Print this pageFluvial Valley Filling during a Transgression through Multiple and Sequential Valley-Scale Cut-and-Fill Events: Can Compound Valley-Fills be Generated Independent of Sea Level Fluctuations?
HOLBROOK, JOHN M.
Compound valley-fills typically result from an episode of regression and incision during an overall transgressive valley filling episode, and are manifest as stacking of multiple unconformity-bound valley-fills within a single incised valley. Valley cut and fill, however, my also result from tectonic or climatic forcing in the absence of sea level control. Such cut-and-fill events may generate lowstand valley-fills which appear as bundles of smaller-scale valley-fill units, that could be confused with sea-level-generated compound valleys. Fluvial architecture of the upper sandstone member of the Dakota Sandstone in southeastern Colorado yields evidence for possible non-sea-level-controlled multistage valley filling.
The upper sandstone member consists entirely of cross-bedded fluvial sandstone, and reflects filling of a dense network of valleys with fluvial strata preceding Early Cretaceous Greenhorn transgression in the southern U.S. Western Interior. Examination of fluvial architecture from these rocks reveals that this units is composed of amalgamated channel-fill elements with width/depth ratios near 14:1, and thicknesses of 7-12 m. These channel fills are bound, however, by larger-scale channel-shaped surfaces that represent the boundaries of minor valleys generated during master valley filling. Individual valley fills thus appear locally as compound. Closer examination, however, reveals that minor valleys within larger valley fills are not regionally significant, and reflect only local cut-and-fill events. They thus do not represent major regional episodes of sea level drop and lowstand incisement.