Conodont Thermal Maturation Patterns in Paleozoic and Triassic Rocks, Northern Alaska--Geologic and Exploration Implications: ABSTRACT

Abstract

The Paleozoic through Jurassic stratigraphic sequence in the Brooks Range consists of platform to intraplatform basin deposits 1-5 km thick. This comparatively thin sequence of heterogeneous lithologies was tectonically disrupted and shortened at least 600 km to form a stack of allochthons that were transported relatively northward and emplaced during the earliest Cretaceous. Thermal patterns in Paleozoic and Triassic rocks, based on conodont color alteration indices (CAI) from about 600 localities, show: (1) a gradual increase in thermal level from the northern margin to about 3/4 of the distance southward across the range (from CAI 1 to 5.5 and higher); (2) a belt of mixed high values (CAI 4.5 to 7) along the south border of the range; (3) thermal levels in surface and ubsurface samples in the range related to tectonic burial and not to pre-thrust burial metamorphism; (4) the same CAI values in rocks above and below the Ellesmerian unconformity in the northeast Brooks Range; (5) an association of anomalously high CAI values with mineralized areas and plutonic rocks; (6) a few anomalously high CAI areas of unknown origin that deserve further study; (7) thermal potential of hydrocarbons (CAI = 4.5) only in the westernmost and northern margins of the Brooks Range; and (9) mineralization potential related to anomalously high CAI values in the southern Brooks Range. Triassic through Mississippian rocks in wells on the north flank of the Colville basin show conventional burial metamorphism patterns within each well and from well to well. All rocks indexed have thermal potential for hydrocarbons (CAI = 1-4.5). The geology of the Seaward Peninsula appears to be a southwestern continuation of that in the Brooks Range. Most of the western one-fifth of the Seaward Peninsula contains 4-5 km of unmetamorphosed platformal, dominantly shallow-water carbonate rocks of Early Ordovician through Devonian age having CAI values of 3-4. Eastward, these rocks are thrust onto a blueschist terrane of mafic volcanogenic and clastic deposits and mixed carbonate and clastic rocks of probable Ordovician through Silurian age. Adjacent to, and possibly infolded with, these rocks are metamorphosed shallow-water carbonates of Early Ordovician through Devonian age similar to the rocks of the westernmost Seward Peninsula. Conodonts from the metacarbonate rocks have CAI values of 5.5-7, indicating temperatures of 350-4 0°C that are consistent with blueschist metamorphism. End_of_Article - Last_Page 666------------