Full article available in PDF version.

Abstract Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.

Abstract Five wells were examined for evidence of cycles in the Middle Carnian Cumnock Formation. Strata in three wells, located in the lake depocenter, show strong periodicities which may be related to astronomical climate forcing as indicated by time series analyses using a modified Cooley-Tukey Fast Fourier Transform of gamma-ray logs from the three wells. The Butler well has strong signals at thicknesses of 4.2, 19.4, and 62 m. The Groce well has strong signals at 6.0, 25.6, and 61.5 m. The Hall well shows strong signals at 6.3 and 51 m. The thicknesses and ratios between them correspond to Van Houten cycles in outcrops of other Newark Supergroup strata. Periodicities may represent present-day 21,700-, 109,000-, and 412,000-year astronomical cycles. Lithofacies sequences and petrology suggest expansion and contraction of the lake, and possibly correlate with the present-day 21,700- and 109,000-year cycles shown in the power spectra. If the 4.2-, 6.0-, and 6.3-m thicknesses represent the present-day 21, 700-year precession cycle, the sedimentation rates of strata in the three wells range from 0.19 to 0.29 mm of rock/yr, west to east. The life span of the lake was at least 1.2 million years. Renewed tectonic activity along the Jonesboro fault system to the east caused an increase in sedimentation rates in that part of the basin, masked lake cycles, and eventually eliminated the Cumnock lake. Two other wells, located in the basin perimeter, do not display obvious cycles. The Dummit Palmer well, to the northwest, did not penetrate the entire Cumnock Formation and is affected by a diabase intrusive, faults, several coal beds and related basin-edge complications. The Gregson well, to the southeast, contains only minor lake-margin strata, and they were not amenable to any of our cycle analyses.

Abstract Shallow-marine and marginal-marine depositional systems of foreland regions between cratons and orogens are sensitive to all the potential influences thought to control stratigraphic cyclicity: autogenic, climatic, tectonic, tectono-eustatic, and glacio-eustatic. In foreland settings, both clastic and carbonate depositional systems of shelf or epeiric seas and adjacent coastal plains are responsive to (a) shifts in shoreline position caused by local or regional variations in sediment supply, and (b) fluctuations in accommodation space controlled by either global eustasy or tectonic flexure that changes relative sea level. For half a century, many have attributed development of prominent Permo-Carboniferous foreland cyclothems to glacio-eustasy in response to Gondwanan glaciations, but dominance of other controls is not easy to exclude. Stratigraphic analysis of key sequences in the southern Cordilleran region supports a primarily glacio-eustatic origin for Permo-Carboniferous cyclothems. This is based on the following observations difficult to reconcile jointly with dominance of other controls: (a) ubiquitous development of stacked cyclothems in both the Permo-Carboniferous foreland of the Ouachita-Marathon orogenic system and in correlative non-foreland settings of Nevada and Utah; (b) absence of comparable cyclicity in older Antler and younger Sevier foreland successions; (c) distinctly diachronous stratigraphic records produced by foreland tectonic flexure and associated migratory forebulges; (d) basinwide distribution of multiple intrabasinal cyclothems in selected Ancestral Rockies basins; (e) provisional interbasinal correlation of individual cyclothems from the midcontinent region through the Ouachita-Marathon foreland; and (f) apparent cyclothem duration within the Milankovitch time band.

Abstract New sedimentological determinations of the water depth and associated sea-level change of midcontinent Pennsylvanian cyclothems shows that they accumulated in water depths ranging from as low as 32 m to as high as 160 m, depending on which model is used to establish the deepest water facies. These depth determinations also indicate that, regardless of model, depth variations existed for different cyclothems both laterally and in time. Average water-depth determinations and sea-level change for models of Heckel (1977) and Gerhard (1991) are 96.4 m and 86.0 m respectively. Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects, in response to regional orogenic movements, partially influenced Pennsylvanian sea-level change in the midcontinent. Organization of Virgilian and Missourian midcontinent cyclothems into four-to fivefold bundles shows that sea-level changes in midcontinent platform areas were influenced both by Milankovitch orbital parameters and longer-term climate change, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence controlled by foreland-basin tectonism. The magnitude of tectonically-contributed change in sea level varied laterally. In the midcontinent, tectonic subsidence accounts for approximately 5 to 20% of the total sea-level change in platform areas, and perhaps as much as 20% in basin depocenters. The remaining change in sea level is controlled by both short-term glacial eustasy (Milankovitch orbital forcing; approximately 70% of sea-level change) and long-term climate change (approximately 15% of sea-level change). These findings suggest that, away from orogenic belts, climatic change is the principal driving mechanism controlling sea level change, whereas within orogenic belts, climate becomes somewhat more subordinate as a driving mechanism for Pennsylvanian sea-level change, even though indicators of climatic change itself are preserved. Methods discussed herein permit calculation of magnitudes of both tectonic and climatic-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition, and may be applicable to other cyclic sequences.

Abstract Stratigraphic analysis of the Lower and Middle Pennsylvanian rocks of part of the central Appalachian basin reveals two orders of cycles and one overall trend in the vertical sequence of coal-bearing rocks. The smallest order cycle, the coal-clastic cycle, begins at the top of a major-resource coal bed and is composed of a sequence of shale, siltstone, sandstone, seat rock, and overlying coal bed which, in turn, is overlain by the next coal-clastic sequence. The major marine-transgression cycle is composed of five to seven coal-clastic cycles and is distinguished by the occurrence of widespread, relatively thick (generally greater than 5 m) marine strata at its base. The Breathitt coarsening-upward trend describes the general upward coarsening of the Middle Pennsylvanian part of the Breathitt Group and includes at least five major marine-transgression cycles. Chronologic analysis, based on averaging relative age dates determined in previous investigations, provides a duration of 20 my for the deposition of Lower and Middle Pennsylvanian strata of the central Appalachian basin. The eight major marine-transgression cycles that occurred in this interval are calculated to represent an average of 2.5 my each. The average duration of the coal-clastic cycle, in contrast, is calculated to be only about 0.4 my. The average duration of coal-clastic cycles is of the same order of magnitude (10 5 yr) as the Milankovitch orbital-eccentricity cycles and matches the 0.4 my second-order eccentricity cycle (Long Earth-Eccentricity cycle). These orbital periodicities are known to modulate glacial stages and glacio-eustatic levels. The calculated periodicities of the coal-clastic cycles can be used to support glacio-eustatic control of the coal-bearing rocks of the Appalachian basin. The 2.5-my periodicity of the major marine-transgression cycle does not match any known orbital or tectonic cycle. The cause of this cycle is unknown, but might represent episodic thrusting in the orogen, propagation of intra-plate stresses, or an unidentified orbital cycle. The Breathitt coarsening-upward trend represents the increasing intensity and proximity of the Alleghanian orogeny.

Abstract Pennsylvanian major marine cyclothems in midcontinent North America comprise the sequence: thin transgressive limestone, thin offshore gray to black phosphatic shale, and thick regressive limestone. Typically, the cyclothems are separated from one another by well-developed paleosols across an area of perhaps 500,000 km 2 from southern Kansas to Iowa and Nebraska. Southward, the offshore shales extend into the foreland basin of Oklahoma, and the regressive limestones and paleosols grade into deltaic to fluvial clastics derived from the Ouachita detrital source. Texas, Illinois, and Appalachian marine cyclothems are detrital-rich like those in Oklahoma, but appear to be separated by paleosols like those in the northern midcontinent. Because the black phosphatic offshore shales of the midcontinent record sediment-starved, condensed deposition below a thermocline in about 100 m of water, sea-level rise and fall of at least that amount is required over the entire northern midcontinent region to account for the widespread dark shale-paleosol cyclicity. Sparsity to absence of deltas between most cycles on the northern shelf rules out delta shifting as a control over major cyclothem formation there. Continuity of all major cyclothems across both the Forest City basin and adjacent Nemaha uplift rules out local differential tectonics on the northern shelf as a major control. Confinement of all reasonable estimates of cyclothem periods within the Milankovitch band of orbital parameters (20 ky to 400 ky), which controlled Pleistocene glacial fluctuation, points to glacial eustasy as the major control over midcontinent cyclicity. Moreover, only the documented late Quaternary post-glacial rates of sea-level rise significantly greater than 3 mm/yr are sufficient to exceed carbonate accumulation consistently and produce the characteristic thin transgressive limestone overlain by the widespread thin subthermocline black shale in each major cyclothem. Although tectonic subsidence helped provide space for sediment accumulation, tectonic control over cyclothem deposition would require both subsidence and uplift of the midcontinent (or an equally large region) at Milankovitch band frequencies. However, currently developed cyclic tectonic mechanisms that can achieve the required depths repeatedly in a cratonic area act at periods at the very least 5 times greater (2 my+) and at rates of sea-level rise at least 30 times too slowly (maximum of 0.1 mm/yr). Firm biostratigraphic correlation of major midcontinent Upper Pennsylvanian cyclothems with similar depositional cycles in Texas and Illinois allows a strong glacio-eustatic signal to be identified in those regions, with little evidence so far of temporally differential tectonism among them. The lithic differences between carbonate-rich midcontinent cyclothems and detrital-rich cyclothems in Texas, Illinois, and the Appalachians are attributable directly to the greater detrital influx in the latter areas, which could relate as much to appropriate climate and accessibility to detrital provenance as to tectonic activity. Preliminary correlations showing that only certain bundles of major marine transgressions extended into the Appalachian basin suggest that the absence of the others may reflect tectonic uplift there, and that with more definite correlation, a longer-term tectonic signal can be isolated in that area.

Abstract The Lower Pennsylvanian Pottsville Formation in the Black Warrior foreland basin of Alabama contains abundant coal and coalbed-methane resources, and because of numerous data from geophysical well logs, offers one of the best opportunities to evaluate causes of cyclicity in Carboniferous coal-bearing strata through subsurface mapping and facies analysis. Twelve regionally mappable transgressive-regressive cycles are present in the Pottsville Formation, which is of Morrowan age. Individual cycles accumulated in an average of 0.2 to 0.5 my and thus represent high-frequency fluctuations of relative sea level. The Black Warrior basin underwent a rapid tectonic evolution related to progressive deformational loading of the Alabama promontory as the Appalachian-Ouachita orogen developed. Subsidence rate averaged approximately 15.3 cm/1,000 yr (0.5 ft/1,000 yr) in the structurally deepest part of the basin, and disregarding sediment influx or sea-level change, could account for an increase of water depth of more than 75 m (250 ft) during deposition of some cycles. This rapid subsidence evidently imparted pronounced asymmetry to relative sea-level variation by amplifying marine transgression and suppressing marine regression. Whether or not extremely rapid subsidence in response to the introduction of new load elements onto the continental promontory caused regional transgression is unclear, but episodes of enhanced loading probably resulted in at least local inundation as some cycles were deposited. Although rapid flexural subsidence may have amplified marine transgression, no tectonic causes of regional marine regression were identified that operated at the time scale of deposition of a single Pottsville cycle. For this reason, glacial eustasy is considered to have been the dominant cause of cyclicity in the study interval. Consistent distribution of fluvial-deltaic sandstone and coal in each cycle mapped indicates that, despite rapidly changing subsidence patterns, a northwest- to west-dipping coastal plain and a single sediment-dispersal system persisted in Alabama. Hence, tectonism and eustasy operated faster than sediment could be dispersed from evolving sources in the advancing orogenic belt, and the resulting paleogeography was much more sensitive to eustatic sea-level variation than to flexural changes of basin geometry.

Abstract Paleodrainage mapping of the Mississippian-Pennsylvanian unconformity in northwestern West Virginia verifies the existence of an incised, sub-Pennsylvanian paleovalley system there that extends for over 130 km. The paleovalleys are filled mostly with quartzose sandstones of the New River Formation. This paleovalley system was carved by a southwest-draining network of rivers that was rejuvenated during the mid-Carboniferous. Bedload-dominated streams that occupied the paleovalleys deposited most of the valley-fill sediment. Regional paleodrainage data indicate that the sub-Pennsylvanian paleovalleys in northwestern West Virginia form the middle reach of a major paleoriver system which includes the Middlesboro paleovalley in eastern Kentucky, the Sharon paleovalley in eastern Ohio, and the Perry paleovalley in southeastern Ohio. This regional paleodrainage network (herein named the Middlesboro-Sharon-Perry paleovalley system) transported sediment from the craton north of the central Appalachian basin to the mid-Carboniferous depocenter in southwestern Virginia. Although it has been previously suggested that maximum erosional development of the Mississippian-Pennsylvanian unconformity occurred during the Early Pennsylvanian, paleoslope considerations rule against an Early Pennsylvanian age for carving of the Middlesboro-Sharon-Perry paleovalley system. Existing biostratigraphic data from the mid-Carboniferous depocenter in southern West Virginia support the existence of an Upper Mississippian (Chokierian-Alportian stages of the Namurian Series) hiatus there, suggesting that incision of the Middlesboro-Sharon-Perry paleovalley system was dominantly a Late Mississippian (Chokierian-Alportian) event. Uplift of the craton north of the central Appalachian basin combined with subsidence within the basin that increased in rate toward the mid-Carboniferous depocenter in southern West Virginia created the generally south-dipping paleoslope which the Middlesboro-Sharon-Perry paleoriver system drained. The regional paleodrainage picture rules against tectonic uplift of the Cincinnati arch as a key factor in driving the incision of the Middlesboro-Sharon-Perry paleovalley system. Carving of this paleovalley system apparently was driven by a previously documented, Late Mississippian (Chokierian-Alportian) eustatic sea-level drop. Regional tectonic uplift during the Early Pennsylvanian may have influenced erosional development of the Mississippian-Pennsylvanian after the Late Mississippian incision of the Middlesboro-Sharon-Perry paleodrainage system.

Abstract Recent computer climate modeling has revealed possible mechanisms which allow Milankovitch orbital parameters to influence climate and sedimentation in tropical areas with monsoonal climates such as the setting of the Devonian Catskill Delta complex of the Appalachians. There is evidence that the shoreline, shelf, slope, and basin sediments of the Catskill Delta complex contain sedimentary cycles with depositional periods of approximately 1-3 ka, 20-40 ka, 100 ka, 400 ka, and, possibly, 1.3, 2.0 and 3.5 Ma, which may be related to orbital precession, obliquity and eccentricity cycles, plus harmonics of these forcing periods. Preliminary correlations suggest that at least some of these rhythms may be traceable throughout the Appalachian basin and some may be of global extent. Until more complete age-dating and improved stratigraphic correlation becomes available, the synchronism of these events will remain unproven.

Abstract During the Late Devonian, a thick clastic wedge, derived from a tectonically active source area to the east, was deposited in the Appalachian basin. The basinal facies of the wedge is composed of blackish-gray or dark-brown organic carbon-rich shales alternating with gray or greenish-gray non-organic shales. Five large (third-order or approximately 2 Ma or longer) black/gray shale cycles have been previously recognized in the New York outcrop belt, as well as in the subsurface throughout the basin. Eleven newly defined higher frequency basinwide organic/inorganic shale cycles occur within the relatively inorganic portion of one of the larger cycles. These higher frequency lithologic cycles represent the basinal expression of fourth-order (0.1-0.3 Ma) cycles of parasequence scale. The stratigraphic position of these cycles can be traced into nearershore silty and sandy facies using subsurface gamma-ray logs, revealing a synchronous cyclicity. These correlations show that the eleven cycles can be subdivided into two stacked sets of basinwide progradational parasequence sets. This pattern is confirmed by correlation into the most distal portions of the basin and examination of patterns of thinning and convergence. A previously recognized major transgression occurs at the top of the interval which rapidly initiated a major period of organic-rich shale deposition (Huron/Dunkirk Shales). These stratigraphic patterns are interpreted to result from a series of minor eustatic sea-level cycles during Upper Devonian time.

Abstract Refined intrabasinal correlation of medial-Silurian strata has led to recognition of discrete eustatic and tectonic controls on the stratigraphic architecture of the Appalachian foreland basin. Major unconformities and disconformities are used to define third- and fourth-order sea-level cycles (depositional sequences and sub-sequences). Although they are asymmetric, most unconformities are present along both basin margins, and their timing corresponds with sea-level lowstands in other Silurian basins. The correlation with apparent, global, sea-level lowstands suggests a eustatic component to Silurian Appalachian basin unconformities. Silurian sequences are divisible into systems tracts that are correlatable across the basin. Transgressive systems tracts are laterally correlative, retrogradational, carbonate and sandstone successions that onlap unconformities on northwestern and southeastern margins of the basin, respectively. Highstand systems tracts are thickest in the basin center and thin laterally toward each margin. They are divisible into early highstand phases, typified by aggradational, fine-grained siliciclastic and argillaceous carbonate successions, and late highstand (or regressive) phases, that characteristically exhibit a general upward-coarsening (progradational) pattern. These regressive deposits are typically divisible into two or more sub-sequences whereas the transgressive and early highstand systems tracts comprise a single subsequence. Smaller-scale, discontinuity-bound, stratal packages that are interpreted to record fifth- and sixth-order sea-level changes, are analogous to parasequences sets and parasequences, respectively. Many of the small-scale cycles can be mapped basin-wide or unti 1 they are truncated under marginal unconformities. High-frequency, eustatic, sea-level changes or climatic oscillations are plausible mechanisms to explain the pervasiveness of these cycles. Documented tectonic controls on stratigraphic architecture include lateral migration of the foreland-basin axis and uplifts along both the cratonic arch and the tectonically active eastern basin margin. These tectonic signatures are imprinted on marginal unconformities and are also recorded by progressive lateral shift in the locus of thickest accumulated sediment and deepest facies. Tectonically imprinted unconformities are more fully developed on one basin margin and are distinguished from purely eustatically generated unconformities by their asymmetry. Consequently, large-scale sedimentary cycles, that are bounded by unconformities and correlate with sea-level changes in other basins, record an interplay between foreland basin geodynamics and eustatic processes of similar rate. In contrast, smaller-scale, sedimentary allocycles, that have recurrence intervals that outpace rates of foreland-basin flexure, more clearly record high-frequency, low-amplitude, eustatic sea-level changes or climatic oscillations.

Abstract Plots of the cumulative aggradation of cyclically repetitive strata were generated by a method similar to that for Fischer plots. Plots were generated for three outcrop sections of the upper part of the Upper Ordovician Juniata Formation, eleven wells penetrating the Juniata Formation, and four wells penetrating the entire Ordovician System. The resultant plots are inferred to represent relative changes in sea level for each locality over the time interval represented. Cycles of relative sea-level change with a periodicity of approximately 5 my are apparent throughout the Ordovician. These cycles are superimposed upon a larger scale relative sea-level curve that reflects the sea-level pattern associated with the Creek holostrome (Tippecanoe I sequence). Each of these sea-level cycles can be correlated from well to well and, because of their regional extent, are possibly the effect of an eustatic cause. Other relative shallowing and deepening events are apparent in some of the wells, and at various stratigraphic positions. These local events are interpreted as tectonic uplifts and downwarps. Many of these localized sea-level events are more long-lived than the regional 5-my sea-level signal, but have a shorter duration than the Creek holostrome.

Abstract The Upper Ordovician stratigraphic succession in New York and Ontario is superficially similar to a very large eustatically-controlled sequence. These strata are bounded by unconformities, and analogs of systems tracts are present. Submarine fan siltstones, sandstones, and shales comprise the lowstand systems tract analog; the transgressive systems tract analog is represented by a stratigraphically condensed section, and the highstand systems tract analog is characterized by an upward-coarsening, and generally upward-shallowing, succession of strata. The diachroneity of the systems tract analogs and of sequence-bounding unconformities suggest, however, that this stratigraphic succession was most strongly influenced by tectonic forces associated with the Taconic Orogeny. In the Taconic foreland basin of New York and Ontario, thrust- and sediment-loading drove subsidence so that the Middle to Late Ordovician Trenton carbonate ramp progressively oversteepened and collapsed. In New York, the oversteepening is represented by a stratigraphic succession in which carbonate-dominated deposits are disconformably overlain by flysch. In the Toronto, Ontario region, which was more distal to the thrust sheets and, presumably, more proximal to the Late Ordovician tectonic hinge, the disconformable relationship between the underlying carbonates and the overlying siliciclastics grades to conformity. In New York and Ontario, progressive southeast to northwest oversteepening of the carbonate ramp resulted in a geographically diachronous shift from carbonate-dominated deposition to organic-rich mud deposition; a sediment-starvation surface (a condensed section) is often associated with this shift. The siliciclastic strata that overlie the condensed section record the prograding Queenston clastic wedge. Paleocurrent and stratigraphic data suggest that movement along Taconic, or older rejuvenated, basement normal faults was an important control on basin subsidence and filling. Smaller scale isochronous stratigraphic changes that cross-cut facies patterns may record eustatic events. These possible eustatic events include: (1) an Early Maysvillian deepening event, recorded by the Collingwood Formation in Ontario; (2) a basal-Pulaski progradational event; and (3) a mid-Queenston transgression.

Abstract Onset of the Taconic Orogeny is generally dated by the age of the last shelf strata beneath Ordovician pelitic sequences. Recent stratigraphic analysis of the shelf sequence in the Champlain Valley, however, suggests that timing of the end of shelf sedimentation in a given area was controlled by eustasy as well as tectonics. Following major eustatic lowstands in Cambrian and Early-Middle Ordovician time, the shelf margin abruptly jumped cratonward and large areas of outer shelf foundered. Only by analyzing subsidence (stratigraphic accumulation) rates can the timing of the orogenic event be determined. Analysis of subsidence rates indicate a simultaneous start of the initial phase of orogenic subsidence in late Early Ordovician time for the entire shelf from Newfoundland to Pennsylvania. Arrival of the orogenic wedge varied from Middle Ordovician (Newfoundland) to early Late Ordovician (Champlain Valley) time.

Abstract Recently developed flexural models suggest that lithospheric responses to early craton-margin orogenies should result in at least one major unconformity, both within and beyond the foreland basin, as well as a distinct sequence of lithologies largely restricted to the foreland basin. Consequently, interpretation of tectonic origin is based on (1) the presence of a distinctive overlying flexural sequence, (2) the coincidence of unconformity formation with the inception of established orogenies or tectophases therein, and (3) the distribution of unconformities relative to probable loci of tectonism. Based on the above criteria, ten of the 13 major interregional and regional unconformities in the Appalachian basin appear to reflect major tectonic control, one is uncertain, one is largely eustatic in origin, and one probably reflects some combination of tectonic and eustatic control. Eight of the ten tectonically related surfaces are concurrent with the initiation of tectophases in the Taconian, Salinic and Acadian orogenies, whereas the other two reflect overlap of Mississippian Ouachita flexural events into extreme southern parts of the Appalachian basin. A widespread Early Pennsylvanian unconformity probably coincides with the initiation of the Alleghanian orogeny but lacks the anticipated overlying flexural stratigraphic sequence. Although this surface could reflect major eustatic influence, the differences in the accompanying sequence might just as likely result from the different style of tectonism accompanying this late-stage orogeny. The only certain, largely eustatically derived unconformity in the Appalachian basin appears to be that at the Ordovician-Silurian boundary, and even it bears some overprint of Taconian tectonic influence. However, a Middle Mississippian unconformity may represent some combination of eustatic lowering and relaxational bulge movement. Inasmuch as typical Appalachian interregional or regional unconformities recur repeatedly in Paleozoic rocks both within and beyond the Appalachian basin, they must be considered cyclic. The cycles, however, are irregular and appear to be largely related to concurrent phases of tectonism.

Abstract Foreland-basin stratigraphy and orogen state are determined by the rate of mass accretion to an orogen by thrust tectonics, the efficiency of mass redistribution by surface processes, and lithospheric flexure. Orogen state can be characterized as constructive, steady, or destructive depending on the mass net balance in the orogen (Jamieson and Beaumont, 1988, Tectonics, v. 7, pp. 417-445). We have constructed a kinematic planform foreland-basin model to look for stratigraphic relationships between synthetic foreland basin stratigraphy and orogen state. The foreland-basin model links thin-skinned tectonic development of an orogen, lithospheric flexure (Beaumont and others, 1988, Tectonics, v. 7, p. 389-416) and mass redistribution by surface processes (Beaumont and others, 1992, Thrust Tectonics, p. 1-18). The tectonic model uses critical wedge principles to construct a two-sided wedge-shaped orogen. Sediments are accreted to the toe of each wedge at a rate proportional to the convergence rate of each leading slip line with the adjacent autochthon. The wedges, which need not be symmetric, grow in proportion to the net rate of mass influx. Their geometry is consistent with flexural adjustment of the lithosphere, conservation of mass, the criticality of each Coulomb wedge and match of wedge heights at their interface. The lithospheric-flexure model includes elastic or thermally activated linear viscoelastic rheologies. The surface process model couples climatic, hillslope (mass diffusion) and fluvial (mass transport) processes to erode, redistribute, and deposit mass across the orogen, its foreland basin and peripheral bulge. Synthetic stratigraphic assemblages are constructed for a range of tectonic, lithospheric, and surface process model parameters, to determine under what circumstances an assemblage can be considered diagnostic of an orogens's state, or change in state.

Abstract Two competing hypotheses can explain distal unconformities in foreland basin stratigraphy: peripheral-bulge migration with stress relaxation in the lithosphere (Quinlan and Beaumont, 1984, Canadian Journal of Earth Sciences, v. 21, p. 973) or orogen tectonics and basin-filling mechanisms (Flemmings and Jordan, 1990, Geology, v. 18, p. 335; Sinclair and others, in review). We investigate synthetically the origin of unconformities using the planform model (Johnson and Beaumont, abstract above). The figure shows an unconformity bounded sequence for one tectonic cycle, on a stress-relaxing lithosphere. I-type erosion occurs when constructive orogenic loading outstrips basin-filling and lithospheric relaxation. F-type erosion occurs when peripheral bulge migration with lithospheric-stress relaxation dominates. I- and F-type unconformities remain distinct when intervening sediments are preserved, otherwise the composite unconformity reflects the superposition of tectonic and relaxation dominated phases that may span several tectonic cycles. Does synthetic basin stratigraphy provide the evidence to distinguish I- from F-type erosion and to determine which dominates?

Abstract I have used >10,000 line-km of multichannel seismic-reflection profiles and 88 key boreholes to map 23 postrift Mesozoic and Cenozoic depositional sequences of the U. S. Middle Atlantic margin. From these data I infer that tectonic uplift was consistently a dominant force in determining the architecture and distribution of the sequences. Relative uplift among three primary source terrains directly determined the location of siliciclastic dispersal routes, rates of sediment accumulation, and the latitudinal position of associated offshore depocenters. The principal role of sea-level change appears to have been to distribute and redistribute sediments once they reached the basin complex. Sea-level change was particularly effective during long-term rises and short-term falls, when it determined the bathymetric position of depocenters. A marked increase in sediment supply (triggered by source-terrain uplift), however, could mask the effects of short-term sea-level rise. Moreover, in the absence of tectonic uplift, major sea-level falls generally did not accelerate sediment accumulation. Paleoclimatic shifts appear to have influenced deposition most effectively when associated with extreme conditions, such as extensive aridity or the buildup of continental ice sheets. But the relative amount of carbonate production on the continental shelves also was responsive to paleoclimatic change.