ABSTRACT

The Castlegate Sandstone is one of several regressive sandstone tongues of the Late Cretaceous Mesaverde Group of east-central Utah. The Castlegate is the rock record of a fluvial-delta complex. The sedimentary facies change seaward (east) from piedmont red-bed conglomerates in central Utah to fluvial sandstones, then delta-plain sandstones, mudstones, and coals, and farther east to shoreline sandstones and finally delta-front interbedded sandstones and mudstones. The Castlegate wedges out as a rock unit into mudstones of the Mancos Shale near the Utah-Colorado state line.

The depositional facies differ significantly in paleocurrent directions, measured on imbricated pebbles in conglomerates and trough cross-beds in sandstones. Paleocurrent directions in piedmont conglomerates are polymodal but with a dominant eastward directions. The average paleocurrent direction for fluvial sandstones is eastward but there is considerable variation among outcrops. Distributary channel sandstone of the delta-plain facies have polymodal paleocurrent directions due to tidal fluctuations. The shoreline sandstones have bimodal cross-bed directions both landward (west) and seaward. Delta-front bottom currents flowed landward.

Grain-size decreases eastward from the piedmont through delta-front facies. Most Castlegate sandstones and siltstones from all facies are texturally similar to Holocene fluvial sands.

Sedimentary rocks uplifted in western Utah and eastern Nevada during the Laramide orogeny were the source for Castlegate detritus. The grains consist largely of quartz plus fragments of chert, orthoquartzite sandstone, carbonate rock, and mudstone. Clay minerals are kaolinite, illite, and montmorillonite. The cements are silica, carbonates, and iron-oxides. Feldspars, micas, and metamorphic fragments constitute only a few percent. Non-opaque heavy minerals are 99% zircon, tourmaline, and rutile, largely reworked from older sandstones.