Abstract

Deconstructing the complex architecture of ancient alluvial deposits requires an understanding of the processes that affect fluvial systems. Allogenic (extrabasinal) processes (e.g. base-level, tectonics, and climate) are considered a primary control on the stratigraphic distribution of channel bodies in the rock record. However, recent studies have indicated that autogenic (intrabasinal) stratigraphic organization may occur within fluvial systems on basin-filling time scales (10⁵-10⁶ years). Zones rich in channel-belt deposits seen in alluvial basin fills can be generated by several different mechanisms including processes that are initially degradational (e.g. incised-valleys produced by base-level changes) and solely aggradational processes (e.g. tectonic damming, climate change, and long-time scale organization of river avulsion, called avulsion clustering).

The fluvial-dominated Lance Formation (Maastrichtian; Bighorn Basin, WY) is characterized by large, sandstone channel belt deposits separated by intervals of mudstone floodplain deposits. Sand-dominated intervals in the basin have been tentatively interpreted by Webb (2001) as incised-valley fills, where valley formation occurs during relative sea-level lowstands and valley infilling takes place during subsequent relative sea level rise. The purpose of this study is to establish and evaluate criteria to distinguish between four models that could explain high concentrations of channel-belt sand bodies in fluvial successions.

The Lance Formation in the southern Bighorn Basin is evaluated using field data and aerial photographs in an effort to determine whether these sand-dominated intervals are truly incised-valley fills resulting from relative base-level changes, or if they were generated by other processes. Sections of the Lance Formation were measured at five localities on the western and southern margins of the Bighorn Basin. Sand body geometries in both areas show average paleoflow depths on the scale of ∼3 meters. Two types of sand body distributions are seen: multistory sand sheet-like deposits and lenticular isolated sand bodies. Multistory sand bodies consist of 2-4 stories and each story is about one paleoflow depth thick. Aspect ratio of the largest sand bodies are about 50:1.

Results of this study indicate that sand-dominated intervals in the Lance Formation are aggradational in origin, rather than erosional. Additionally, there is no strong stratigraphic evidence for either tectonic damming or climate as controlling distribution of sand bodies. Rather, the alluvial architecture observed in the Lance Formation could have formed in the same way as the Ferris Formation (Maastrichtian/Paleogene, Hanna Basin, WY). Closely-spaced sand bodies in the Ferris Formation are interpreted as channel-belt deposits of solely aggradational origin, and have been compared to autogenic avulsion stratigraphy produced in experimental basins. Heterogeneities in sand body distribution identified in the Lance Formation Bighorn Basin may be comparable to that observed in the Ferris Formation, although at a larger spatial scale. The implications of this study show that groupings of sand bodies in fluvial successions do not necessarily indicate the influence of allogenic processes.