GLACIO-EUSTATIC VERSUS TECTONIC ORIGIN OF HIGH-FREQUENCY SEQUENCES IN THE MID-LATITUDE CRETACEOUS FORELAND BASINS OF NORTH AMERICA

Strata of the Cretaceous Western Interior Seaway of North America have been central in the development of high-resolution sequence stratigraphy; however, there remains debate as to whether these sequences are primarily tectonic or eustatic in origin. Subtle tectonic unconformities at a 0.4 Ma frequency in the Frontier Formation of the Powder River basin, Wyoming, indicate differential tilting and tens of meters of erosion over distances of tens of kilometers, forming subtle angular discordance. Significantly, erosion occurred in more central parts of the basin, within muddy marine strata and is interpreted to be the result of marine erosion versus erosion by rivers.

In contrast, documentation of shoreline trajectories in the age-equivalent Ferron Sandstone Member of the Uinta Basin in Utah shows higher-frequency Milankovitch-scale (20 - 100 Ka) sequences with sea-level falls on the order of 10-16 meters. These base-level falls are indicated by nested compound-incised valleys, and stepped forced regressive shoreline deposits. Unlike the basin-distal Powder River Basin, erosion in the Ferron is primarily by rivers, and the area of erosion appears to shift as shorelines migrate, which is incompatible with basement control. However, within the Ferron, broad angular discordance is also observed at a couple of levels suggesting that the high-frequency Milankovitch sequences occurred over a basin that experiences localized tectonic uplift.

Many have argued that glacio-eustasy operated during the mid-Cretaceous in which 15-30 m eustatic changes were driven by growth and decay of ephemeral Antarctic ice sheets, which match well with the magnitudes of sea-level falls documented in the Cretaceous examples presented above. We suggest that both Tectono and climato- eustatic sequences can be recognized. The observation of high-frequency climatic cycles ultimately requires comparison with age-equivalent basins worldwide to understand the importance of glacial activity during the Cretaceous Greenhouse.