Shallow-marine and marginal-marine sandstone bodies constitute important reservoirs for oil and gas in the Western Interior. The geometries and physical characteristics of these bodies reflect the hydrographic regimes of their paleoenvironmental settings.

Tidal range is a primary determinant of hydrographic regime and is, therefore, an important factor in paleoenvironmental reconstruction and in delineation of exploration targets. Two opposed models exist for tidal patterns in epeiric seas: (1) tidal ranges were small, with the amplitude of the tidal wave generally decreasing away from apertures with the oceans; and (2) tidal ranges were large, the tidal wave being amplified with increasing distance from the apertures and attaining maximum amplitudes in the interior parts of the seas. A variety of evidence indicates that the second model is more applicable to the interior Cretaceous seaway. In the Western Interior Cretaceous, features that indicate tides include tidal inlets, tidally influenced reaches of river systems, tidal flats, li ear sand bars, and thick foreshore sequences. These paleoenvironments are characterized by suites of physical and biogenic structures. The distribution of these features indicates that tides were present throughout the interior Cretaceous seaway. Microtidal conditions prevailed in the area of its connection with the proto-Caribbean. Tidal range increased northward along the western shoreline, attaining mesotidal amplitudes in Wyoming, in Montana, and probably in southern Alberta. Tidal patterns farther to the north cannot be discerned on the basis of physical evidence. The eastern

End_Page 778------------------------------

shoreline was probably everywhere microtidal as a result of dissipation of tidal energy across the very shallow eastern shelf of the basin.

The Arafura Sea displays a tidal pattern that may be similar to that of many ancient epeiric seas. Though smaller and shallower, it is thought to be a good modern analog for the interior Cretaceous seaway.

End_of_Article - Last_Page 779------------