Paul L. Heller

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.

Interpreting avulsion process from ancient alluvial sequences: Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado)

Alluvial deposits of the Guadalope-Matarranya system (Oligocene, Ebro basin, Spain) and the Wasatch Formation (Eocene, western Colorado), provide time-integrated records of the process of river-channel avulsion. These sequences consist of isolated channel-belt sandstones incised into, and abruptly overlain by, flood-plain siltstones, indicating deposition by avulsive river systems. The geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics, climate, or base-level changes, but formed by autocyclic processes. Measurements from 221 channel fills in the Guadalope-Matarranya system and 38 from the Wasatch Formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions. Paleoflow depths in both systems average 1.4 to 1.6 m. Aggradation height (superelevation) of channel margin levees are, on average, 0.6 and 1.1 times paleoflow depth in the Guadalope-Matarranya and Wasatch systems, respectively. These results are consistent with values from recently avulsed modern rivers and suggest that (1) flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion; and (2) the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful. Numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels. These reoccupation channels indicate that inherited flood-plain topography, here abandoned channel forms, was an important control on the arrival site of newly avulsed channels. Comparison of our results to others suggests two end-member types of avulsion can take place. Incisional avulsion, seen here, is characterized by an early incision phase followed by infilling by migrating bar forms. Aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel. We suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained. In both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion.

Downstream Changes In Alluvial Architecture: An Exploration of Controls on Channel-stacking Patterns

ABSTRACT Various, but related, models have been proposed to explain the architectural arrangement of channel stacking patterns in avulsion-dominated alluvial sequences. The early models published by Leeder, Allen, and Bridge (LAB) addressed the role of changes in sedimentation rate (a proxy for subsidence rate) as a control on stacking patterns. The models decouple avulsion frequency from sedimentation rates, resulting in an inverse relationship between stacking density (or interconnectedness) and sedimentation rates. A key element missing from these models is the likely dependence of avulsion frequency on local sedimentation rate within the active channel belt. We consider a simple model whereby avulsion takes place only when a minimum, critical, relief is developed between a channel bank and the adjacent flood plain. If avulsion frequency increases at rates slower than the increase in sedimentation rate, then stacking density increases with decreasing sedimentation rate, similar to that predicted by the LAB models. However, if avulsion frequency increases linearly with sedimentation rate, then there is no change in stacking pattern with changes in sedimentation rate. If avulsion frequency increases faster than sedimentation rates, as seen in some data sets, then stacking patterns become more dense with increasing sedimentation rates, a result that is the exact opposite of that predicted by the LAB models. Therefore sensitive dependence on the relationship between avulsion frequency and sedimentation rate calls into question the veracity of some previous interpretations of re ative subsidence made in alluvial architecture studies. We provide an alternative, simple geometric model that links changes in subsidence rate to downstream rate of change in stacking pattern as seen in three dimensions within sedimentary basins. Other controls that are considered include: the geometry of subsidence; whether avulsions take place locally along a river or regionally affect the basin; whether local sedimentation rate or flow depth controls the thickness of sand bodies; and the exact relationship between avulsion frequency and sedimentation rate. The primary result of the model is that subsidence strongly influences the rate at which alluvial architecture changes in the downstream direction, but other controls dictate whether the stacking pattern becomes more dense or less dense downstream. Hence, we suggest that subsidence e erts an influence on stacking patterns not necessarily evident in individual vertical sections, but may be recorded in three dimensions as downstream changes in alluvial architecture. Unfortunately any model of alluvial architecture in avulsion-dominated sequences is limited by our lack of understanding of the processes controlling avulsion. As a result any model of alluvial stacking patterns is at best a working hypothesis that should not be taken as proof of changes in tectonic subsidence rates or sea-level changes.

The relative contribution of accretion, shear, and extension to Cenozoic tectonic rotation in the Pacific Northwest

Large Cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the Pacific Northwest, and many tectonic models have been proposed to explain them, including (1) rotation of accreted oceanic microplates during docking, (2) dextral shear between North America and northward-moving oceanic plates to the west, and (3) microplate rotation in front of an expanding Basin and Range province. Stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations. Coast Range structures, Basin and Range extension, and paleomagnetic data from middle Miocene (15 Ma) Coast Range rocks indicate that dextral shear is responsible for at least 40% of the post-15 Ma rotation of the Coast Range and that Basin and Range extension is responsible for the remainder. Reconstructions based on extrapolation of this ratio back to 37 and 50 Ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has, been a significant contributor to rotation during most of Tertiary time. Changes in the dextral-shear rotation rate over the past 50 m.y. correlate directly with changes in the velocity of the Farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism. Continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost Basin and Range region and suggest 17% extension since 15 Ma, 39% since 37 Ma, and 72% since 50 Ma near latitude 42°N.

Time of initial thrusting in the Sevier orogenic belt, Idaho-Wyoming and Utah

Reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the Sevier thrust belt suggests that initial thrust movement may be no older than Aptian age. This interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the Idaho-Wyoming and Utah thrust belts. A major episode of middle Cretaceous (Aptian–Cenomanian) subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent Sevier orogenic belt. An earlier subsidence event took place during Middle Jurassic time, more than 30 m.y. prior to the Cretaceous event, and may be the result of tectonic events to the west that are unrelated to thrust deformation in the Idaho-Wyoming and Utah thrust belts. We find no verifiable evidence to support previous interpretations that Sevier belt deformation and uplift began in Late Jurassic time.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.

Plate tectonics and basin subsidence history

Tectonic setting exerts fi rst-order control on basin formation as refl ected in basin subsidence history. While our approach ignores the effects of fl exural loading and eustatic sea-level change, consistency of backstripped subsidence histories (i.e., with local loading effects of sediment removed) suggests consistent tectonic driving mechanisms in each tectonic setting, with the possible exception of forearc basins. Based on published subsidence curves and open-fi le stratigraphic data, we show the subsidence characteristics of passive margins, strike-slip basins, intracontinental basins, foreland basins, and forearc basins. Passive margin subsidence is characterized by two stages, rapid initial, synrift subsidence and slow post-rift thermal subsidence, with increasing subsidence rates toward the adjacent ocean basin. Subsidence of intracontinental basins is similar in magnitude to that seen in passive margin settings, but the former is generally slower, longer lived, and lacks initial subsidence. Long-lived subsidence for many intracontinental basins is consistent with cooling following thermal perturbation of thick lithosphere found beneath old parts of continents. Basins associated with strike-slip faults are usually short lived with very rapid subsidence. Changes in local stress regimes as strike-slip faults evolve, and migrate over time, coupled with three-dimensional heat loss in these small basins likely explain this subsidence pattern. Foreland basin subsidence rates refl ect the fl exural response to episodic thrust loading. Resultant subsidence curves are punctuated by convex-up (accelerating) segments. Forearc basins have the least consistent subsidence patterns. Subsidence histories of these basins are complex and may refl ect multiple driving mechanisms of subsidence in forearc settings. Second-order deviations in subsidence suggest reactivation or superimposed tectonic events in many basin settings. The effects of eustatic sea-level change may also explain some deviations in curves. For many of these settings, subsidence histories are suffi ciently distinctive to be used to help determine tectonic setting of ancient basin deposits.

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

Fluvial response to abrupt global warming at the Palaeocene/Eocene boundary

Climate strongly affects the production of sediment from mountain catchments as well as its transport and deposition within adjacent sedimentary basins. However, identifying climatic influences on basin stratigraphy is complicated by nonlinearities, feedback loops, lag times, buffering and convergence among processes within the sediment routeing system. The Palaeocene/Eocene thermal maximum (PETM) arguably represents the most abrupt and dramatic instance of global warming in the Cenozoic era and has been proposed to be a geologic analogue for anthropogenic climate change. Here we evaluate the fluvial response in western Colorado to the PETM. Concomitant with the carbon isotope excursion marking the PETM we document a basin-wide shift to thick, multistoried, sheets of sandstone characterized by variable channel dimensions, dominance of upper flow regime sedimentary structures, and prevalent crevasse splay deposits. This progradation of coarse-grained lithofacies matches model predictions for rapid increases in sediment flux and discharge, instigated by regional vegetation overturn and enhanced monsoon precipitation. Yet the change in fluvial deposition persisted long after the approximately 200,000-year-long PETM with its increased carbon dioxide levels in the atmosphere, emphasizing the strong role the protracted transmission of catchment responses to distant depositional systems has in constructing large-scale basin stratigraphy. Our results, combined with evidence for increased dissolved loads and terrestrial clay export to world oceans, indicate that the transient hyper-greenhouse climate of the PETM may represent a major geomorphic ‘system-clearing event’, involving a global mobilization of dissolved and solid sediment loads on Earth’s surface.

Significance of channel-belt clustering in alluvial basins

The distribution of channel deposits in alluvial basins is commonly used to interpret past changes in climate, tectonics, and sea level. Here we present preliminary evidence that longtime scale (~10 3 –10 5 yr) self-organization in fl uvial systems may generate structured stratigraphic patterns spontaneously, in the absence of, or independent from, changing basin boundary conditions. A physical experiment and an ancient alluvial succession (Ferris Formation, latest Cretaceous–Paleogene, south-central Wyoming) both show stratigraphy where clusters of many closely spaced channel deposits are separated from each other by extensive intervals of overbank mudstones. Analysis using spatial point process methods shows that channel deposits in both basins are statistically clustered over intermediate basin length scales. In the experiment, external controls (base level, subsidence rate, and sediment/water supplies) were not varied, and therefore not factors in cluster formation. Likewise, the ancient system lacks stratigraphic and sedimentologic evidence of external controls on channel clustering. We propose that channel clusters, as seen in this study, refl ect a scale of flself-organization that is not usually recognized in ancient deposits. This type of internally generated stratigraphy should be considered when reconstructing tectonic, climate, and sea-level changes from