Controls on the development of clastic wedges and growth strata in foreland basins : examples from Cretaceous Cordilleran foreland basin strata, USA
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Tectonic signatures such as growth strata, clastic progradation, detrital composition, thickness trends, paleoflow shifts, lithofacies distribution, and vertical stratigraphic stacking patterns provide the basis for a range of tectonic/structural interpretations. Complete understanding of the application and limitations of tectonic signatures is important to maintain consistency and reduce uncertainty of interpretations that use them. This study provides insight into the external controls on two frequently used tectonic signatures in foreland basins: (1) growth strata, and (2) clastic wedge progradation. First, two syntectonic unconformity types are recognized in non-marine, Cenomanian growth strata adjacent to the Sevier thrust-belt in southeastern Nevada, USA. Unconformities with larger angular discordance (>10°, “Traditional Type”) developed when uplift outpaced sediment accumulation. More subtle unconformities with less discordance (2-10°, “Subtle Type”) developed when sediment accumulation nearly kept pace with uplift. Increasing sediment supply with positive net accommodation, allows syntectonic deposits to aggrade above a growing structure, with no change in uplift rate. Hence, sediment supply and regional accommodation impart an important control over growth strata geometries that are often interpreted on the basis of tectonics alone. Identification of unconformity types in growth strata can therefore document additional phases of uplift, particularly for intervals where sediments aggraded above an active structure due to higher sediment supply during regional subsidence, or sea level rise. Second, an anomalous, Campanian clastic wedge is identified in Cordilleran Foreland basin fill, Utah and Colorado. The complex internal architecture, tide-dominated facies and characteristic flat-to-falling shoreline stacking patterns of the wedge reflect rapid progradation of wide (60-80 km), embayed, tide-influenced shorelines; these characteristics distinguish the anomalous wedge from the underlying and overlying clastic wedges in the basin. A high-resolution regional correlation and isopach maps for the anomalous wedge provide evidence that extensive clastic progradation was coeval with both Sevier- and Laramide-style deformation. Stratigraphic relations suggest that development of the anomalous character of Wedge B was due to uplift of a Laramide structure within the foredeep, and possibly enhanced by reduced dynamic subsidence.