Charles L. Angevine

Two-phase stratigraphic model of foreland-basin sequences

Application of flexural models to nonmarine foreland-basin evolution indicates that two different stratigraphic styles of basin fill may develop over time. Basin subsidence is most rapid during times of thrust-load emplacement; associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine-grained deposits that cover most of the basin. The distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt. Subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin. During this postorogenic phase of adjustment, a regional unconformity develops in the proximal part of the foreland basin. Proximal deposits, along with thrust-derived sediment, are redeposited in the distal foreland basin and beyond. This two-phase model of foreland sedimentation predicts that coarsening-upward sequences in the proximal and distal parts of the basin have reciprocal significance: the proximal sequence represents thrust-belt advance, whereas the distal sequence represents thrust-belt cessation.

Quantitative Sedimentary Basin Modeling

This publication is designed to introduce the concepts and techniques of quantitative modeling of basin subsidence histories. The book also describes some of the methods and results of modeling the development of sedimentary sequences generated by the interaction of subsidence, sediment supply, and sea-level changes. It concentrates on the theory and application of subsidence and stratigraphic modeling by working through specific examples from real or artificial basin sequences

Postdepositional tilt of the Miocene-Pliocene Ogallala Group on the western Great Plains: Evidence of late Cenozoic uplift of the Rocky Mountains

The Rocky Mountains and adjacent western Great Plains share a common history of late Cenozoic stream incision. Both epeirogenic uplift and climate change (with no associated uplift) have been proposed as the cause of thissubcontinental-scale erosional episode. However, the lack of a well-defined Cenozoic paleoelevation history for the region has hampered our ability to distinguish between the two. A tilt analysis of the Cheyenne Tablelands in the western Great Plains of Wyoming and Nebraska provides us with a datum from which postdepositional changes in slope can be determined. Miocene to earliest Pliocene gravels of the Ogallala Group (17.5-5 Ma) cap the tablelands, which currently tilt down to the east at slopes as great as 10 - 2 . However, paleohydraulic analysis of the Ogallala gravels indicates depositional slopes of 10 - 3 to 10 - 4 , implying a postdepositional increase in tilt. If a hinge point at the eastern edge of the Cheyenne Tablelands is assumed, this tilt translates into differential uplift of 680 m (815-410 m) at the western edge of the Great Plains. Flexural isostatic rebound of the tablelands due to known young erosion in surrounding basins only produces a few hundred meters of uplift. Therefore, even if all of the recent erosion in the region can be attributed to climate change, the resulting rebound is insufficient to account for the observed uplift of the tablelands. Thus, the tilting of the Cheyenne Tablelands is most consistent with broad-wavelength tectonic uplift centered under the Rocky Mountains initiated during Ogallala deposition and continuing since deposition ceased.

Sea-level cycles during the growth of Atlantic-type oceans

Abstract First-order changes in global sea level are commonly thought to be the result of short-term fluctuations in oceanic spreading rate. However, eustatic high stands have occurred only twice in Phanerozoic time, and in both cases have followed supercontinent break up by a lag period of 50–100 Ma. Therefore, if pulses in spreading rate were the primary control of eustasy, then, for some reason, rate increases would have had to have occurred with predictable periodicity with respect to major rifting events. Alternatively, we propose that the major control on first-order sea-level cycles is the formation of Atlantic-type oceans with its attendant changes in the area/age distribution of the global ocean floor and the formation of extensional passive margins. Our calculations indicate that both the approximate magnitude and relative timing of sea-level highstands relative to supercontinent break up are predicted by this mechanism. Thus, the importance of short-term spreading rate variations in eustatic cycles becomes secondary.

Is the middle Cretaceous pulse of rapid sea-floor spreading real or necessary?

It is commonly accepted that the major control on long-term eustasy is variation in sea-floor spreading rate. The middle Cretaceous sea-level highstand, in particular, has been correlated with a postulated pulse of rapid spreading in the Pacific basin. This inferred event took place during the Cretaceous normal superchron (CNS) and has also been used as evidence for the existence of superplumes from the deep mantle, giving a causative link between core-mantle interactions and the record of sea-level change. There are reasons, however, to question the foundation upon which this linkage is based. Recent studies of marine geochemistry show no evidence for large hydrothermal fluxes during middle Cretaceous time that require major increases in oceanic spreading rates. Plate reorganizations in the Pacific during the CNS show evidence of ridge jumps, indicating that more of the sea floor of this age may be preserved than has been presumed. Improvements in the Mesozoic time scale indicate that the CNS was longer than previously assumed, reducing the need for rapid spreading rates in the Pacific basin. Middle Cretaceous rapid plate generation rates are not needed to explain the history of sea-level change. Long-term eustasy can easily be accounted for by supercontinent breakup, variations in the distribution of crust consumed at subduction zones, and other effects. Because the inferred pulse of spreading was, in part, used to argue for the existence of a superplume, the basis for this hypothesis is diminished. The phenomena attributed to deep mantle plumes can be accounted for by plate tectonic forces and plate reorganization. Hence, we question the basis and need for the Cretaceous pulse of rapid sea-floor spreading.

Cause of tectonic reactivation and subtle uplifts in the Rocky Mountain region and its effect on the stratigraphic record

Stratigraphic studies show that widespread, albeit subtle, topographic uplifts have occurred throughout the U.S. Rocky Mountain region during middle and late Mesozoic time. Individual paleotopographic highs generally have wavelengths on the order of tens of kilometres and amplitudes on the scale of metres. In places, these features correspond to sites of previous or later deformation and so suggest the possibility of tectonic reactivation along pre-existing zones of structural weakness in the crust. Observations of modern features and modeling studies indicate that uplifts of small magnitude are both a common and expected result of changes in intraplate stress levels. Finite-element modeling suggests that broad-wavelength, low-amplitude topography can develop in a heterogeneous elastic plate under high-stress magnitudes along preexisting faults and under low-stress magnitudes where zones of high and low strength are juxtaposed. Although these features are small, they have sufficient topography to bring about large changes in paleocurrent patterns for low-gradient streams and significantly affect isopach patterns. Although subtle uplifts from this cause are difficult to predict, they are likely a common occurrence and may be the most dramatic features in an area that is otherwise tectonically quiescent. Thus, subtle uplift may be a background noise rather than a harbinger of regional orogenesis.

A TEST OF A QUARTZ ECLOGITE SOURCE FOR PARENTAL ALEUTIAN MAGMAS: A MASS BALANCE APPROACH'

The observation that magmatic conduits evolve with time has provided a new framework within which to interpret Aleutian compositional and isotopic data. Accordingly, a two-stage physical model of Aleutian magma genesis has been developed and used to model quantitatively geochemical and isotopic mass balance relations. Parental magmas, approximated by high-alumina basalts, are presumed to be generated by partial melting of eclogite and subducted sediment. The incorporation of sediment produces elevated parental magma Sr and Pb isotopic ratios but lowered

Comment and Reply on "Thrusting and gravel progradation in foreland basins: A test of post-thrusting gravel dispersal"

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