ABSTRACT

The upper Cretaceous Little Muddy Creek conglomerate in southwestern Wyoming is an often-cited example of a thin-skinned, thrust-derived conglomerate that has been used to date absolute and relative timing of thrust sheet motion in the Idaho-Wyoming-Utah fold and thrust belt. Despite the regional tectonic significance of the Little Muddy Creek conglomerate, no quantitative sedimentological or compositional analyses are available in the literature.

Sedimentological analysis indicates that the Little Muddy Creek conglomerate was produced by forelandward (eastward) progradation of a marine fan-delta that emanated from the thrust ridge above the old Absaroka thrust. Fan-delta deposits are overlain by nearshore, inner-shelf, outer-shelf and slope deposits which record sedimentation as motion on the old Absaroka thrust began to wane. A two-dimensional provenance modeling technique described herein compares hypothetical, synorogenic-sediment compositions with actual compositional data from the Little Muddy Creek conglomerate. The two-dimensional modeling is a refinement of previous, one-dimensional models in that it is based upon the reconstruction of syn-thrusting hanging wall and footwall geometries. The provenance mod ling shows that compositional data from the Little Muddy Creek conglomerate record uplift followed by gradual cessation thereof on the old Absaroka thrust. Sedimentological and compositional interpretations are integrated, allowing for an evaluation of the tectonic controls on deposition of this thin-skinned, thrust-derived conglomerate.