Abstract

The Smithwick is a unit approximately 400 ft thick of dark gray claystone that grades upward in flysch-like interbedded sandstone and claystone. The claystone is composed chiefly of illite with minor amounts of quartz and muscovite silt and minute plant fragments. The silt occurs in laminae except where it has been randomly scattered by burrowing animals. Fossils in the claystone other than plants and Lebensspuren are exceedingly scarce sponge spicules and arenaceous Foraminifera. The claystone was deposited below common wave base in a marine basin. The sandstone beds are considered to have been deposited by strong submarine currents, most probably turbidity currents. The chief evidences are: 1) occurrence of numerous sandstone beds intercalated with pelagic claystone, 2) presence of gravel-size material in some sandstone beds that indicates the depositing currents had high competency, and 3) persistent sequence of structures within sandstone beds that indicates the beds were deposited by currents of continually decreasing velocity. This sequence of structures is, from bottom to top: 1) massive bedding, 2) horizontal laminations, and 3) current-ripple cross-laminations. The orientation of directional structures, chiefly groove and flute casts, indicates the currents flowed chiefly down a regional paleoslope toward the S. The mineralogy of the sandstone indicates the source area was composed largely of sedimentary and granitic plutonic rocks with minor amounts of low-grade metamorphic and volcanic rocks. Most detritus was derived from source lands (Llanoria) on the NE. The abundance of slump structures adjacent to the NNE.-trending normal faults that cut the formation suggests the faults were active during deposition of the Smithwick. Slump overfolds indicate sediment slid eastward (at right angles to the major paleoslope), probably down local submarine fault scarps that also served to channel turbidity currents flowing southward.

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