NEW DIGITAL CONODONT COLOR ALTERATION INDEX (CAI) MAPS OF NEVADA

Conodont color alteration indices (CAI) from 2,617 conodont collections from Nevada were used to provide constraints on the geologic history of Paleozoic and Triassic rocks throughout the state and to help identify post-Triassic heat-flow patterns that affected these rocks. While Tertiary volcanism and Mesozoic plutonism have had important local affects on CAI values of conodonts from Upper Cambrian through Triassic rocks, many of the values also record older events relating to the stratigraphic and tectonic history of the pre-Jurassic rocks. The CAI values help to define tectonic domains, regions of accreted terranes, areas of hydrothermal alteration or mineralization, and other structural and stratigraphic complexities throughout the state.

CAI values in Mississippian through Triassic rocks in southern and eastern Nevada are uniformly low with a few notable exceptions likely due to high heat flow from nearby igneous or hydrothermal sources. In western, northern, and far northeastern Nevada, Mississippian through Triassic rocks are within highly tectonized accreted terranes and generally have high or very high CAI values (4.0-8.0).

Deeper stratigraphic burial of Devonian through Ordovician rocks in southern Nevada produced moderate CAI values (3.0-4.0) that agree with field observations of stratigraphic thickness and apparently average geothermal gradient. In the east-central part of Nevada the lower Paleozoic stratigraphic section was not thick enough to alter the conodont color. Very high CAI values (5.0-8.0) occur in Cambrian and Ordovician rocks in a north-south belt in the center of the state. These rocks bear evidence of a tectonic history of burial, deformation and movement that is not reflected in coeval rocks to the east. This additional tectonism is interpreted as the source of the unusually high CAI values in these rocks.

Notable places where mineralizing systems have likely affected the CAI value include the Independence Mountains, the Tuscarora Mountains, the Fish Creek Range, and the Bullfrog Hills. A number of examples of anomalously high CAI values from tectonic loading and other high heat flow sources can also be identified.

Correlations of conodont CAI values with geologic and geophysical data are another valuable tool for interpreting the complex geology and earth resources of Nevada.