Abstract

This paper examines the petrographic and chemical compositions of sandstones and the lithologies of conglomerate clast populations from sedimentary sequences representing a 1.5 Ga period in the history of the Colorado Province cratonic block. On average, compositions of Colorado Province sandstones become more mature in younger sequences. The proportion of framework quartz increases through time at the expense of less robust constituents. The chemical data show that SiO 2 increases through time and that all other major oxides decrease progressively. Normative sandstone compositions, calculated by combining petrographic and chemical data, indicate progressive decrease in the proportion of lithic fragments and feldspar. Conglomerate clast populations in younger sequences tend to become richer in sedimentary clasts. These data are interpreted to reflect an increase through time in the input of sedimentary-rock-derived detritus relative to that derived from crystalline basement rocks. As the craton matures, and increasing volumes of sediment collect on its surface, crystalline rocks are buried more deeply. For a given amount of uplift, the ratio of exposed basement to sedimentary cover will be higher, on average, for a newly formed piece of continental crust than for a stable cratonic block (with the exception of shield areas). Therefore, older sandstones will contain a larger proportion of first-cycle debris relative to recycled material. In addition, as the basement contribution to the sedimentary system decreases, sedimentation on a mature continental block becomes increasingly dominated by recycling, and the proportion of multiply cycled, and hence highly mature, sedimentary material in the source area will also increase. The net result is progressive growth of a more mature clastic sediment mass through time.

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