Abstract

The composition of syntectonic conglomerates in foreland-basin sequences is strongly controlled by the lithology of strata exposed to erosion in uplifted thrust plates. Consideration of the styles and mechanisms of uplift in thrust belts, combined with knowledge of sediment provenance and dispersal in eroding thrusted terranes, leads to the concept of interior ramp-supported uplift as a potential mechanism for generating syntectonic conglomerates in foreland basins. Interior ramp-supported uplift occurs when an older, inactive thrust plate that is part of the upper plate of a younger, active thrust is transported over a ramp in the younger thrust. Uplift and folding of the inactive thrust plate re-establish the older thrusted terrane as a sediment source area.

This mechanism is utilized to account for the deposition of a thick accumulation of quartzite-clast conglomerate (Harebell and Pinyon Formations) in the Sevier foreland basin of northwestern Wyoming. Other hypotheses of uplift-erosion-deposition suggested to explain the origin of these deposits include (1) basement-involved uplift, (2) progressive clast recycling through multiple episodes of fluvial transport during uplift on successive thrust plates, and (3) direct transport from the toe of active thrusts. Although these mechanisms all undoubtedly operate in eroding thrusted terranes, each has limitations to its potential validity as source for quartzite debris in the northwestern Wyoming foreland basin. The concept of interior ramp-supported uplift alleviates many of the inconsistencies of other models and is compatible with observations elsewhere in the Cordilleran thrust belt.

The interior uplift model also implies that uplift of an older, inactive thrusted terrane over ramps of active thrusts may be the primary factor in the development of topographic relief in fold-thrust belts.

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