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The third type of lag forms during sea-level fall as the shelf is eroded by fluvial channels forming the incised valleys. During incision, finer-grained shelf sediments are flushed through the valley system. Coarser-grained particles eroded from the shelf strata are concentrated as a basal lag as much as several feet thick on the sequence boundary in the valley. The lag derived from the shelf strata commonly consists of a wide variety of grain types including intertidal and open-marine shells, shark teeth, glauconite, phosphorite pebbles, shale rip-up clasts, and bones.

2 (Figure 29). The systems tracts are identified in the well logs using parasequence-stacking patterns and facies interpretations from outcrops and cores. 2 km) north of the outcrops. The Sego, Buck Tongue, Castlegate, and Desert members were cored continuously in this well. A measured section through the Sego, Buck Tongue, Castlegate, and Desert members at Thompson Canyon is illustrated in Figure 30. The measured section documents the vertical-facies associations and the sequence stratigraphy of these units.

Broad, sheet-like geometries, well-developed truncation, and a basinward shift in facies are associated with these incised valleys (Figure 22). The sandstones within the incised valleys are interpreted as fluvial, possibly braided stream to estuarine in origin. 6- to 3 m-) thick fining-upward units. Based on biostratigraphic data, the marine mudstones below the sequence boundaries were deposited in inner- to middle-neritic water depths. Thin sandstones within or at the top of 32 Sequence boundary characteristics the mudstones but below the sequence boundaries are interpreted to be distal delta front or lower shoreface, based on well-log shape, thickness, areal distribution, and association with open-marine mudstones.