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Distinguishing Syntectonic Unconformity Types to Enhance Analysis of Growth Strata: An Example from the Cretaceous, Southeastern Nevada, U.S.A.

¹ Department of Geology and Geological Engineering, 1516 Illinois Street, Colorado School of Mines, Golden, Colorado 80401 U.S.A.; jaschoff{at}mines.edu
² Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, U.S.A.

Two syntectonic unconformity types are recognized in nonmarine Upper Cretaceous growth strata adjacent to the Willow Tank thrust in southeastern Nevada, U.S.A. Unconformities with larger angular discordance (6–90°, "traditional-type") developed when uplift outpaced sediment accumulation. More subtle unconformities with less discordance (2–5°, "subtle-type") developed when sediment accumulation nearly kept pace with uplift. Traditional-type unconformities are the focus of most studies of growth strata because the discordances are obvious in the field. By contrast, subtle-type unconformities are more difficult to identify. Subtle-type unconformities are mappable surfaces of erosion or nondeposition located within progressive, flattening-up stratigraphic successions, and are associated with clustering of large-scale (0.5–4 m) soft-sediment deformation features (possible seismites), better-developed or clustered paleosols, greater reworking, and/or grain-size increase. Increasing sediment supply, in the presence of positive net accommodation, allows syntectonic deposits to aggrade above a growing structure despite no change in uplift rate. Aggradation across a structure is punctuated when uplift is slightly higher than sedimentation, producing subtle discordances (subtle-type) with evidence of slope destabilization, seismicity, or longer residence time, instead of the traditional-type angular unconformities. Identification of unconformity types in growth strata can document additional phases of uplift, particularly for intervals where sediments aggraded above an active structure because of higher sediment supply, and regional subsidence or sea-level rise. Although these syntectonic unconformity types are identified in nonmarine, compressional growth strata, they are likely present in many other depositional and structural settings.