bypass with concomitant development of detached falling-stage and lowstand shoreface-deltaic sand bodies, and no incised-valley fill deposits in the Desert-Castlegate interval. Shoreface-incised channels occur in the uppermost proximal parts of most Desert-Castlegate parasequences, not just the parasequence-set top, as predicted by conventional models. Shoreface-incised channel fills are temporally and spatially linked to the laterally adjoining nearshore terrestrial and shallow marine facies belts, as demonstrated by correlation of coal beds and flooding surfaces. Parasequence-scale interfingering of nearshore terrestrial and shallow marine facies belts are common, with thin packages of channels, coal-bearing coastal plain with associated white-capped foreshore-shoreface sandstones, and flat-topped, rooted foreshore sandstones along most parasequence tops. Multiple, parasequence-scale, shoreface-incised channel fills are often lumped together and miscorrelated as sequence-scale incised-valley fills. Similar shoreface-incised channels occur at a variety of parasequence levels in all other members of the Blackhawk Formation. An alternative sequence stratigraphic model showing temporally and spatially linked, laterally adjoining, nearshore terrestrial and shallow marine facies belts is the best fit for the Campanian Book Cliffs strata. This mod