IDENTIFICATION OF SUBTLE TECTONIC SIGNATURES IN NON-MARINE STRATA UTILIZING REGIONAL CROSS-SECTION CONSTRUCTION

In the Upper Cretaceous and Lower Tertiary strata of the Hanna Basin area, active tectonic uplift of several surrounding Laramide structures during simultaneous deposition resulted in progressive down-cutting of older strata during sediment accumulation. The use of mammalian fossils and palynostratigraphy to correlate across the basin allows key stratigraphic intervals to be followed from the basin center onto the flanks of the uplifts. Resulting correlation panels reveal the nearly sheet-like surface of some unconformities, to the exclusion of alternate reconstructions. In the absence of biostratigraphic data and obvious relation to active tectonic features, many of these unconformities could potentially be reconstructed as having 100s or even 1000s of meters of incision. In stratigraphic successions for which adequate time control on potentially relevant tectonic features is not available, differentiating the original surface morphology of unconformable surfaces is problematic. In such cases, construction and manipulation of stratigraphic correlation panels that span wide geographic areas to render multiple possible correlations may reveal patterns of stratigraphic stacking or potential onlap/offlap surfaces that are not readily observable at smaller, outcrop scale or from measured stratigraphic sections at one or two localities.

An example of such a succession is within the Upper Cretaceous Castlegate Formation in the Book Cliffs of Utah. There, a succession of fluvial/estuarine strata can be traced basinward into time-equivalent open marine strata. Regional cross-sections utilizing a flooding surface datum would indicate significant relief on the unconformity (~100 m) at the top of the succession, just below the Bluecastle Tongue. Accompanying this relief is variable thickness in the middle Castlegate interval from 50 m up to approximately 160 m. An alternative reconstruction using the unconformity as a datum suggests that the thickness changes are related to folding associated with uplift on the San Rafael Swell. This type of reconstruction would indicate beveling of the fold crest with subsequent deposition of the Bluecastle Tongue.