Vertical block tectonics significantly affected the deposition of the Frontier Formation of Late Cretaceous age. This syntectonic relationship was identified by detailed facies mapping which recognized seven major lithofacies on the basis of lithology, sedimentary structures, sandstone geometries, fossils, and trace fossils. Each lithofacies is a process-controlled genetic unit and can be related to a depositional environment within a wave-dominated deltaic system. The ascending sequence of environments and lithofacies is: (1) prodelta shales and siltstones; (2) hummocky-stratified, distal delta-front sandstones; (3) low-angle trough to tabular-stratified, delta-front and mouth-bar sandstones; (4) lenticular, medium to coarse-grained, distributary-channel sandstones; (5) agoonal and delta-plain carbonaceous shales and coals; (6) medium-grained fluvial sandstones; and (7) bioturbated, offshore and shelf sandstones and shales.

These lithofacies change laterally across two east-west-trending basement faults. Delta-front and distributary-channel deposits thicken between the faults, indicating a topographic low or graben at the time of deposition. Within the graben, sandstone

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Figure

beds have scoured bases and become finer grained upward, suggesting traction-load deposition. Sand-transport directions are normal to the paleoshoreline. Outside the graben, chenier plain and interdistributary sequences were deposited. Likewise, hummocky stratification is more common and sand transport parallels the paleoshoreline, suggesting wave and longshore transport of sand material.

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