Base Level Drop Research Articles

Implications of bank failures and fluvial erosion for gully development: Field observations and modeling

Gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop. Field observations attribute the headward growth and widening of many gully systems to gravitational mass‐wasting processes of oversteepened sidewalls. Soil saturation, groundwater sapping, and tension crack development contribute to the instability. Recent landscape evolution models treat such mass failures as slope‐dependent continuous sediment transport processes, sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient. In this study we first present an explicit physically based theory for the stability analysis of gully heads and walls. The theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall, with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face. Then, we test the theory against field data collected in our field site in Colorado and against other published data. Second, the theory is implemented in a one‐dimensional hillslope profile development model and the three‐dimensional channel‐hillslope integrated landscape development (CHILD) to study the effects of soil cohesion, erosion thresholds, and stochastic climate on the tempo of gully development and morphology. Preliminary results indicate that wider and shallower gullies develop and integrate, forming wide valleys, when soil cohesion is small. As soil cohesion increases, erosion slows down, gullies become deeper with vertical walls, and episodic mass failures occur. Differences in storm intensity‐duration characteristics and erosion thresholds are predicted to have a significant impact on gully development. Vertical gully walls develop rapidly, and gullies enlarge by slab failures in a climate characterized by high‐intensity, short‐duration storm pulses. However, under low‐intensity, long‐duration storms, gullies quickly stabilize, and vertical walls are eliminated and rounded, forming diffusion‐dominated hilltops. Erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction. Lowering the erosion threshold enhances gully widening by slab failures. Gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls.

Quaternary palaeogeomorphologic evolution of the Wadi Faynan area, southern Jordan

The Faynan area comprises a complex assemblage of deposits and landforms of varying nature, age and position in the landscape. The dominant facies are fluvial, slope and aeolian. At least eight palaeochannels are present at different heights within the Wadis Dana and Ghuwayr. Fewer terraces are preserved in the Wadi Faynan. Planated surfaces leaving low-gradient straths, rock-cut channels and fluvial deposits are all evident. The continuing drop in regional base level as a result of subsidence of the Dead Sea trough has driven incision and aided in the retention of many of the terrace deposits as they have been abandoned as the wadi systems have eroded downwards. Calcretisation has enhanced the preservation potential of many of the fluvial deposits. A large proportion of these deposits are comparable to those found in the modern wadi systems, and it is thought that they formed under arid conditions similar to those that exist today. Two of the terrace deposits appear to reflect slightly wetter conditions as they consist of finer grained, better sorted materials and display evidence of biological activity. There are at least four distinct alluvial fan and slope deposits that have formed at the edge of various mountain fronts. The fans have been entrenched to varying degrees and are likely to have formed at different times in the Quaternary, possibly when conditions were more humid or during periods of increased tectonic activity. Aeolian deposits indicate a marginally drier phase in the Late Glacial maximum and similar sediments of mid-Holocene age could either reflect increasing aridity at this time as has been suggested by Frumkin et al. (1991) and/or be a result of the increasing impact of humans on the landscape. Many of the deposits are beyond the age range of optically stimulated luminescence (OSL) and radiocarbon dating techniques, but a proposed stratigraphy is supported by some numerical dates on the younger deposits and associated archaeological artefacts.

Long-term changes to river regimes prior to late Holocene coseismic faulting, Canterbury, New Zealand

Two sites are described from range front faults along the foothills of the Southern Alps of New Zealand, where apparently a period of 200–300 years of accelerated river incision preceded late Holocene coseismic ruptures, each probably in excess of M w 7.5. They relate to separate fault segments and seismic events on a transpressive system associated with fault-driven folding, but both show similar evidence of off-plane aseismic deformation during the downcutting phase. The incision history is documented by the ages, relative elevations and profiles of degradation terraces. The surface dating is largely based on the weathering rind technique of McSaveney (McSaveney, M.J., 1992. A Manual for Weathering-rind Dating of Grey Sandstones of the Torlesse Supergroup, New Zealand. 92/4, Institute of Geological and Nuclear Sciences), supported by some consistent radiocarbon ages. On the Porters Pass Fault, drainage from Red Lakes has incised up to 12 m into late Pleistocene recessional outwash, but the oldest degradation terrace surface T I is dated at only 690±50 years BP. The upper terraces T I and T II converge uniformly downstream right across the fault trace, but by T III the terrace has a reversed gradient upstream. T II and T III break into multiple small terraces on the hanging wall only, close to the fault trace. Continued backtilting during incision caused T IV to diverge downstream relative to the older surfaces. Coseismic faulting displaced T V and all the older terraces by a metre high reverse scarp and an uncertain right lateral component. This event cannot be younger than a nearby ca. 500 year old rock avalanche covering the trace. The second site in the middle reaches of the Waipara River valley involves the interaction of four faults associated with the Doctors Anticline. The main river and tributaries have incised steeply into a 2000 year old mid-Holocene, broad, degradation surface downcutting as much as 55 m. Beginning approximately 600 years ago accelerating incision eventually attained rates in excess of 100 mm/year in those reaches closely associated with the Doctors Anticline and related thrust and transfer faults. All four faults ruptured, either synchronously or sequentially, between 250 and 400 years ago when the river was close to 8 m above its present bed. Better cross-method checks on dating would eliminate some uncertainties, but the apparent similarities suggest a pattern of precursor events initiated by a period of base level drop extending for several kilometres across the structure, presumably in response to general uplift. Over time, deformation is concentrated close to the fault zone causing tilting of degradation terraces, and demonstrably in the Waipara case at least, coseismic rupture is preceded by marked acceleration of the downcutting rate. Overall base level drop is an order of magnitude greater than the throw on the eventual fault scarp. The Ostler Fault (Van Dissen et al., 1993) demonstrates that current deformation is taking place on similar thrust–fault driven folding in the Southern Alps. Regular re-levelling since 1966 has shown uplift rates of 1.0–1.5 mm/year at the crest of a 1–2 km half wave length anticline, but this case also illustrates the general problem of interpreting the significance of rates derived from geophysical monitoring relative to the long term seismic cycle. If the geomorphic signals described can be shown to hold for other examples, then criteria for targeting faults approaching the end of the seismic cycle in some tectonic settings may be possible.

Long-term fluvial incision rates and postglacial river relaxation time in the French Western Alps from 10Be dating of alluvial terraces with assessment of inheritance, soil development and wind ablation effects

Studying river long-profile development as a response to tectonic and climatic controls requires reliable age-dating of palaeo-profile remnants preserved as river terraces. Cosmic ray exposure (CRE) dating often represents the only method available to date river terraces, but the interpretation of cosmogenic nuclide concentrations is complicated by pre-depositional inheritance and post-depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation. Here, we use cosmogenic 10Be measurements to date alluvial terraces in the French Western Alps, in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing. We assess inheritance by constructing 10Be concentration vs. pebble depth profiles and use a Monte Carlo technique to estimate terrace ages. We find that inheritance is negligible on all terraces, enabling us to date terraces as young as 5 kyr. Terraces that predate the last glaciation experienced intense pedogenesis and wind ablation, which led to significant scatter in the 10Be concentrations of surface samples. We assess these effects using a model of 10Be ingrowth and show that the oldest CRE ages of surface clasts are close to the probable terrace age. We study two catchments which have undergone varying degrees of glacial disruption. The Buëch River experienced variations in runoff and sediment flux during the last glaciation, as well as occasional ice-dammings of its outlet. Its upper terraces record incision rates averaged over 190 kyr of ∼0.8 mm yr −1, consistent with denudation rates estimated in the surrounding areas and suggesting long-term stability of river incision rates. Climatic forcing is well documented for the Drac River, which was repeatedly dammed by glaciers during cold periods. Its postglacial incision history was triggered by an 800 m drop in base level following ice-dam disappearance. Long-profile development by knickpoint propagation explains the time-lag of 2–5 kyr between this base-level drop and terrace abandonment upstream, as well as subsequent peak incision rates of >6 cm yr −1 followed by a gradual decrease in incision rates. The present knickpoint location, ∼55 km upstream from the glacial damming site, enables us to calculate a fluvial response time of 15–20 kyr, controlled by knickpoint propagation rates of several metres per year, within the Drac River.

Plio-Quaternary uplift rates in the outer zone of the central Apennines fold-and-thrust belt, Italy

Geological, seismic and borehole data permitted the reconstruction of the Plio-Quaternary uplift/subsidence and erosion/sedimentation history for the outer sector of the central Apennines. During the latest contractional stages (Middle–Late Pliocene), the frontal sector of the belt experienced a net subsidence locally compensated by the growth of the fold-and-thrust system. This tectonic uplift was localized at the axial culmination of the anticlines and, particularly for the major structures, was responsible for exhumation and erosion of the Neogene succession and for near-flat planation surfaces development, on Meso-Cenozoic carbonate rocks, at the crest of the anticlines. Since the Early Middle Pleistocene, a regional doming involved the outer part of the belt (Peri-Adriatic Domain). This process interacted with Quaternary eustatic sea level fluctuations and caused the basinward migration of shallow marine deposits, forming an overall prograding system. Quaternary shoreline retreat, base level drop and climatic changes were mainly associated with valley deepening and fluvial terrace formation. The present study outlines that fold/thrust growth and regional doming driving processes, characterized by different amplitude, wavelength and uplift-rate amounts, are both responsible for the morphological setting of the central Apennines external zones.

Late Miocene to present exhumation in the Ligurian Alps (southwest Alps) with evidence for accelerated denudation during the Messinian salinity crisis

Detrital apatite (U-Th)/He and fission-track thermochronometry are applied to constrain the exhumation history of the Ligurian Alps (southwest Alps). Exhumation of the Ligurian Alps is recorded from ca. 11 Ma and continues until present. More than 4 km of exhumation is recorded at an average rate of 0.4 ± 0.2 mm/yr. Exhumation is accommodated along a north-dipping thrust at the foot of the Ligurian margin. Combined (U-Th)/He and fission-track thermal modeling show a phase of accelerated denudation between 7 and 5 Ma, associated with the base-level drop during the Messinian salinity crisis.

Seaward pinching out and internal stratigraphy of the Gironde incised valley on the shelf (Bay of Biscay)

An incised paleovalley offshore the estuary mouth of the Gironde River (Southwest France) was evidenced recently (Comptes Rendus de l'Académie des Sciences de Paris 326 (1998) 701). Nevertheless its seaward extension to the shelf break was still unknown. New high-resolution geophysical data were acquired on the continental shelf in order to follow the incision until the shelf break. After processing the seismic lines, the shape of the valley was identified and the valley-fill in terms of sequence stratigraphy was interpreted. The valley-fill contains a single sequence corresponding to a fifth-order cycle (20 kyr). The three Systems Tracks described onshore by Allen and Posamentier (Journal of Sedimentary Petrology 63 (1993) 378; SEPM (Society for Sedimentary Geology) Special Publication, Tulsa (1994) 225) are present offshore. The Lowstand is poorly developed and is only present in the proximal part of the incised-valley. The Transgressive Systems Tract (TST) constitutes the bulk of the valley-fill, it is composed of landward-prograding estuarine clinoforms in the distal part of the valley (early TST, phase 1) and of landward-migrating tidal inlet-fill deposits in the proximal part of the valley (late TST, phase 2). The thin early Highstand Systems Tracts (HST) strata downlap seaward onto the maximum flooding surface and caps the TST. As expected in this wave- and tide-dominated environment, the tidal -and wave-ravinement surfaces, which bound the TST, are particularly well developed. The valley incision tapers 50 km offshore of the estuary mouth: the depth of the basal sequence boundary gradually decreases seaward and is locally truncated by the wave-ravinement surface around −70 m below the mean sea-level. These observations indicate that during the Lowstand times, rivers do not always generate continuous cross-shelf incised-valleys. They may adjust to a rapid drop in base level by modifying their channel morphology (i.e. deep large channels located upstream change downstream to shallow anastomosed channels with a weaker erosional ability). Therefore, the Lowstand fluvial channels will be poorly developed, and will eventually died out in the proximal part of the shelf because they are unable to keep pace with a phase of rapid sea-level drop. Similar observations have already been reported for the English Channel paleoriver (Geological Society Special Publication, London (1996) 203; Thèse de Géologie Marine, Université de Bordeaux 1 (1997) 265).

THE GEOLOGY AND HYDROCARBON HABITAT OF THE SARIR SANDSTONE, SE SIRT BASIN, LIBYA

The Jurassic — Lower Cretaceous Sarir Sandstone Cformerly known as the Nubian Sandstone) in the SE Sirt Basin is composed of four members which can be correlated regionally using a lithostratigraphic framework. These synrift sandstones unconformably overlie a little known pre‐rift succession, and are in turn unconformably overlain by post‐rift marine shales of Late Cretaceous age.Within the Sarir Sandstone are two sandstone‐dominated members, each reflecting a rapid drop in base level, which are important oil reservoirs in the study area. Between these sandstones are thick shales of continental origin which define the architecture of the reservoir units. This four‐fold lithostratigraphic subdivision of the Sarir Sandstone contrasts with previous schemes which generally only recognised three members.The sandstones below the top‐Sarir unconformity host in excess of 20 billion barrels of oil in‐place. The dominant traps are structural (e.g. Sarir C field), stratigraphic (e.g. Messla field), hanging‐wall fault plays (e.g. UU1–65 field) and horst‐block plays (e.g. Calanscio field). Three Sarir petroleum systems are recognised in the SE Sirt Basin. The most significant relies on post‐rift (Upper Cretaceous) shales, which act as both source and seal. The Variegated Shale Member of the Sarir Sandstone may also provide source and seal; while a third, conceptual petroleum system requires generation of non‐marine oils from pre‐rift (?Triassic) source rocks in the axis of the Sarir Trough.The intrabasinal Messla High forms a relatively rigid block at the intersection of two rift trends, around which stress vectors were deflected during deposition of the syn‐rift Sarir Sandstone. Adjacent troughs accommodated thick, post‐rift shale successions which comprise excellent source rocks. Palaeogene subsidence facilitated oil generation, and the Messla High was a focus for oil migration. Wrenching on master faults with associated shale smear has facilitated fault seal and the retention of hydrocarbons. In the Calanscio area, transpressional faulting has resulted in structural inversion with oil entrapment in “pop‐up” horst blocks. Elsewhere, transtensional faulting has resulted in numerous fault‐dependent traps which, in combination with stratigraphic and truncation plays, will provide the focus for future exploration.

Palaeomagnetic chronology of the evaporitic sedimentation in the Neogene Fortuna Basin (SE Spain): early restriction preceding the `Messinian Salinity Crisis'

The magnetostratigraphic study of the evaporitic Rı́o Chicamo section (240 m) in the Neogene Fortuna Basin (Murcia, southeast Spain) has identified the record of five magnetozones. The most probable correlations with the standard geomagnetic polarity time scale (GPTS) imply that the marine evaporitic sedimentation of this basin was not coeval with the Messinian evaporites of the Sorbas Basin (Almeria, southeast Spain) and the Caltanissetta Basin (central Sicily) (assigned to the reverse Chron C3r, late Messinian by Gautier et al., 1994). The marine evaporites and diatomites from the Fortuna Basin are older (late Tortonian to early Messinian) than the evaporites of those basins. The chronological framework for the sedimentation in the Fortuna Basin together with the isotopic data from the sulphates in these evaporitic units indicate the following. (1) Restriction and confinement in the basin initiated as early as uppermost Tortonian, leading to deposition of evaporites under mixed (marine–continental) conditions. (2) The subsequent sedimentation of marine evaporites and diatomites in this basin occurred in a period between the Tortonian and Messinian transition and the early Messinian: the onset of this sedimentation pre-dates similar sediments of restricted marine environments in the eastern Betics basins, and possibly of the western Mediterranean region also. (3) Episodes of restriction and reflooding in the basin would have occurred in response to periodic fluctuations of the oceanic level under local tectonic controls. In the Fortuna Basin, the global base level drop associated with the `late Messinian Salinity Crisis' was recorded by the progradation of alluvial fans leading to thick clastic deposits overlying the youngest evaporites. These observations hint to: (a) the peculiar characteristics and sensitiveness of some of the marginal intramontane basins in the eastern Betics to reflect structural controls framed in the late Neogene global climatic changes; and (b) the diachronism of the beginning of the marine evaporitic deposition in the Mediterranean region linked to the salinity crisis during the Messinian.

Chronology of the late Turolian deposits of the Fortuna basin (SE Spain): implications for the Messinian evolution of the eastern Betics

The magnetostratigraphy of the mammal-bearing alluvial fan–fan delta sequences of the Fortuna basin (SE Spain) has yielded an accurate chronology for the late Turolian (Messinian) basin infill. From early to late Messinian (at least between 6.8 and 5.7 Ma), the Fortuna basin records the sedimentation of alluvial–palustrine deposits over a confined shallow basin. Changing environmental conditions in the latest Messinian are illustrated by the retreat of palustrine facies. A rapid progradation of the marginal clastic wedges and the initiation of an efficient basin drainage at ∼5.8 Ma (lower part of chron C3r) most likely represents the onshore response to the drastic drop of base level taking place during the Messinian salinity crisis. This study further provides improved age estimates for the late Turolian land mammal events in southern Spain. The oldest MN 13 locality in the studied sections is correlated to chron C3Ar at an age of 6.8 Ma. The entry of camels and the murid Paraethomys in southern Spain occurs in chron C3An.1n at 6.1 Ma, and gives further support for land mammal exchange between Africa and the Iberian peninsula prior to the salinity crisis, in good agreement with results from northern Africa [M. Benammi, M. Calvo, M. Prévot, J.J. Jaeger, Magnetostratigraphy and paleontology of Aı̈t Kandoula basin (High Atlas, Morocco) and the African–European late Miocene terrestrial fauna exchanges, Earth Planet. Sci. Lett. 145 (1996) 15–29]. The age of the studied sequences provides important constraints on the understanding of the sedimentary evolution of the eastern Betic margin, and shows that previous interpretations of the evaporitic–diatomitic sequences of the Fortuna basin, as being coeval to the late Messinian salinity crisis in the Mediterranean, are not correct. The confinement leading to the emergence of the Fortuna basin occurred in the late Tortonian to earliest Messinian, similar to other intramontane basins in the Betics. Therefore, the inclusion of the Fortuna basin in a hypothetical marine Betic Corridor during the late Messinian is no longer tenable.

NICKPOINT RECESSION IN KARST TERRAINS: AN EXAMPLE FROM THE BUCHAN KARST, SOUTHEASTERN AUSTRALIA

Nickpoint recession in the Buchan karst, southeastern Australia, has resulted in the formation of an underground meander cut-off system in the Murrindal River valley. Three nickpoints have been stranded in the surface channel abandoned by the subterranean piracy, and these can be correlated with river terraces and epiphreatic cave passages in the nearby Buchan River valley. The presence of palaeomagnetically reversed sediments in the youngest cave passage in the Buchan valley implies that the topographically lowest nickpoint in the Murrindal valley is more than 730 ka old, and the other nickpoints are probably several million years old. The nickpoints are occasionally active during floods, but the diversion of most surface flow underground has slowed down their retreat to the extent that they have been effectively stationary for several million years. Underground nickpoint migration has been by both incision within major phreatic conduits and their abandonment for lower-level passages. The nickpoints are all present in the upstream part of the cave system, but have not migrated past the sink in the river channel, despite the long period of time available for this to happen. The sink is characterized by collapsed limestone blocks; these filter out the coarse bedload from the river channel. As a result, erosion within the cave passages is dominantly solutional and therefore slower than in the surface channel, where it is mostly mechanical. In addition, to transmit a drop in base level the cave system requires the removal of a larger volume of rock than for the surface migration of a nickpoint, because any roof collapse material in the subsurface system must be removed. These factors have slowed the migration of the base-level changes through the subsurface system, and may be a general feature in caves that have diffuse sinks as their main inputs.

A detachment‐limited model of drainage basin evolution

A drainage basin simulation model introduced here incorporates creep and threshold slumping and both detachment‐ and transport‐limited fluvial processes. Fluvial erosion of natural slopes and headwater channels is argued to be dominantly detachment‐limited. Such slopes undergo nearly parallel retreat and replacement with alluvial surfaces under fixed base level, in contrast with gradual slope decline for transport‐limited conditions. The arrangement of divides and valleys is sensitive to initial conditions, although average morphology is insensitive. Dissected, initially flat surfaces in which downstream concavity is slight exhibit nearly parallel drainage, compared to very wandering main valleys when concavity is great. Steady state is reached after a cumulative base level drop approximately 3 times the final relief. Simulated valley systems are similar to those predicted by a previous model of optimal drainage basins. A critical value of slope divergence normalized by average slope gradient is a useful criterion for defining the valley network.

Regional stratigraphy, sedimentology, and tectonic significance of Oligocene-Miocene sedimentary and volcanic rocks, northern Baja California, Mexico

Upper Oligocene (?) to middle Miocene sedimentary and volcanic rocks in northern Baja California were deposited along the western margin of North America during subduction of the Guadalupe plate and southward migration of the Rivera Triple Junction. Regional mapping and compilation of stratigraphic data reveal a sequence of three regionally traceable stratigraphic units. (1) Oligocene (?) to lower Miocene Mesa Formation: basal quartz-rich fluvial sandstone, grus, conglomerate, and accessory facies, whose detrital compositions reflect the composition of local pre-Tertiary basement rock. (2) Lower to middle Miocene Comondú Formation: laterally variable sequence of volcaniclastic conglomerate, breccia, sandstone, tuff and minor volcanic flow units. (3) Widespread mesa-capping rhyolite tuff, typically welded and crystal-rich, probably upper Miocene in age. The Mesa Formation overlies a highly irregular and deeply dissected erosional surface developed on pre-Tertiary basement rock. The shift from pre-Mesa erosion to widespread (though localized) deposition and valley-filling records the final phase of late Cretaceous to middle Tertiary regional subsidence and eastward transgression that resulted from slow cooling and thermal contraction of Cretaceous arc crust during a temporal gap in magmatic activity along the western Cordilleran margin. Nonmarine sediments of the Mesa Formation were deposited in small, steep-walled paleovalleys and basins that gradually filled and evolved to form through-going, low-energy ephemeral stream systems. The gradational upward transition from the Mesa to Comondú Formation records the early to middle Miocene onset of subduction-related arc magmatism in eastern Baja California and related westward progradation of alluvial volcaniclastic aprons shed from high-standing eruptive volcanic centers. Pre-existing streams were choked with the new influx of volcanic detritus, causing the onset of rapid sediment deposition by stream flows and dilute to viscous sediment gravity flows. Deposits of the Comondú Formation thin and fine systematically westward, from proximal volcanic conglomerate and breccia with thin basalt and andesite flows in the east, to distal volcaniclastic fluvial sandstone in the west. These proximal—distal relationships help to define the location and paleogeography of active arc-flanking volcaniclastic alluvial aprons of the Miocene magmatic arc in northern Baja California. A substantial late Miocene drop in regional base level (relative sea level) is best attributed to regional uplift caused by the renewal of magmatic and thermal activity in northern Baja California, which has continued to the present day.

Evolving Fluvial Style in the Kekiktuk Formation (Mississippian), Endicott Field Area, Alaska: Base Level Response to Contemporaneous Tectonism

The hydrocarbon reservoir at Endicott field, offshore from the Sagavanirktok delta in northeastern Alaska, is found in the Upper Mississippian (Visean) Kekiktuk Formation. The trap is bounded by the northwest-southeast-trending Tigvariak and Mikkelson Bay faults, and the younger east-west-trending Niakuk fault. The Kekiktuk comprises sandstones, siltstones, and mudstones with locally significant coals and conglomerates, and is subdivided into three major reservoir zones based on the relative dominance of these lithologies. Zone 1 (the lowest) comprises mudstones, siltstones, and coals with subordinate isolated sandstones, and represents deposition within a low-lying swamp plain. With very low net-to-gross ratio, this zone is considered to be effectively nonreservoir. Major movement on the Mikkelson Bay and Tigvariak faults created a half-graben with uplift to the north and east and a very significant drop in base level. This change resulted in fairly widespread deposition of zone 2, medium- to coarse-grained sandstones from sandy bed-load braided streams. However, those sandstones thin significantly to the west, away from the faults. Continuing tectonism at this time created local shifts in base level, manifest as diversion of streams, widespread abandonment, and subsidence leading to the creation of a arge lake. The lacustrine deposits are widespread, and effectively divide zone 2 into two subzones. Those subzones are mappable sheet sandstones, and constitute the best reservoir at Endicott field in terms of net-to-gross ratio, porosity/permeability, and sandstone connectivity. Base level shows evidence of rising dramatically at the end of zone 2. This rise is attributed to continuing tectonism within the basin, and is particularly associated with the east-west-trending mid-field fault system at Endicott field, as well as ongoing activity on the Tigvariak/Mikkelson Bay fault system. Thus, at the base of zone 3 (subzone 3A), in the southern and eastern parts of the field, the rocks are mudstones and widespread coals, and contain at least one unequivocally marine mudstone. These rocks are lower delta-plain interdistributary bay-fill deposits, with rare distributary channel sandstones. North and west of the mid-field fault system, the marine mudstone is absent and the facies changes significantly with the loss of dominant coal and a concomitant increase of dist ibutary channel sandstones. Reservoir properties of subzone 3A, including connectivity, are significantly different in this area. Subzone 3B is a thick accumulation of fine-grained alluvial overbank deposits within which occur rare, isolated anastomosed channel sandstones of limited connectivity. This subzone makes minimal contribution to Endicott field reserves. Subzone 3C is characterized (particularly in its upper part) by well-developed fining-upward cycles indicating mature meandering streams. The relationships between these two highest subzones of the Kekiktuk at Endicott field indicate they represent an aggradational episode initiated by tectonically induced avulsion and set against a background of rising base level. To the west, at some distance from the Tigvariak/Mikkelson Bay f ult system, zone 3 is much thinner than at Endicott field, and is dominated by mudstones and coals. This evidence emphasizes the control that contemporaneous tectonism related to those faults had on sediment (especially sandstone) distribution within the Kekiktuk Formation. Base level changes can be identified within the End_Page 1723------------------------------ Kekiktuk Formation in the North Slope subsurface by careful lithofacies analysis and an awareness of key constraining indicators, such as are revealed by palynology, paleontology, and structural geology. By analogy with shallow-marine rocks, sequence stratigraphic concepts may be extended to continental successions. However, one must approach such problems with a full awareness of the impact of tectonism on base level and basin development.

Character of hearwaters adjustment to base level drop, investigated by digital modeling

Using a computer model written for dynamic river profile modeling (Snow and Slingerland, 1987), long-term profile adjustments to constant-rate base level lowering/tectonic block uplift have been simulated with varied conditions of outflow discharge (10 and 100 m 3/s), sediment diameter (0.1, 0.4 and 2.0 mm) and sediment load (concentrations of 1200 and 400 ppm by weight). Tweleve sets of initially graded profile data cover all possible combinations of the variables. Each modeled stream has been subjected to each of three base level drop rates (0.1, 1.0 and 10.0 mm/yr), for 10,000 years of model time. Modeled histories of headwaters erosional response differ in two ways. The first is length of lag time of response, characterized by a period of quiescence in erosion at the headwaters prior to the channel steepening sufficiently for erosion to commence at this upstream point. Variations in discharge and sediment concentration affect the lag time of response in all cases. The second variable aspect of response is the relative magnitude of eventual headwaters erosion. The main control is the drop rate; a high drop rate yields a low relative magnitude whereas a low drop rate yields a high relative magnitude of response. The rate of base level drop also controls to what degree the other controlling variables of discharge, sediment concentration, and sediment diameter affect the eventual magnitude of headwaters erosion. Rates of trunk stream and headwaters erosion indicate that 21 out of 36 of the modeled stream systems attain a condition approximating parallel profile erosion after 10,000 years. Some examples, notably those with low power to erode and high imposed drop rates, exhibit headwaters erosion rates that continue to be well below rates of base level lowering, creating profile steepening with no apparent time limit.

Depositional history of Lower Cretaceous strata in northeastern New Mexico: Implications for regional tectonics and depositional sequences

Lower Cretaceous strata in northeastern New Mexico reflect deposition during marine transgression and regression associated with the Kiowa-Skull Creek cycle. Development of regionally significant unconformities can be directly related to shoreline shifts during this cycle. Topography related to Early Cretaceous tectonic activity on crystalline basement structures, including the Dalhart Basin, Tucumcari Basin, and the intervening Bravo Dome, also had a direct effect on Early Cretaceous deposition and is reflected by thickness trends in marine strata. The Kiowa-Skull Creek transgression was preceded by infilling of paleovalleys represented by the Long Canyon and Campana sandstone beds of the Glencairn Formation and Tucumcari Shale, respectively. The Long Canyon and Campana sandstone beds are equivalent to the Plainview Formation of central Colorado, and together they represent an extensive drainage network, developed over a correlative unconformity, that is preserved throughout most of the southern Western Interior. Glencairn and Tucumcari marine shales represent clastic deposition during Kiowa-Skull Creek transgression and early regression. Base-level drop associated with Kiowa-Skull Creek regression resulted in dramatic shoreline regression and deposition of fluvial channel sandstone in the Dalhart Basin, which maintained a low offshore gradient. This produced an erosional unconformity at the base of Mesa Rica channel sandstone that correlates with the well-documented erosional surface at the base of the Muddy Sandstone in central Colorado. The Tucumcari Basin, which maintained a steeper offshore gradient, accumulated marine-deltaic Mesa Rica Sandstone at this time. Marine strata in both the Tucumcari and Dalhart Basins thin toward the Bravo Dome. Mesa Rica and upper Tucumcari strata represent the only extensive marginal-marine lowstand deposits found on the High Plains to date, associated with the southern arm of the Kiowa-Skull Creek sea. Major Mesa Rica deposition was followed by transgression that infilled only those Mesa Rica fluvial channels that were active at that time and, also, that deposited marine-influenced strata of the basal Pajarito Formation. This event most likely represents onset of the Greenhorn cycle in northeastern New Mexico. Other crystalline basement structures in the northern part of the United States Western Interior Basin were active during Early Cretaceous time. Tectonism in these areas may be related to structures in northeastern New Mexico and thus may be expressions of broad-scale crustal strain in the Early Cretaceous Western Interior that is undocumented by present tectonic models.

Low Stand Seismic Facies – A Carnarvon-Browse Basin Comparison

Similarities between eleven Mesozoic and Cainozoic supercycles in the Carnarvon and Browse Basins of Western Australia may be exploited in exploring for low stand turbidites, located just above sequence boundaries. Seismic and well sequence analyses are the tools used. Local well calibration is vital as geometrically similar units are being tested which are lithologically dissimilar, namely slumped shale slope units versus low stand clastic fans. Most low stand fans are associated with tectonically influenced base level drops, especially in the late Jurassic and Neocomian ? supercycles 3 and 4. These packages are the synrift phases of two major rift periods on the west coast. A particular boundary may have a low stand unit but its details can be quite different between basins.

Outcrop and Seismic Stratigraphic Criteria for Documenting Relative Sea Level Changes in the Devonian of the Canning Basin, Western Australia: ABSTRACT

Four Devonian (and Carboniferous) unconformity bounded carbonate sequences have been identified by outcrop and seismic sequence analyses: the Pillara, Nullara, Yellow Drum, and Laurel sequences. Each sequence, characterized by a lowstand wedge in a marine onlap setting, probably resulted from a tectonically induced base-level drop with concomitant carbonate platform exposure and incision/bypass. The clastic wedges (lowstand systems tract) are followed by fine grained basin/slop and nearshore carbonate sediments of the transgressive system tract. These, in turn, are followed by the highstand systems tract with a well-developed carbonate platform, and sometimes a well-established reef facies. Each sequence is identified on seismic in the Yarrada/Meda and Blina areas of the northern margin of the Canning Basin and some pertinent geometries mapped. The sequence boundary separating the oldest two cycles (Nullara/Pillara) crops out, along with the Nullara lowstand wedge, in the McWhae Ridge area, as well as on the classic face in Windjana Gorge. The latter location clearly depicts the differing highstand character of the early sequences, namely the accreting Pillara sequence vs. the prograding Nullara sequence. Geometrical analogies are found between these Devonian sequences and Permian carbonate sequences in the Delaware basin of west Texas, Cretaceous/Tertiary sequences in Australia, and Triassic sequencesmore » in the Dolomites of northern Italy.« less

Geomorphic and hydrologic implications of the rapid incision of Afton Canyon, Mojave Desert, California

Afton Canyon is a >150-m-deep canyon that formed as a result of overflow and drainage of Lake Manix, an ∼215 km2 late Wisconsin pluvial lake in the central Mojave Desert. Because the canyon age is within the range of conventional radiocarbon dating, it is possible to provide a time-based chronology of events and resultant landscape changes due to a >120 m base-level drop. Analysis of erosion volumes upstream of Afton Canyon and a review of public and private well logs downstream of the canyon mouth indicate that late Wisconsin surfaces are deeply buried downstream of Afton Canyon. Burial depths are ∼55 m at the mouth of Afton Canyon, 27 m in the Cronese basin, and 18 m at Crucero. The well logs also indicate that Soda Lake was far more areally extensive prior to late Wisconsin time. Stratigraphic and geomorphic evidence suggests that Afton Canyon was cut rapidly sometime after 14,230 ±1325 yr B.P. The rapid draining of Lake Manix and subsequent basin dissection have important implications for late Quaternary lake fluctuations in Death Valley, base-level changes and resultant alluvial-fan adjustments in the Cronese basin, the sand source for the Kelso Dunes, and the absence of old artifacts in the Cronese basin and the Mojave River wash region.

Fluvial Ribbon Deposits of Channel and Valley Origin and Their Relationship to Petroleum Reservoir Development: Glauconite Member (Lower Cretaceous), Southern Alberta: ABSTRACT

Stratigraphic fluvial ribbon deposits within the subsurface Glauconite Member (Mannville Group) have been mapped over a 60 x 30-km area of southern Alberta. These ribbons truncate regional markers and have fills comprised of discontinuous sandstone bodies isolated by shales. Ribbons of two distinct types are recognized. Type 1 ribbons are up to 2 km wide and 25 m thick and have relatively simple fills with pod-shaped sandstone bodies distributed preferentially along the ribbon margins. These ribbons are interpreted as superimposed paleochannels which incised into preexisting sediments during river or distributary avulsion. Sandstone bodies represent lateral bars deposited within active channels, whereas the intervening shales were deposited during abandonment. Type 2 ribbons are up to 7 km wide and 40 m thick and have complex fills. Sandstone bodies are pod or ribbon-shaped, occur in lateral or mid-ribbon positions, and locally show vertical or lateral amalgamation. These ribbons are interpreted as paleovalleys excavated in response to base level drop associated with northward regression of the Lower Cretaceous Boreal sea. Valleys were filled with deposits from several distinct, confined, underfit streams resulting in highly complex internal facies relationships. Paleosols are present within strata laterally adjacent to the top of Type 2 ribbons andmore » formed during subaerial exposure associated with valley excavation. Similar surfaces have not been recognized in association with Type 1 ribbons.« less