Post-lease sale exploraton of the Navarin Basin, Bearing Sea, Alaska
Although the basin had optimistic beginnings, interest in further exploration of the Navarin Basin is waning because of disappointing d r i g results. Eight exploratory wells were drilled in the Navarin Basin in the year following Lease Sale 83 in 1984. All of these wells tested Tertiary fill that drapes or folds over simple, broad basement highs. Significant reservoir rocks were absent in all of these wells. Immature to mature Eocene kerogen with possible hydrocarbon-generating characteristics was found in a stratigraphic test well and two exploratory wells located on the flanks of the Pervenets subbasin. However, no signs of oil or oil migration were evident. A third exploratory well d r i ed on the southeast flank of the Pinnacle Island subbasin encountered thermogenic gas from a thin, probably lenticular Paleocene sandstone. The gas was probably sourced from interbedded, resinous coal. The geologic information collected from seven of the eight exploratory wells has now been released to the public. Of the leases acquired in Sale 83, 74 percent have been relinquished as of January 1988.
Review of Petroleum Geology of Anadyr and Khatyrka Basins, USSR
Two sedimentary basins bordering the Bering Sea in northeastern Siberia, the Anadyr and the Khatyrka, have been the targets of recent Soviet exploration for oil and gas. Drilling in the Anadyr basin has produced noncommercial quantities of gas and condensate from Tertiary volcaniclastic rocks. The Anadyr basin lies due west of Norton basin, a large unexplored sediment-filled Tertiary basin that underlies Norton Sound. Geologically, Anadyr basin lies between a late Mesozoic belt of volcanic and plutonic rocks similar to the Sierra Nevada of California and a complex foldbelt on the south that is structurally and lithologically similar to Franciscan terrane of the California Coast Ranges. Less extensive exploration in the Khatyrka basin has discovered only noncommercial quantities of oil and gas. The Khatyrka basin extends offshore between Capes Rubicon and Navarin near the northwest end of the Navarin basin on the Bering Shelf, and contains Tertiary sediment deposited within a structurally complex Mesozoic foldbelt that parallels the edge of the continental shelf.
Tectonic contact between the Franciscan Mélange and the Great Valley Sequence—Crustal expression of a Late Mesozoic Benioff Zone
Late Mesozoic Franciscan rocks of the California Coast Ranges, chaotically deformed and consisting of graywacke, micrograywacke, shale, chert, and mafic pillow lava, are considered to have been deposited in and adjacent to a northwest-trending oceanic trench. Principally on the east, contemporaneous, well-bedded conglomerate, lithic sandstone, siltstone, and shale of the Great Valley sequence evidently were laid down on a continental shelf, slope, and sea floor environment. The junction between these two sequences, one the ensimatic eugeosynclinal melange of Franciscan rocks, the other the more orderly Great Valley miogeosynclinal strata overlying chiefly sialic-type basement, is marked by the South Fork Mountain-Stoney Creek-Ortigalita (–Sur-Nacimiento) fault system. This structural break is interpreted as the crustal expression of a Late Mesozoic Benioff zone. Episodic or relatively continuous underthrusting of the trench melange beneath this former seismic shear zone is held responsible for the contrasting characteristics of the Franciscan rocks and the simultaneously deposited Great Valley sequence, as well as for their ubiquitous tectonic juxaposition. A speculative four-state tectonic model is proposed that involves: (1) rapid Late Jurassic relative northeastward or eastward spreading of the Pacific Ocean floor, coupled with westward or southwestward encroachment by the continental lithospheric plate; (2) mid-Cretaceous buoyant uplift of portions of the eugeosynclinal prism during a period of less intense spreading; (3) accelerated post-Cretaceous convergence between oceanic and continental lithospheric plates; and (4) Miocene-Pleistocene diapiric uplift of the Franciscan melange related to northwestward seafloor spreading.