Abstract

The Sphinx Conglomerate is an uppermost Cretaceous and Paleocene(?) synorogenic clastic wedge derived from a Laramide uplift in southwestern Montana. Conglomerate petrology documents unroofing of the thick Phanerozoic section that covered the uplift. The active tectonic setting resulted in close correspondence between source lithologies and derived conglomerates and sandstones. Sandstone detrital modes determined using the Gazzi-Dickinson point-counting method further document the unroofing sequence. All Sphinx sandstones consist of mixtures of siliciclastic and carbonatoclastic detritus. Detrital modes correspond to the recycled orogenic provenance of other workers. In addition to these general modal compositions, vertical trends in QFL%Q and QFL%L, and in proportions of types of sedimentary lithic fragments mimic trends in the inverted Phanerozoic stratigraphic section. Total quartz (Q) decreases upsection in the Sphinx, corresponding to decreases of both sandstone (the only source for Qm) and chert (source for Qp) in the inverted Phanerozoic section. Upward increases occur in unstable lithic fragments in the Sphinx, and in carbonate units in the inverted Phanerozoic section. Carbonate sedimentary lithic fragments (Lsc) increase vertically in proportion to silicic sedimentary lithic fragments (Lss) and chert (Qp) fragments, again mimicking the inverted Phanerozoic section. Detailed modal analysis of recycled orogenic sandstones should include study of subcategories of sedimentary lithic fragments, for example, QpLssLsc and QLssLsc plots. Standard QFL and QpLvmLsm plots are useful in assessing the overall tectonic setting of foreland sandstones but are insensitive to temporal changes in source areas. Supracrustal sources contribute most or all of the detritus to foreland sandstones. Therefore, proportions of sedimentary lithic fragments record tectonic and erosional processes in source areas through time.

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