Early Cenozoic Time Research Articles

Mesozoic/cenozoic calcite compensation depth and the global distribution of calcareous sediments

From late Jurassic to early Cenozoic time the calcite compensation depth (CCD) in the world oceans was shallow (above 4000 m). About 40 m.y. ago it dropped to about 4500 m, sharply in the Indian and Pacific oceans and gradually in the Atlantic. In the early Miocene it began to rise again, reaching a very shallow peak 10–15 m.y. ago, then descended to its present deep position. In the Pacific, fluctuations during the last 40 m.y. appear to result mainly from changes in bottom-water structure related to the progressive glaciation of the Antarctic and Arctic regions. Analogous explanations hold for CCD fluctuations during this period in the Indian and Atlantic oceans. The very shallow CCD prior to about 50 m.y., on the other hand, cannot be explained in this manner but must be attributed either to a low input of calcium in the oceans from deeply weathered continents or, more probably, to large changes in the distribution of carbonate deposition between shallow and deep seas.

Tectonic Framework of Petroliferous Rocks in Alaska: ABSTRACT

Alaska, comprising 3.6 × 106 sq km (about 28%) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey to contain about 20% of total petroleum liquids and natural gas resources of the nation. Some 15 billion bbl of petroleum liquids and about 31 trillion cu ft of natural gas have been discovered. In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits and some ophiolitic rocks of the Brooks Range orogen were thrust northward over the depressed south margin of the Paleozoic and Mesozoic Barrow platform, on which a foredeep (the Colville geosyncline) developed in Early Cretaceous time. Cretaceous and Tertiary sediments from the Brooks Range filled this foredeep and prograded northwest and northeast to form the Chukchi and Colville delta systems and to fill the Camden coastal basin. A series of arc-trench systems developed on oceanic rocks in southern Alaska during the Jurassic and Cretaceous. These arcs were subparallel with the Mesozoic continental margin of southern Alaska. Between the arcs and the metamorphic (continental) terranes of east-central Alaska and the southern Brooks Range, a large marginal ocean basin received thick Jurassic and Cretaceous volcanic and detrital deposits. These deposits were extensively deformed and disrupted by widespread mid-Jurassic to Tertiary plutonism, Late Cretaceous and early Tertiary (Laramide) oroclinal bending, wrench faulting, and arc-related compression. The Laramide events continentalized the late Mesozoic marginal basin deposits and welded them to the older continental terranes. Subsequent sedimentation was more localized and nonmarine, except in basins along the Pacific, Arctic, and Bering coasts where thick mixed marine and nonmarine sections are present. The active Aleutian arc and associated Queen Charlotte transform-fault system were superimposed obliquely across the southern continental margin of Alaska in early Cenozoic time and have since dominated structural and depositional patterns in southern Alaska. The largest petroleum reserves in Alaska and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by upper Paleozoic to Tertiary shelf and slope clastic and carbonate deposits. The pre-Tertiary arc and marginal-sea deposits in southern and interior Alaska are either too intensely deformed or too low in porosity to offer more than modest local prospects. The Tertiary coastal onshore and offshore basins with End_Page 1446------------------------------ thick marine and nonmarine clastic rocks, and locally many large folds, are attractive for exploration. These little explored basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet. End_of_Article - Last_Page 1447------------

The Uralides and the Motion of the Russian and Siberian Platforms

The Uralides—the late Precambrian and Paleozoic orogenic terrane between the Russian and Siberian Platforms—in part are exposed in the Ural Mountains, in the central Soviet Arctic, along the west edge of the Siberian Platform, and in southern Siberia and Kazakhstan, and in part are buried beneath the fill of the West Siberian Lowlands and other basins. Paleomagnetic orientations suggest that the Russian and Siberian Platforms were far apart during the early Paleozoic, converged during the middle Paleozoic, and collided in the Permian or Triassic. The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones. The medial eugeosyncline of the Uralides consists largely of what may be oceanic material scraped off against the edges of the opposed subcontinents. Basalt-and-spilite belts may represent ocean-floor abyssal tholeiite, and the manganiferous cherts and other sediments upon them may be pelagic oozes. Andesite belts may have formed as island arcs within the ocean, swept subsequently against the continents. Fossil subduction zones are recorded by great faults soled by, or containing tectonic injections of, mafic and ultramafic rocks from the lower oceanic crust and upper mantle, and containing high-pressure metamorphic rocks. Granitic and silicic-volcanic rocks may have formed above the subduction zones in the accreted parts of the continental plates. Both these continental-margin magmatic rocks and the island-arc complexes display ratios of potassium to silicon that vary across strike and so indicate the directions of dip of the subduction zones. From the distribution of such indicators of various ages, a history of the continental margins can be deduced. An active subduction zone dipped beneath the Siberian Platform during at least parts of late Precambrian and early, middle, and late Paleozoic time. The late Precambrian and Cambrian history of the Russian side is unclear, but in the Ordovician and Silurian the Russian continental margin was stable, while somewhere offshore an island arc was present whose trench was on the Russian side; the last of the intervening oceanic plate vanished down the subduction zone in about the Early Devonian, and the island arc became part of the continental margin. During the remainder of the Devonian and during the Carboniferous and Early Permian, a subduction zone was present along the margin of the enlarged Russian continent and dipped beneath it. Each subcontinent grew oceanward as oceanic material was accreted against it, and the subduction zones stepped oceanward correspondingly. The continental magmatic zones migrated oceanward behind the accreting edges of the continental plates, so the tectonic and magmatic progression with time at any one place is analogous to the variations present across the entire orogenic belt at any one time. Severe right-lateral deformation of the Uralides, the Russian side having moved northward relative to the Siberian side during Mesozoic and early Cenozoic time, is inferred from structural and magnetic-anomaly patterns. The deformation was accomplished by oroclinal folding, strike-slip faulting, and tensional thinning of the crust. The Uralides may have been continuous in early Mesozoic time with the Ellesmerides of North Greenland and the Canadian Arctic islands. The Cenozoic (and late Mesozoic?) opening of the Arctic Ocean was accomplished by spreading of the Eurasia Basin, and by opening of the Canada Basin behind a counterclockwise-rotating Alaska.

Upper Cretaceous and Tertiary Stratigraphy of Guatemala

Upper Cretaceous and Tertiary sediments cover more than half of Guatemala and were deposited in the Chapayal basin, an extension of the Gulf Coast embayment. The basin's foredeep lies between a southern borderland and the Yucatan foreland. Deposition in the Caribbean was connected during late Mesozoic and early Cenozoic time with the deposition in the Chapayal basin by the Sarstun portal. The first age-identifiable sediments are the Carboniferous and Permian. The earliest Gulf Coast embayment deposition in the Chapayal basin was probably that of the late Jurassic Todos Santos Formation, followed by the thick Lower Cretaceous carbonates and evaporites of the Coban Formation. Overlying the Coban, the Upper Cretaceous units--which have an aggregate thickness of approximately 1,750 meters--include Senonian limestones of the Campur Formation and younger clastics of the Verapaz group. The latter consists of the Chemal, Sepur, and Lacandon Formations deposited in early episodic Laramide stages, with the Chemal and Sepur in the foredeep region and the Lacandon on the foreland. The Chemal and Sepur are an argillaceous facies, but the Chemal was deposited only in the region of the present-day Cuchumatanes Mountains and is preserved under distinct environmental conditions in synclinal and downfaulted valleys at elevations exceeding 10,000 feet. The shelfal Lacandon Formation is detrital limestone. Five early Tertiary and six late Tertiary formations occur in the Chapayal basin and Amatique embayment, and cover more than one-third of Guatemala. Previously, only the Rio Dulce and Toledo have been mentioned formally in the literature. The Sepur, thought by Sapper (1937) to be Eocene, is now known to be Cretaceous at the type locality and in other areas. The early Tertiary formations are assigned to the lower Eocene Peten Group. They were deposited contemporaneously and are differentiated on lithology and basin position. The Reforma, Cambio, and Toledo are the shaly formations in the foredeep region. The Toledo occurs only in the Amatique embayment. Shales with conglomeratic lenses characterize the Cambio, and softer clay shales identify the Reforma. The shelfal formations are the anta Amelia and Buena Vista, defined separately by an extensive basal gypsum bed in the overlying Buena Vista. Carbonates and evaporites occur in both formations, but evaporites predominate in the Buena End_Page 425------------------------------ Vista. The thickness of the Peten Group is 1,500 meters. Mid-Tertiary non-deposition divided Tertiary sedimentation into early and late periods. Late Tertiary sedimentation is represented by Oligocene to Pliocene deposits in the Chapayal basin and by Miocene to Pliocene(?) deposits in the Amatique embayment. The Chapayal basin formations are the Desempeno conglomerate, the Lacantun, and the Caribe which have an aggregate thickness of 1,500 meters of terrestrial and deltaic claystones, sandstones, and conglomerates. The Amatique embayment is a graben continuation of the Bartlett trough. The Miocene Rio Dulce Limestone is its oldest late Tertiary formation. Following late Miocene uplift, clastics of the Pliocene(?) Herreria and Armas Formations were laid down. The Rio Dulce has a thickness in excess of 1,000 meters, the Herreria 240 meters, and the Armas 2,500 meters. The seas withdrew at the end of the Pliocene. Quaternary depo its mask older rocks in some areas.

Maps of Cenozoic depositional provinces, western United States

Major provinces of Cenozoic deposits in western United States were controlled largely by the Cordilleran tectonic framework. As in late Mesozoic time, the principal sites of Cenozoic aggradation were the eastern and western flanks of the Cordillera. In contrast, the axial part received limited deposits throughout much of Cenozoic time, except for accumulation of volcanic debris. In early Cenozoic (Paleocene and Eocene) time, active aggradation prevailed along the western margin of the eugeosynclinal belt (thick marine deposits of the Pacific border) and in the Rocky Mountain miogeosynclinal belt (nonmarine deposits centered in Rocky Mountain basins and spreading across the northern Great Plains). Middle Cenozoic (Oligocene to middle Miocene) time was marked by restricted flooding along the Pacific border and by widespread volcanic-rich nonmarine sedimentation in the central and northern Rocky Mountains and Great Plains. During this interval the axial part of the Cordillera was the site of extensive volcanic deposits - andesitic volcanic debris along the Cascade-Sierra Nevada axis, welded tuff in the Great Basin, and basaltic lava on the Columbia Plateau. Smaller patches of volcanic material accumulated in the Rocky Mountain miogeosynclinal belt. During late Cenozoic (late Miocene and Pliocene) time submergence of the Pacific border was accompanied by deposition of thick marine sequences in actively sinking basins. An extensive volcanic-rich nonmarine mantle continued to accumulate across the eastern Cordillera. The axial part was also the site of widespread volcanic-rich nonmarine sedimentation in many newly formed basins.