Differences in framework composition of first-cycle sandstones within coarse-grained delta systems of the Fountain and Minturn Formations (Pennsylvanian) near Colorado 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 (Q50F48R2). 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 meandering-fluvial 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 composition to their parent alluvial sand, suggesting that sand in the Minturn deltas had low residence time in the beach and that the beaches experienced low wave activity. Overall, the marine-influenced fades of meandering-fluviodeltaic origin are more distinct in composition than those of braided-fluviodeltaic origin. Although variation in grain size between fades accounts for most of the observed difference in composition of sandstones of the meandering-fluviodeltaic system, weak correlation between grain size and QFR composition indicates that grain size by itself cannot explain all of the compositional variation in sandstones of the braided-fluviodeltaic system.
Differences in compositional modification between the wave-dominated (Fountain) and river-dominated (Minturn) coarse-grained deltas were due largely to subtle differences in the composition and grain size of alluvial sand brought to the shoreline and in both the rigor and duration of reworking in the marine environments of the two types of deltas. Largely as a consequence of the latter, the composition of beach sandstones of the Fountain was more distinctly modified relative to other facies in the Fountain. Also, more modification occurred in Fountain beach sandstones than in shoreline sandstones of the Minturn Formation where full-scale development of beaches and their long-term existence were prevented because of the dominance of fluvial processes on the delta plains and delta fronts.
