CHEMICAL TRENDS BETWEEN CUTLER SANDSTONE, PRECURSOR SAND, AND CRYSTALLINE SOURCE ROCK

Sandstone facies of Permian Cutler Formation, derived from the Uncompahgre uplift in Gateway Colorado, were deposited in an alluvial fan setting in arid climate under periods of tectonic quiescence. Similar geological conditions exist in the area as first-cycle Holocene sands, derived from the same crystalline source as the Cutler sandstones, are being deposited in an alluvial-fan setting. Therefore, the Holocene sand can be considered as an analog for the precursor sand of Cutler Formation.

Fifty-five samples from crystalline source-rock, first-cycle sand, and Cutler sandstones, were collected for chemical analysis in order to observe if there is a chemical trend between these three suites of samples. Total gain and loss was calculated for these three suites, which shows a slight loss of silica content in Holocene sand ~5.5% and less than 1.3% for the rest of the major oxides compared to the crystalline source. On the other hand, Cutler sandstones show gain in most of the major oxides and loss only in SiO₂ and Na₂O, while LOI shows gain in both Holocene sand and Cutler sandstones. In a triangular Si₂O:K₂O+Na₂O:CaO+MgO+FeO plot, the average of crystalline and Holocene sand show nearly identical composition, while significant gain in CaO+MgO+FeO is observed in Cutler sandstones.

Minor changes in bulk chemistry between the crystalline source rocks and Holocene sand imply chemical weathering at a much reduced intensity as expected in arid climates, while the gain of CaO, MgO, and FeO in Cutler sandstones suggests the incremental input of these elements through groundwater during diagenesis over a long period of time.