Differences in framework composition of first-cycle sandstones within coarsegrained delta systems of the Fountain and Minturn Formations (Pennsylvanian) nearColorado Springs and McCoy, Colorado, largely are a function of variable mechanical disaggregation and hydrodynamic sorting characteristic of different depositional environments within the deltas. Modification of composition occurred in spite of deposition in a tectonically active setting where rates of sediment supply and burial were relatively high.Within a wave-dominated delta in the Fountain Formation near Colorado Springs foreshore sandstones are most mature (Q69F28R3) and offshore/transition sandstones are most immature (Qso^s^z)-Differences in maturity reflect shoreline reworking processes. Feldspar is mechanically broken and abraded in the foreshore due to swash/ backwash processes. Smaller grains of feldspar are winnowed from the foreshore and transported in suspension to the offshore during storms. The average composition of shoreface sandstone (Q62F34R4) closely resembles the composition of its precursor alluvial sandstone (Q61F35R4), suggesting most shoreface sand was derived directly from the alluvial channels and underwent little or no compositional change. However, the overall variability in the composition of shoreface sandstones is greater than that of alluvial sandstones, suggesting a more complex derivational history. Some samples of shoreface sandstone are enriched in feldspar, presumably derived directly by winnowing from the foreshore; other samples are enriched in quartz, indicating total reworking of coarser mature sand from the foreshore.The Minturn Formation near McCoy contains facies of river-dominated deltas, fed by both meandering-and braided-river systems. Differences in depositional processes between the two systems caused distinctly different modification of the framework composition of sandstones deposited in the two systems. Sand delivered by meanderingfluvial systems presumably formed under more intense weathering conditions and contains up to 8% fewer rock fragments and as much as 12% more feldspar than braided-fluvial sandstones. Compositions of fluvial sandstones were subsequently modified by marine processes and the compositions of the sandstones in the braided and meandering fluviodeltaic systems diverged even further. The primary variation in composition is reflected by a decrease in abundance of rock fragments, with resulting enrichment of quartz and feldspar. In both systems, beach facies are very similar in •Present address: Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252. Kairo, S., Suttner, L. J., and Dutta, P. K., 1993, Variability in sandstone composition as a function of depositional environment in coarse-grained delta systems, in Johnsson, M. J., and Basu, A., eds., Processes Controlling the Composition of Clastic Sediments: Boulder, Colorado, Geological Society of America Special Paper 284. 263 on June 21, 2015 specialpapers.gsapubs.org Downloaded from 264
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