AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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PALMETTO FORMATION

Type Section Information

The Palmetto Formation was defined by Turner (1902) for a sequence of cherts and shales exposed in the Palmetto Mountains in Esmarelda County. The name has been applied to a variable lithologic assemblage of autochthonous or para-autochthonous shale, phyllite, quartzite, limestone and chert exposed in the Toiyabe and Toquima Ranges in northern Nye Counties (Ferguson and Cathcart, 1954), originally broken out as the Mayflower Schist, Zanzibar Limestone, and Toquima Formation (Ferguson, 1924).

Geologic Age

The Palmetto Formation is commonly considered Middle to Upper Ordovician (Caradocian to Ashgillian) in age based on stratigraphic sequence and sparse fossil evidence. It is equivalent with the Broad Canyon and Crane Canyon Sequences of Means (1962), and probably partially equivalent with the Vinini Formation. Conodonts of Cambrian age have been found in the formation at the mouth of West Northumberland Canyon in the Toquima Range (Kleinhampl and Ziony, 1985). On the Geologic Map of Northeastern Nevada these rocks are considered part of a Cambro-Ordovician sequence.

General Lithology

In general, the Palmetto Formation is composed of mildly to moderately metamorphosed (as a result of contact effects of nearby plutonic bodies) phyllite, schist, argillite, slate, small amounts of limestone locally metamorphosed to marble, and greenstone. In the Manhattan district of the southern Toquima Range, Ferguson (1924) divided the Palmetto into a lower knotty to fine-grained chloritic schist of hornblende and pyroxene-hornfels facies, which he called the Mayflower Schist. This andalusite-cordierite-quartz-mica schist contains knots of cordierite which increase in number and size near plutons and become more phyllitic outward from the plutons (Ferguson, 1924; Kleinhampl and Ziony, 1985). Intercalated within the schist are thin interbeds of gray to black limestone, phyllite, slate, and rare pebble beds.

These schists grade upwards in the overlying Zanzibar Limestone which is a blue-gray, thin-bedded, platy to flaggy cliff-forming limestone, intercalated with and grading laterally and vertically into, dark gray to brownish-black weathering siliceous, slaty and cherty beds (Ferguson, 1924; Kleinhampl and Ziony, 1985). The overlying lithologies were originally lumped by Ferguson (1924) into the Toquima Formation which is composed primarily of dark slate and chloritic schist. Minor lithologies include white to gray quartzite in beds and lenses up to 50 feet in thickness, graptolitic slates, and massive brown to gray chert-bearing limestones (Kleinhampl and Ziony, 1985). In the Monitor Range, about 1,000 feet of sheared dark-gray dolomite and limestone and shaley limestone interbedded with gray chert, and gray slate and quartzite occupy this interval (Kleinhampl and Ziony, 1985).

Average Thickness

The Palmetto is commonly several thousand feet in thickness, with isoclinally folded and thrusted sections making estimates of depositional thickness speculative. Near Manhattan in the Toquima Range, the Palmetto is about 5,000 feet thick (Ferguson and Cathcart, 1954), and is about 2,300 feet thick a few miles to the north (Kleinhampl and Ziony, 1985).

Areal Distribution

Within the evaluation area, the Palmetto Formation is exposed in the southern Monitor, Toquima and Toiyabe Ranges.

Depositional Setting

Stewart (1980) suggests that the Palmetto Formation in the Toquima and Toiyabe Ranges is autochthonous and or para-autochthonous and not part of the Roberts Mountains allochthon. Ross (1967) felt the Palmetto rocks in these ranges were part of the Roberts Mountains allochthon, as did Kleinhampl and Ziony (1985). McKee (1968c) considered the recumbently folded and imbricately thrusted Palmetto farther to the west in Esmeralda County to be an allochthonous sequence related to the Roberts Mountains allochthon.

Stanley and others (1977) suggested that the limestones, shales and cherts of the Palmetto represent pelagic and hemi-pelagic sediments deposited in a slope setting with cross-bedded siltstones and sandstones representing deposition by clear-water bottom currents.


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Last modified: 09/12/06