Timothy A. Cross

Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions

Most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors. Combinations of (1) rapid absolute upper-plate motion toward the trench and active overriding of the subducted plate, (2) rapid relative plate convergence, and (3) subduction of intraplate island-seamount chains, aseismic ridges, and oceanic plateaus (anomalously low-density oceanic lithosphere) cause low-angle subduction. Under conditions of low-angle subduction, the upper surface of the subducted plate is in contact with the base of the overlying plate, the wedge of low-density asthenosphere is replaced by subducted lithosphere, and the width of the arc-trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate. The fourth factor is age of the subducting lithosphere. Subduction of young lithosphere produces two opposing tendencies: (1) low-angle subduction and increased arc-trench distance, owing to its low density; and (2) decreased arc-trench distance, owing to its higher temperature. Two factors of secondary importance contribute to variation in subduction-zone geometry and arc-trench distance. Accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward. Prolonged subduction thickens the upper plate, depresses the isotherms in the subducted plate, and may create a broader arc. Both factors increase the arc-trench gap. The four primary factors also control development of other tectonic elements, such as regional subsidence (for example, the Amazon basin and a portion of the Cretaceous Interior Seaway of western United States), intra-arc extension (for example, the Basin and Range province), foreland fold and thrust belts, and Laramide-style tectonics.

Architecture and stratigraphy of alluvial deposits, Morrison Formation (Upper Jurassic), Utah

This article presents a unique cross section of a 13.5-km (8.3-mi)-long by 150-m (492-ft)-thick stratigraphic interval containing braided stream and associated flood-plain deposits. The cross section is oriented approximately parallel to depositional strike. This cross section is a resource for geoscientists and engineers interested in the measurements of stratigraphic architectural elements, such as dimensions and continuity of facies tracts and facies associations, stratigraphic and geographic changes in sandstone/mudstone proportions (net to gross), and frequency and cause of vertical fluid communication between superposed reservoirs.In addition to presenting this rich data resource, observed and documented stratigraphic relationships add to our conceptual understanding of certain attributes of the stratigraphic process-response system. For example, we show that, like meandering rivers, aggradational braided river systems also build levees and alluvial ridges, providing the supra-elevation above the adjacent floodplain to create extensive crevasse splay and channel complexes. We show that superposed channel sandstone reservoirs may be brought into physical contact not only by the erosion of an upper sand into a lower sand but also by the vertical aggradation of the lower sandstone, producing a pyramid on which a younger sandstone is deposited.Four stratigraphic cycles of increasing and decreasing accommodation/sediment supply (A/S) regimes are identified. These cycles are recognized from systematic vertical changes in stratigraphic and sedimentologic attributes. At the largest scale, there is a progressive downhill shift in facies tracts recording a basinward stepping of the four stratigraphic cycles. At the intermediate scale, there is a systematic change in channel types, from laterally amalgamated braided channels, to vertically building braided channels, to steer-head distributary channels encased within lacustrine-dominated fines. At the smallest scale, systematic and repeated vertical successions of facies occur within the three types of channel belts. These systematic changes are related to progressive changes in the A/S regime that occur during superimposed stratigraphic base-level cycles of three different scales.

Amanz Gressly’s Role in Founding Modern Stratigraphy

This paper discusses Amanz Gressly’s (1814-1865) fundamental contributions to stratigraphy in three areas: facies concepts and applications, stratigraphic correlation, and paleogeographic reconstruction. To facilitate access to his discoveries, we present an English translation of his paper (Gressly, 1838) on facies and stratigraphic correlation. We discuss excerpts from this translation which demonstrate that many of the fundamental principles of modern stratigraphy were understood and expressed by Gressly. We put this into the context of subsequent development and refinement of current stratigraphic principles. Gressly explained the genesis of sedimentary facies by processes operating in depositional environments. He demonstrated regular lateral facies transitions along beds which he interpreted as mosaics of environments along depositional profiles. He recognized the coincidence of particular fossil morphologies with particular sedimentological facies, and distinguished “facies fossils” from those which had time value and which were useful for biostratigraphy (“index” or “zone” fossils). He discussed the equivalency of vertical facies successions through a series of strata and lateral facies transitions along a bed, developing the same principle that later became known as Walther’s Law of the Correlation of Facies. He distinguished between the time value of strata and properties which reflect their genesis, and introduced specific terms to reflect this distinction. He used this understanding to show how stratigraphic successions should be correlated across different facies tracts. Gressly derived an internally consistent, logical and comprehensive definition of a new stratigraphic paradigm which was the basis for further developments and refinements. The five remaining principles of contemporary stratigraphic thought include: (a) the stratigraphic process-response system conserves mass; (b) sediment volumes are differentially partitioned into facies tracts within a space-time continuum as a consequence of mass conservation; (c) cycles of facies tract movements laterally (uphill and downhill) across the earth’s surface are directly linked to vertical facies successions, and are th e basis for high-resolution correlation of stratigraphic cycles; (d) stratigraphic base level is the clock of geologic time, and the reference frame for relating the energy of space formation with the energy of sediment transfer; and (e) facies differentiation is a byproduct of sediment volume partitioning.

Late Cretaceous sea level from a paleoshoreline

The contemporary elevation of an ancient strandline provides a measure of eustasy at a single time in the geologic past. If the elevation of a paleoshoreline has not changed since deposition (or if it has changed by a known amount), then the measured (or corrected) elevation equals the sea level at that time. The ideal location for measuring an ancient sea level is a paleoshoreline deposited high on the shoulder of a stable craton where postdepositional elevation changes through tectonic movement, lithospheric compensation to loads, and sediment compaction are minimized. The contemporary elevation of a Late Cenomanian (≈93 Ma) shoreline was determined at five localities along the tectonically stable, eastern margin of the Cretaceous Western Interior Seaway, North America. This shoreline, represented by marine-to-nonmarine facies transitions in strata of the Greenhorn sequence (UZA-2 cycle of Haq et al. (1987)), was identified from outcrop and borehole data. Biostratigraphic zonations constrained the geologic age at each locality. Sequence stratigraphic correlations, based on identifying discrete progradational units and the surfaces that separate them, were used to refine age correlations to better than 100 kyr between localities. A single Cenomanian shoreline was correlated within a single progradational unit, and its elevation was determined at five localities. This paleostrandline occurs 265–286 m above present-day sea level, at an average elevation of 276 m. Isostatic and flexural corrections were applied to remove the effects of postdepositional vertical movement, including sediment compaction by loading, uplift due to erosion, and glacial loading and rebound. Errors inherent in each measurement and each correction were estimated. Corrections and their cumulative error estimates yield a Late Cenomanian elevation of 269±87 m above present sea level. The corrected elevation approximates sea level at 93 Ma and provides a measure of Late Cenomanian eustasy prior to the Early Turonian highstand. Establishing the absolute value for eustasy at a single point in geologic time provides a frame of reference for calibrating relative sea level curves, as well as constraining the magnitudes of tectonic subsidence, sediment flux, and othervariables that controlled water depth and relative sea level.

Large-Scale Cycle Architecture in Continental Strata, Hornelen Basin (Devonian), Norway

ABSTRACT Nine large-scale stratigraphic cycles, each about 100 m thick, along the northern margin of the Hornelen basin, western Norway, record systematic expansions and contractions of alluvial-fan, braidplain, and lake facies tracts. The braidplain facies tract occupies the basin center, from time to time expanding toward the margins, and constitutes the deposits of axial or longitudinal drainage from an eastern source. The alluvial-fan facies tract, which comprises the deposits of high-gradient transverse drainages, occupies a narrow belt close to the fault-bounded basin margin. The lake facies tract is between the alluvial-fan and braidplain facies tracts. Physical correlation of strata shows that the large-scale cycles are symmetric in all facies tracts. The symmetry of the large-scale cycles, in which the base-level rise and fall hemicycles of each cycle are of approximately the same thickness, indicates that sediment accumulated in all environments during the entire base-level cycle. Absence of regional unconformities (sequence boundaries) also indicates approximately continuous sediment accumulation in the basin during base-level cycles. The base-level fall-to-rise turnaround is represented by an interval of strata, rather than by a sequence boundary, at the maximum expansion of the alluvial-fan facies tract. Condensed sections are also absent. They are represented mainly by intervals of lake strata at base-level rise-to-fall turnarounds. Strata in the base-level fall hemicycle (decreasing accommodation) accumulated as the braidplain and alluvial-fan facies tracts expanded from the basin center and the northern basin margin, respectively. Expansions of these two facies tracts filled the basin, while the lake facies tract contracted. Strata in the base-level rise hemicycle accumulated as the alluvial-fan and braidplain facies tracts contracted sourceward, respectively toward the northern and eastern basin margins, during times of increasing accommodation. This sourceward retreat of the alluvial-fan and braidplain facies tracts was coincident with expansion of the lake facies tract. At the rise-to-fall turnarounds, alluvial-fan and braidplain sediments are stored near their respective sources toward the basin margins, and the centers of mass of these facies tracts are at basin-margin positions. During base-level fall time, the centers of mass of the alluvial-fan and braidplain facies tracts migrate basinward. The alluvial-fan and braidplain facies tracts usually expand toward each other simultaneously, and then move away from each other simultaneously. This in-phase relationship is replaced by a reciprocal stacking pattern when the two facies tracts abut each other near base-level fall-to-rise turnarounds. When the two facies tracts are in contact, the higher-gradient alluvial-fan facies tract initially blocks expansion of the braidplain. As the alluvial-fan facies tract retreats toward the basin margin and fan-margin gradients are reduced, however, the braidplain expands across former fan margins. In the reciprocal configuration, the two facies tracts move in tandem in the same direction. This reciprocal pattern plus changes in aggradation-to-progradation ratio of the alluvial-fan facies tract suggest that alluvial-fan morphology changes during base-level cycles. Large-scale cycle stacking patterns show significant changes in basin-fill architecture through time, including syndepositional structural changes accompanied by changes in stratal geometries, a change from ephemeral to permanent lakes, permanent increase in alluvial-fan gradients, and permanent reduction of alluvial-fan radii and volume at the northern basin margin. Consideration of these changes and the largely in-phase relationships between facies tracts sourced by two separate sediment supplies suggest that an interplay of climate, self-regulatory, and tectonic factors controlled sedimentary accumulation within the Hornelen basin.

An Inverse Stratigraphic Simulation Model – Is Stratigraphic Inversion Possible?:

Inversion is a systematic method of determining values of process parameters of a forward model that allow a match between observed and modeled data. Historically, geologists have considered the stratigraphic record to be nonunique. That is, geologists have assumed that it is impossible to determine values for and separate stratigraphic process variables such as eustasy, tectonics and sediment supply that operated to form the stratigraphic record. If stratigraphic data are nonunique, then inversion of stratigraphic data is impossible. In an influential paper. Burton et al. (1987) argued that inversion of stratigraphic data using a stratigraphic forward model is not possible. The purpose of this study was to determine if inversion of stratigraphic data using a stratigraphic forward model is theoretically possible. In this study, we designed a stratigraphic inverse simulation model using a forward stratigraphic model capable of simulating realistic temporal and spatial distributions of fades tracts and stratigraphic surfaces. For numerical optimization, we used a gradient descent method that minimizes errors in the least squares sense. We tested this inverse model on synthetic stratigraphic data which act as a proxy for real-world stratigraphic data, to test multiple aspects of the inverse model. In these experiments, we inverted synthetic stratigraphic data for eustasy, sediment supply, tectonic subsidence, lithosphere flexural rigidity, and initial basin topography. Results from these inversion experiments establish that inversion of stratigraphic data is theoretically possible. We determined limits of convergence, degrees of parameter separatability, nonuniqueness of data, and types of data necessary for inversion. Results suggest that using distributions of facies tracts and stratigraphic surfaces within a genetic sequence stratigraphic framework is necessary for inversion. Results from inverse model experiments also suggest that nonuniqueness of these data types with respect to stratigraphic process parameters such as eustasy, tectonics, sediment supply and depositional topography is bounded. Moreover, the bounds of nonuniqueness are quite small. The next phase of our research is to first test an inverse algorithm that is more appropriate for stratigraphic inversion, and then to test an inverse stratigraphic model using a real stratigraphic data set.

Variations in Plate Kinematics and Subduction Geometries: Unifying Explanation of Mesozoic and Cenozoic Deformation in Rocky Mountains Region: ABSTRACT

The variety of late Mesozoic through early Cenozoic tectonic elements and events in the Rocky Mountains region shows temporal and spatial correspondence with inferred variations in kinematics of plate interactions and geometries of subducted oceanic lithosphere. From this space and time correspondence and current understanding of subduction processes and responses, we suggest a unified explanation for the occurrence and genesis of these features. The following tectonic elements and events are regarded as genetic expressions of variations in subduction modes and geometries: (1) the history of igneous activity in the western United States, (2) the contrasting styles and loci of deformation along the foreland fold and thrust belt (Sevier style) and the basement-cored uplifts (Laramide style) bordering the northern and eastern margins of the Colorado Plateau, (3) the development and maintenance of the Colorado Plateau as a relatively rigid tectonic block, (4) the timing and geometry of End_Page 845------------------------------ subsidence in the foreland basin, (5) the disjunct history of subsidence and subsequent uplift of the Colorado-Wyoming-Utah (CWU) region beyond the foreland basin, and (6) the initial stability and subsequent subsidence of the High Plains region. During normal subduction, thin-skinned crustal deformation was continuous opposite the convergent margin. During the ensuing period of low-angle subduction, the Colorado Plateau region was underpinned by subducted lithosphere, anomalous subsidence occurred in the CWU locus, and deformation was transferred to the position of greatest contrast in mechanical properties of the crust (the eastern and northern boundaries of the plateau). Decoupling of subducted lithosphere from overlying lithosphere caused uplift and erosional stripping of the CWU region, crustal flexure to the east, and sediment accumulation on the High Plains. End_of_Article - Last_Page 846------------