--> --> ‘Clay mineral assemblage of a mudstone facies, Upper Jurassic Curtis Formation, east-central Utah’

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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‘Clay mineral assemblage of a mudstone facies, Upper Jurassic Curtis Formation, east-central Utah’

Abstract

The Curtis Formation, a lithostratigraphic unit of the San Rafael Group of east-central Utah, was deposited in a nearshore depositional system along the eastern shelf of a Late (?) Jurassic seaway. This study complements previous work on the Curtis by assessing composition and origin of clay minerals in a slope-forming mudstone facies in the lower 6 to 8 meters of the Curtis interval. Samples were collected at 3 locations along the western San Rafael Swell, a Laramide structure in east-central Utah, and at 1 location on the eastern side of the Swell; at half meter intervals upward from the unconformable contact with the underlying Entrada Sandstone to where the facies grades into a cliff-forming sandstone facies. Samples were disaggregated, carbonate cement was removed, and clay-sized particles were separated from silt-sized particles in a settling tube. From the < 2 µm clay-mineral suspension, a clay fraction was drawn and the clay slurry was smeared on to glass slides and allowed to dry. Clay smears were analyzed by X-ray diffraction in 4 preparations: untreated, glycolated, 400 degrees C heat treated, and 550 degrees C heat treated. Clay minerals identified are: smectite, illite, and chlorite; non-clay minerals identified are quartz and the zeolite phillipsite. Smectite probably formed from chemical weathering of volcanic glass, as suggested indirectly by andesitic rock fragments in local conglomeratic units associated with the mudstone facies. Phillipsite corroborates an origin for smectite by chemical weathering of volcanic material. Illite and chlorite could not have originated at the expense of smectite during burial diagenesis; diffractometer traces show the proportion of illite decreasing relative to smectite. Chlorite abundance remains constant. Chlorite probably originated by chemical weathering of either volcanic or metamorphic minerals. Illite was chemically altered to form smectite as weathering conditions shifted from cooler, drier to warmer, more humid over time. Results from this study suggest that the clay minerals identified are not products of diagenesis, but rather are detrital; having formed from chemical weathering of minerals in exposed source rocks.