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Petrology of the Late Precambrian Tillite(?) Association in Northern Utah

Dept. of Earth Sciences, Washington University, St. Louis, Missouri

The late Precambrian subgraywackes of northern Utah and adjacent areas are poorly sorted and commonly graded, and contain bent and twisted, reworked subgraywacke fragments. Such features suggest a turbidity current origin. The laminated subgraywackes differ strikingly from Pleistocene varves of the same thickness in that they contain a considerably larger amount of coarse particles and that their carbonaceous matter is not concentrated at the top of each bed. The presence of bent and twisted subgraywacke fragments and contorted beds in the conglomeratic subgraywackes (tillites?) and the common lateral gradation of these rocks into subgraywackes suggests that they are of subaqueous mudflow origin.

Although textural and structural evidence in the subgraywackes and conglomeratic subgraywackes suggests a turbidity current and subaqueous mud-flow origin, the possibility of contemporary glaciation cannot be eliminated inasmuch as glacial sediments may be redeposited by these agents. The possibility of reworking, mixing, and redepositing of sediments derived from contemporary (or extinct) glaciers with sediments of nonglacial origin greatly complicates and temporarily renders insoluble the problem of determining the prevailing climatic conditions at the time these rocks were redeposited.

The subgraywackes and conglomeratic subgraywackes consist chiefly of rounded clastic quartz and minor feldspar grains together with rock fragments in a diagenetic or metamorphic matrix of predominantly mica, chlorite, and quartz. Although the preserved clastic components suggest plutonic- and quartzite-rich source areas, the nature and origin of the original matrix minerals is unknown. It is suggested that they were derived from erosion and redeposition of a weathered mantle on plutonic source areas.

Differences in mineralogy and major and trace element concentrations occur between unmetamorphosed and low-grade metamorphic subgraywacke and conglomeratic subgraywacke sections. Such differences appear to be related to a combination of variability in original sediment compositions and compositional changes accompanying progressive metamorphism. Widespread compositional uniformity of low-grade metamorphic plagioclase, chlorite, and to a lesser degree muscovite and paragonite, suggests that chemical equilibrium was approached in these phases during metamorphism.

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