Deep-sea origin of Ordovician pillow basalt and associated sedimentary rocks, northern Nevada
Greenstone pillow lavas interlayered with pelagic sedimentary rocks of Ordovician age in the Independence Mountains, in the northern Shoshone Range, and at Battle Mountain, Nevada, accumulated at depth in a relatively quiet marine environment. The pillows, commonly 20 to 90 cm across, form layered deposits a few metres to 100 m thick within lower or middle parts of Ordovician formations. Outer parts of some pillows have vesicles averaging only 0.06 to 0.08 mm in diameter, forming 0.05% or less (by volume) of the rock; at many localities, no vesicles were found. Pillows at one locality are conspicuously vesicular from core to rim and have vesicles averaging 0.7 mm in diameter that make up 13% to 15% by volume of the rock. These data, when compared with data from modern lavas, indicate that the nonvesicular and slightly vesicular pillows were emplaced at depths of 4 km or more and that highly vesicular pillows accumulated at depths of about 600 m. Well-developed varioles occur only at the outer edge of the weakly vesicular and nonvesicular pillows; well-formed varioles and abundant vesicles are apparently not compatible in Ordovician pillows of northern Nevada. Corroborative evidence of the deep-water origin of most of the greenstone is found in the associated sedimentary rocks, which belong to a belt of graptolitic shale and chert extending from Nevada to Alaska. Formations along the western part of this belt contain more greenstone and less shale than the rocks farther east, which grade eastward into limestone and shale that is transitional into carbonate rock and quartzite of the miogeosyncline. This transition indicates deposition in progressively shallower water from west to east, resembling facies changes across modern continental margins. Rocks along the eastern side of the graptolitic shale and chert belt have abundant current- and gravity-produced structures that suggest a turbidite origin on the continental slope. The scarcity of these structures in the sedimentary sections along the western part of the belt, the abundance of thin beds, and the depth of water indicated by the pillow lavas suggest that these rocks are quiet-water deposits laid down across the lower part of the continental rise and onto the abyssal ocean floor.