Steven Schamel

Fault-related rocks: Suggestions for terminology

Many traditional terms for fault-related rocks have undergone recent dynamic metamorphism under high-pressure discussions by various groups of specialists. A generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic, engineering, and legal purposes. Such a framework is proposed here, focusing on a rate-of-strain versus rate-of-recovery diagram and relating this framework to the products of brittle and ductile deformation along faults.

Surface expression of an accommodation zone within the Gulf of Suez rift, Egypt

The Gebel Gharamul region in the western Gulf of Suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination. The surface expression of the accommodation zone is dominated by a basement promontory, which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half-grabens, both of which face the Gulf of Suez. The southern half-graben represents the breakaway fault of the southwest-dipping tilt-block domain, and the northern half-graben represents the collapse of the upper plate above the northeast-dipping tilt-block domain. Basement and Nubian rocks in the hanging walls above the opposing low-angle detachments on either side of the accommodation zone are broken by gulf-parallel synthetic normal faults and orthogonal transfer faults, which act as block terminations and incrementally accommodate the large-scale rotation and displacement between adjacent blocks. The prerift and synrift stratigraphic succession is draped over this fault template, resulting in a complex and variable distribution of structures, the product of an inhomogeneous stratigraphic succession within an accommodation zone.

Normal-fault boundary of an Appalachian basement massif?: Results of COCORP profiling across the Pine Mountain belt in western Georgia

Recent seismic reflection profiling in western Georgia and adjacent eastern Alabama, conducted by the Consortium for Continental Reflection Profiling (COCORP), shows that reflections commonly associated with the Appalachian detachment do not continue southeastward beneath the Pine Mountain belt. Rather, these reflections terminate abruptly on the north side of the belt, along the downdip projection of the Towaliga fault. This observation is difficult to reconcile with the basement duplex interpretation traditionally applied to the Pine Mountain belt and to all other Appalachian interior basement massifs. An alternative interpretation, consistent with the reflection data and with local surface geologic relations, is that the Towaliga fault is, at least in its later evolution, a large northwest-dipping normal fault that cuts the Piedmont allochthon, Appalachian detachment, and Grenville basement. Where crossed by the COCORP profile, this fault has an inferred average dip of about 54° and offsets Grenville basement about 9 km. This interpretation is consistent with the view that the Pine Mountain belt is a structural window through the Piedmont allochthon. However, it implies that much of the structural relief on the basement exposed in the window is due to late normal faulting rather than to thrust imbrication alone. If correct, this has several important implications for Appalachian geology: (1) It implies that normal faulting of late Paleozoic and/or Mesozoic age has played a much more important role in the development of the exposed southern Appalachians than has generally been considered to date. (2) Grenville basement exposed in the Pine Mountain belt has been attached to North America since Precambrian time; it does not represent a Paleozoic accreted terrane. (3) The Appalachian detachment may be exposed around the periphery of the Pine Mountain belt and hence may be available for direct observation at this locality.

The role of low-angle normal faulting and isostatic response in the evolution of the Suez rift, Egypt

Abstract A new geometric model for the Suez rift suggests that isostatic uplift of an underlying low-angle normal fault system accompanying early block faulting localized subsequent deep-seated extension. This, in turn, created a superimposed late-stage graben system offset to the east from an earlier locus of extension. Initial extension in the Oligocene and earliest Miocene drove minor rotation on wide tilt blocks over an eastward-dipping low-angle normal fault system and formed a series of wide half-grabens. With increasing extension, more closely spaced faults formed smaller rotating tilt blocks, fragmenting the larger, earlier half grabens and creating numerous sub-basins. In response to this brittle thinning of the upper crust, isostatic uplift of the lower crust arched the underlying low-angle fault surface, limiting gravity-driven tilt-block movement in the west by flattening the low-angle fault surface and creating a broad gulf-parallel high in the fault surface and Moho under the eastern portions of the rift. This process diminished faulting in the west and localized continuing deep-seated extension under the crest of the arch, possibly through induced mantle convection. Through-going faults thus initiated cut the old low-angle fault system and bound a system of down-dropped grabens offset to the east of the most extended portions of the older tilt-block array. In the Red Sea, immediately to the south, this process has continued, with the increased separation allowing the formation of mantle-derived oceanic crust. This model suggests that early movement over low-angle detachments and accompanying isostatic response may play an important role in localizing deformation within rifts, and are probably critical elements in the early formation of passive margins.

Sedimentological evidence for early Miocene fault reactivation in the Gulf of Suez

The sedimentary response to early Miocene fault reactivation and marginal uplift in the Gulf of Suez is strikingly evident within the Gemsa half-graben. This mid-Rudeis event occurred in the late Burdigalian (16.5 Ma) approximately 8 m.y. after the onset of rifting. In outcrop, complicated lateral facies transitions reflect the change from a ramped margin to a structurally controlled depositional environment. Angular unconformities and truncation mark this event atop marginal tilt-block crests. Distal downlapping sequences are present in adjacent topographic lows that acted as sand depocenters. Channels draining the rift margins acted as point sources for basinward alluvial-fan development. Sand-ratio isopach maps reveal the presence of four active upper Rudeis Formation fan systems within the Gemsa half-graben.

Stratigraphic Distribution of Oil and Gas Resources of the Timan-Pechora Basin

The bulk of the hydrocarbon resources of the Timan-Pechora basin on the northeastern margin of the Russian platform is hosted in reservoirs and traps in the Middle Devonian-Triassic stratigraphic interval, the second of three Phanerozoic first-order cycles. The stratigraphic positions of source rocks, reservoirs, and seals within this important first-order cycle bear specific and predictable relationships to stages within second-order transgressive-regressive cycles. Anticlinal traps constitute 12-15% of the surface area of the basin and host about 75% of the discovered reserves. In addition, numerous styles of stratigraphic and combination traps are recognized. Understanding of the stratigraphy of the basin points to many additional reservoirs and trap styles that as yet are underexplored. The prospects for significant future discoveries of oil and gas in the Timan-Pechora basin are high.

Carboniferous-Lower Permian Carbonate Reservoirs of the Timan-Pechora Basin

The Carboniferous-Lower Permian carbonate succession of the Timan-Pechora basin is a major hydrocarbonbearing complex, hosting about half of the oil and nearly a third of the gas reserves of the basin. The succession represents the last episode of carbonate deposition on the northeastern margin of the Russian platform before the closure of the Ural seaway in the mid- Permian. The lower part of the succession (upper Visean-Moscovian) contains three major transgressive-regressive sequences. Depositional facies ranged from nearshore carbonate-shale-evaporite through shallow shelf detrital carbonates to outer-shelf carbonate-siliceous shale. The most pronounced regression during this interval occurred during the Serpukhovian, when marine sabkhas covered vast portions of the carbonate platform. Late Carboniferous-Early Permian sedimentation was complicated by the onset of Uralian tectonism. Flysch from the encroaching orogen accumulated initially in the east, advanced westward across the passive margin, and fi...

Characterization of Appalachian faults

This study presents a classification/characterization of Appalachian faults. Characterization factors include timing of movement relative to folding, metamorphism, and plutonism; tectonic position in the orogen; relations to existing anisotropies in the rock masses; involvement of particular rock units and their ages, as well as the standard Andersonian distinctions. Categories include faults with demonstrable Cenozoic activity, wildflysch-associated thrusts, foreland bedding-plane thrusts, premetamorphic to synmetamorphic thrusts in medium- to high-grade terranes, postmetamorphic thrusts in medium- to high- grade terranes, thrusts rooted in Precambrian basement, reverse faults, strike-slip faults, normal (block) faults, compound faults, structural lineaments, faults associated with local centers of disturbance, and geomorphic (nontectonic) faults.

Effects of global eustatic sea level variations and tectonism on stratigraphy of Iraq

The stratigraphy of Iraq is marked by complex vertical and lateral facies sequences controlled predominantly by two factors: (1) eustatic sea level variations, and (2) tectonic movements. Analysis of the sedimentary cycles provides a framework for evaluating the relative economic importance of transgressive versus regressive facies within the Iraq stratigraphic succession. Most reservoir rocks, principally reefal and neritic limestones and to a lesser extent deltaic facies, were deposited during relatively high sea level stands. Source rock depositional environments in Iraq were typically either deep subsiding or shallow restricted intrashelf basins. These environments were not controlled by sea level, but primarily by local tectonics. Applying modern theories of plate tectonics and sea level control of facies to this well-studied petroleum province allows new interpretations of the regions geologic evolution.

Mesozoic Evolution of the Northeast African Shelf Margin, Libya and Egypt: ABSTRACT

The present tectonic features of the northeast African shelf margin between the Nile delta and the Gulf of Sirte are products of (1) precursory late Paleozoic basement arches, (2) early Mesozoic rifting and plate separation, and (3) Late Cretaceous structural inversion. The 250 km-wide and highly differentiated Mesozoic passive margin in the Western Desert region of Egypt is developed above a broad northwest-trending Late Carboniferous basement arch. In northeastern Libya, in contrast, the passive margin is restricted to just the northernmost Cyrenaica platform, where subsidence was extremely rapid in the Jurassic and Early Cretaceous. The boundary between the Western Desert basin and the Cyrenaica platform is controlled by the western flank of the basement arch. In the middle Cretaceous (100-90 Ma), subsidence accelerated over large areas of the Western desert, further enhancing a pattern of east-west-trending subbasins. This phase of rapid subsidence was abruptly ended about 80 Ma by the onset of structural inversion that uplifted the northern Cyrenaica shelf margin and further differentiated the Western Desert subbasin along a northeasterly trend.