Abstract

Paleocene crayfish burrows are present locally in great abundance in the Greater Green River, Hanna, Wind River, and Piceance basins of Wyoming and Colorado. In the Washakie sub-basin of the Greater Green River Basin, burrows are found in (1) lenticular, cross-bedded sandstones of fluvial-channel origin, (2) massive sandstones and mudrocks of floodplain origin that surround and interfinger with the cross-bedded sandstones, and (3) thin, sandy ironstone beds of overbank and crevasse-splay origin. The burrows, which represent crayfish activity in proximal to distal floodplain settings, are assigned to Camborygma symplokonomos, C. eumekenomos, and C. litonomos on the basis of their architectural morphologies. C. araioklados is not present. Crayfish burrows are part of pedogenically modified channel and overbank deposits.

In all the basins studied, burrows are abundant only in non-coal-bearing rocks. The presence of elongate burrows indicates that the landscape was imperfectly drained (on a seasonal basis), and the burrow depths suggest the paleo-water table was 1-4+ m below the paleosurface. Swampy conditions prevailed in areas where carbonaceous shales and coals formed, and thus crayfish did not construct deep burrows. The crayfish burrows and associated lithologies suggest a warm, humid, wet-seasonal climate. In the eastern Washakie Basin, the vertical and lateral distribution of burrows and their abundance reflect changes in the paleohydrologic regime over space and time. Burrow depth and overall abundance decreases towards local basin lowlands, where they are rare. The paleohydrologic regime coupled with a progressively less seasonal climate through time allowed the formation of the Cherokee coal zone in the late Paleocene.

In an otherwise lithologically homogeneous, non-variegated deposits, stratigraphically significant surfaces may be indicated only by the tops of burrowed intervals and by other types of paleosols. The tops of burrowed intervals represent paleosurfaces of varying environmental stability on which there are different frequency and magnitude of sedimentation events. Immature cumulative paleosols characterized by less intensely burrowed beds or bedsets record higher sedimentation rates. More intensely burrowed beds or bedsets reflect the activity of many generations of crayfish; hence, the tops of these layers record longer durations of exposure and pedogenic modification with infrequent or no sedimentation. Paleosols represented by burrowed intervals, in conjunction with sandstone and mudrock stacking patterns, can be used to identify significant stratigraphic surfaces, such as parasequences and sequence boundaries, in fully continental environments.

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