Notom Delta Research Articles

Correlative conformity or subtle unconformity? The distal expression of a sequence boundary in the Upper Cretaceous Mancos Shale, Henry Mountains Region, Utah, U.S.A.

ABSTRACT In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability. Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall. Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.

Sequence stratigraphic interpretation in marginal marine settings by the approach of parasequence-thickness-to-sandstone-fraction ratio: Case studies of the Gallup and Ferron outcrops in the Western Interior Basin, U.S.A.

ABSTRACTThe parasequence-thickness-to-sandstone-fraction ratio (TSF) is a simple but potentially powerful tool to identify stratal stacking patterns and associated systems tracts in siliciclastic paralic sequences. Parasequence thickness (T) reflects accommodation, and the sandstone fraction (SF) may serve as a proxy for the rate of sediment supply. Although previous research shows the effectiveness of applying TSF techniques to sequence stratigraphic analyses in siliciclastic depositional environments, constraints and analytical procedures of the technique are still not clearly illustrated. The paleogeography and sequence stratigraphy of the Cretaceous Gallup system and Ferron Notom deltaic complex in the Western Interior Basin have been extensively studied, providing an opportunity to explore the applicability and detailed workflow of the TSF method. TSF analyses are conducted first on two representative measured sections from the Gallup and the Ferron outcrops, respectively. The TSF analyses of the two one-dimensional (1D) sections are capable of identifying transgressive–regressive (T-R) cycles of the two deltas. However, the 1D sections can reflect the accommodation and sediment supply change only at single locations, which limits their usefulness in identification of stratigraphically consecutive parasequences and composite bounding surfaces. The utility of TSF analyses on cross sections is then tested on a depositional-dip transect of the Gallup delta, as well as dip-oblique and strike-oblique cross sections of the Ferron Notom delta. Parameters of T and SF are acquired from measured sections as well as interpolated virtual sections. For both the Gallup system and the Ferron Notom deltaic complex, the cross-sectional TSF analyses are more effective in recognizing systems tracts and associated bounding surfaces than the TSF analyses in 1D sections. Because dip-oriented cross sections usually encompass both proximal and distal parts of parasequences, and preserve stratigraphically continuous successions, they can provide more complete information for TSF interpretations than strike-oriented cross sections. Above all, TSF analysis via both measured and virtual sections along a depositional-dip profile is inclined to result in sequence stratigraphic categories that best match those based on the full set of geological observations.TSF analyses can also be used to identify general direction of shoreline trajectories in both the Gallup and Ferron Notom deltas. Parameterization of T, SF, and maximum progradation distance for both the Gallup and Ferron Notom parasequences are used to indicate shelf gradients throughout their deposition. The prominent differences of the shelf gradients between different deltaic parasequences indicate differences in allogenic and autogenic controls on the development of T-R sequences in these two deltas.

Facies architecture and time stratigraphic relationships of a confined trunk-tributary valley fill and unconfined fluvial system in the backwater of the Turonian Ferron-Notom Delta, Utah, U.S.A.

ABSTRACTThis paper documents a tidally incised, mudstone-prone tributary valley fill linked to a trunk valley in the backwater limit of the Turonian Notom Delta of the Ferron Sandstone Member, Utah. High-resolution 3D photogrammetry models were used to correlate a 20-m-deep valley between 32 measured sections over a 1 km2 area. A GPS survey and GIS geostatistical tools were used to restore the morphology of the tributary valley. The restored valley floor is interpreted as a surface of tidal erosion, based on the overlying facies and surface morphology. Morphological similarities exist between this tributary valley and modern analogs observed in northern Australia, the Memramcook tributary in the Bay of Fundy, and Pleistocene sediments in the Gulf of Thailand.40Ar/39Ar dating of sanidine crystals using multi-collector mass spectrometry allow for a re-evaluation of depositional rates and timing of 32 fluvial aggradation cycles (FACs) and 9 fluvial-aggradation cycle sets (FAC sets) in this sequence. The new dates show that the entire sequence was deposited in 15 ± 5 kyr, and show that Milankovitch cycles cannot account for the internal complexity of this fluvial stratigraphy, indicating likely autogenic control of the FAC sets. The lateral extent of FACs in floodplain deposits mapped in outcrop are correlated over tens to hundreds of meters, and scale to estimated channel widths reflecting the autogenic control. FAC sets can be correlated for up to 10 km along depositional strike, which suggest controls unrelated to the dynamics of individual channels and may show some elements of allogenic climate-driven processes.

Stratigraphy of the Fluvial-To-Marine Transition Zone Associated With A Forced-Regressive Compound Incised-Valley System In the Turonian Ferron Notom Delta, Utah, U.S.A.

Abstract This paper examines facies and stratigraphic relationships at the tip-out of a sequence boundary along the fluvial-to-marine transition zone of the Cretaceous Ferron Notom delta, Utah. Correlation of 11 measured sections shows that the sequence boundary underlies amalgamated fluvial deposits in more proximal parts of the valley, whereas the valley fill splits into a pair of fluvial channel facies separated by transgressive shoreface deposits in the transition zone. This indicates at least two episodes of downcutting and valley filling, and suggests a compound valley system. Thick sandy deltaic deposits are interpreted as the falling-stage and lowstand deposits farther downdip, fed by the rivers that cut the valley. Paleohydraulic calculations indicate that the valley-forming trunk channels were on the order of 5.5 m deep, and carried coarse sand with rare pebbles over a slope on the order of 0.0005. Corresponding backwater lengths were on the order of 10 km. Recent models show that channels in the backwater zone should experience greater levels of aggradation, versus rivers landward of the backwater, which may show greater lateral migration or incision, and these predictions are matched by our observations. In addition, tidal backwater effects are predicted to be on the order of a few kilometers. The valley-fill shows increasing aggradation and tidal facies in sections a few kilometers from the mapped shorelines, which suggests a short backwater length. The relatively short backwater also allows the rivers to bring coarser sand and gravel close to the shoreline.

Re-evaluating the Paleogeography of the River-dominated and Wave-influenced Ferron Notom Delta, Southern Central Utah: an Integration of Detailed Facies-architecture and Paleocurrent Analysis

Abstract This study re-evaluates the paleogeography of a river-dominated and wave-influenced deltaic system in a single parasequence (6a) of the Cretaceous Ferron Sandstone, south-central Utah, which was previously interpreted as a typical asymmetric delta. The conceptual asymmetric-delta model of Bhattacharya and Giosan (2003) has been widely applied to interpret ancient deltaic systems. However, reconstructed paleogeographies relying on this conceptual model are commonly not scaled to the architectural elements, which could possibly lead to inaccurate interpretations, especially when data are relatively sparse. The extensively exposed Ferron Sandstone outcrops provide an opportunity to demonstrate how detailed facies-architecture work can be integrated with regional facies relationships to elucidate a properly scaled paleogeography. Seven facies associations (FA) are identified in parasequence 6a and are arranged into two distinctively different successions. The river-dominated succession consists of prodelta, distal delta-front, proximal delta-front, and terminal-distributary-channel FA, while the wave- and storm-dominated succession comprises offshore-shelf, lower-shoreface, and upper-shoreface FA. The river-dominated succession is muddier and less bioturbated than the wave- and storm-dominated succession. Facies-architecture analysis shows that the river-dominated proximal delta-front facies consists of mouth-bar and terminal distributary-channel elements that are not seen in the wave- and storm-dominated facies. The scales of mouth bars and terminal distributary channels suggest that they are rather small features compared with the river-dominated delta lobe, which is incompatible with the simple bar-elongation asymmetric-delta model presented by Li et al. (2011). Instead, parasequence 6a is interpreted to be a valley-fed, river-dominated symmetric delta protected from waves by an updrift barrier system, which is analogous to the modern Mongky river delta on the west coast of Madagascar. The progressive change in orientation of west-directed mouth bars, to north-directed and east-directed lobes around the periphery of the river-dominated delta, indicates a highly radial Lafourche-type delta. Moving away from the protection of the barrier system, there is a gradual change from highly river-dominated facies to increasingly wave- and storm-influenced facies toward the southeast. Due to the entrenched nature of the feeder river associated with an incised-valley system, the parasequence 6a delta was bifurcation-dominated rather than avulsion-dominated. This study shows that it is critical to integrate detailed facies-architecture analysis in order to reconstruct properly scaled paleogeographies and hypothesizes that stable bifurcation-dominated deltas should be more common than avulsion-dominated deltas in a lowstand delta system due to the entrenched nature of the feeding trunk river, which inhibits avulsion.

Basin‐axial progradation of a sediment supply driven distributive fluvial system in the Late Cretaceous southern Utah foreland

AbstractThe Turonian‐Coniacian Smoky Hollow Member of the Straight Cliffs Formation in the Kaiparowits basin of southern Utah records a stratigraphic transition from isolated fluvial channel bodies to increasingly amalgamated channel belts capped by the Calico bed, a sheet‐like sand‐gravel unit. Characteristics of the Smoky Hollow Member are consistent with a prograding distributive fluvial system including: up‐section increases in average grain size, bed thickness, channel‐body amalgamation, a fan‐shaped planform morphology and a downstream increase in channel sinuosity. The system prograded to the northeast based on thickness and facies patterns, and palaeocurrent indicators. This basin‐axial sediment‐dispersal trend, which was approximately parallel to the fold‐thrust belt at this latitude, is supported by provenance data including detrital zircons and modal sandstone compositions indicating sediment derivation mainly from the Mogollon Highlands and Cordilleran magmatic arc to the southwest, with episodic input from the more proximal Sevier fold‐thrust belt to the west. Progradation occurred during a eustatic still‐stand, relatively stable climatic conditions, and continuous tectonic subsidence, thus suggesting increased extrabasinal sediment supply as a primary control on basin‐fill. Progradation of the Smoky Hollow Member fluvial system culminated in a ~2–3 My hiatus at the top of the lower Calico bed. Correlation with the Notom delta of the Ferron Sandstone, 80 km northeast in the Henry basin, is proposed on the basis of facies relationships and geochronology. The Calico bed unconformity is linked to regional tectonically driven tilting and erosion observed in both basins.

Facies analysis and its relation to point-sourced growth faults in river-dominated prodeltaic delta front deposits of the Cretaceous Ferron Notom Delta, Utah, USA

This study evaluates the evolution of a small-scale growth fault within a deltaic parasequence of the Ferron Sandstone, Utah. Analysis is based on 4 stratigraphic logs, and a panorama covering 100 m of lateral exposure.Five growth-fault blocks are displayed in strike and dip views. The listric faults dip basinward, and sole out into bedding parallel decollement surfaces associated with deformed prodelta shales, which also form diapirs. Throw on individual faults averages 4 m and heave increases with depth from ∼2.5 to ∼5 m. Strikes on fault planes average 160°, and fault dip decreases with depth from 50° to 0°, where they become parallel to bedding.Pre-growth facies comprise heterolithics with abundant graded beds, current-ripples, wave-ripples, small-scale soft sediment deformation, and a low bioturbation index. This indicates deposition in a river-dominated, storm-influenced delta that was supplied primarily by storm-triggered flood events and prograded rapidly. Growth strata are primarily planar and quasi-planar fine sandstones deposited as upstream and downstream accreting mouth bars.Faulting initiates with the building of a “critical load” reflecting a low-angle delta front foreset, probably linked to deposition in a mouth bar. Mobile prodelta muds form diapirs that breached the sediment-water interface, providing accommodation for the fault blocks. Termination of faulting occurred when sandy growth strata contacted and locked into higher strength pre-growth heterolithics deeper in the section. Minor deformation of growth strata is demonstrated by synthetic microfaulting in growth sands.This study emphasizes a localized stress field (i.e., point sourced) associated with sandy mouth bar deposits, which acted as fault initiation points. This contrasts with the distributed stress regimes commonly associated with regional-scale growth faults along continental margins. This can lead to expanded, but localized, reservoir compartments within river-dominated deltaic systems, especially those in epeiric seas or other shallow-water sedimentary basins, such as Prudhoe Bay Field, Alaska.

Plan-view Paleochannel Reconstruction of Amalgamated Meander Belts, Cretaceous Ferron Sandstone, Notom Delta, South-central Utah, U.s.a.

Abstract Few studies reconstruct paleohydraulic parameters, such as meander amplitude and sinuosity, of ancient rivers based on plan-view outcrop exposures, but rather rely on interpolation from cliff exposures. In this study, extensive plan-view exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, Utah, USA, allow paleohydraulic reconstruction, evaluation of grain-size variability in meander belts, and their component unit and compound bars by incorporating the variability of paleocurrents, facies architectural-element analysis, and bar migration patterns. Three channel belts were identified, marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest channel was about 2.1 m deep and 118 m wide, and had a meander wavelength of 1 km and a sinuosity of 1.1–2.9. The middle channel was about 3.4 m deep, with a meander wavelength of 2.6 km, and a sinuosity of 1.2. The oldest channel was insufficiently exposed to document its plan-view style. Grain-size trends showed systematic upwards fining in each channel story. In plan view, coarsening occurs towards the bend apex along the bend axis, and fining occurs downstream within some meander scrolls. Plan-form grain-size trends were not obvious, partially due to grain-size variation within unit bars, perhaps also reflecting the fact that the exposures capture variable vertical position in each channel belt. Paleocurrents showed systematically varying trends within individual channel belts (from NE, to SW, to E), and abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop shows a dominance of a meandering-river style, with significant lateral amalgamation of successive point bars within each belt. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, and are probably associated with channel abandonment. Independent measurements of meander wavelength, based on plan-view exposures, match results estimated from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Qw = 1.6 ∼ 2.8 × 102 m3/s). Architectural-element analysis shows that the last stage, and the most sinuous channel belt (A1), filled largely with downstream accreting unit and compound braid bars, similar to those seen in high-sinuosity meander loops of the modern Red River at the Texas–Oklahoma border. This suggests that both braid bars and point bars may occur within a single meander channel and that braid bars may evolve into compound point bars as they migrate and attach to a channel margin.

Thin-bedded reservoir analogs in an ancient delta using terrestrial laser scanner and high-resolution ground-based hyperspectral cameras

Ground-based terrestrial laser scanning and hyperspectral sensors were used to image fine-scale heterogeneity in outcrops of prodeltaic heterolithic facies of Parasequence 6 of the Cretaceous Ferron Notom delta in Southern Utah. Previous work shows that Parasequence 6 is an upward coarsening fluvial-dominated, wave-influenced deltaic deposit containing heterolithic thin-bedded facies representing distal delta front and proximal prodelta environments. Primarily, the thin beds have been interpreted as turbidites, storm beds (tempestites), and hyperpycnites. These deposits represent analogs for thin-bedded unconventional pay zones that lie at the margins of conventional deltaic sandstone reservoirs.The terrestrial laser scanner was used to create a centimeter- to decimeter-scale, digital representation of the outcrops in three dimensions. Hyperspectral sensors record electromagnetic radiation reflected off the outcrops in 840 contiguous bands, which were then used to generate a spectral signature for each pixel sampled. The spectral signatures are a function of mineralogy, chemistry, surface alteration, grain-size, and cements, and were used to distinguish thin mudstones from sandstones within an interbedded succession at the base of a deltaic parasequence. Comparison between the spectral signatures recorded from the outcrop and those of reference materials, and with previous facies architecture studies, enables lithofacies to be identified and subsequently accurately mapped. Hyperspectral data are then draped over the terrestrial laser scanner model to generate a spatially-accurate detailed three-dimensional (3D) geologic map of the heterogeneity.Approximately 100m of outcrop was imaged laterally with the hyperspectral camera and terrestrial laser scanner on the previously mapped distal delta front and prodeltaic facies of Parasequence 6. Bed thickness data, based on measurements made along depositional dip versus strike, show that bed geometries are anisotropic. Reconstruction of the plan-view geometry shows that the thin-beds are lobate to elongate in plan-view profile and extend over distances of about 1km, indicative of the dispersive waning sediment transport processes found in storm-influenced fluvial-dominated deltas. The lobate geometry suggests that beds are laterally discontinuous, with a width to thickness ratio on the order of <10,000 (i.e. a 10cm bed extends for <1km). The combined terrestrial laser scanner and hyperspectral data provide continuous 3D maps of grain-size and lithology. The maps which emphasize the continuity and dimensions of thin beds are among important factors which impact reservoir behavior.

Sequence Stratigraphic Analysis of the Youngest Nonmarine Sequence In the Cretaceous Ferron Notom Delta, South Central Utah, U.S.A.

Abstract This study evaluates the nature and internal complexity of high-frequency fluvial aggradational cycles and linked paleosols that were previously not recognized in nonmarine sequences of the Turonian Ferron Notom Delta in Central Utah. Detailed facies analysis of the ∼ 33-meter-thick youngest nonmarine sequence (Sequence 1) reveals 11 depositional facies that build 33 fluvial aggradational cycles, 9 fluvial aggradation cycles sets, and 3 sequences, in what was previously classified as a single 100 ka, 4th- or 5th-order sequence. Fluvial aggradation cycles in the succession are either simple floodplain bedsets or single channel stories that are bounded by paleosols or erosional surfaces. They represent depositional events that likely do not span more than 2 ka. Depositional periods are followed by short hiati (10 to < 400 a) characterized by subaerial exposure, and soil development, possibly reflecting floodplain abandonment or short-term river avulsions. Fluvial aggradational cycle sets show systematic stacking of fluvial aggradational cycles, and are analogous to parasequence sets. They are represented by either compound floodplain bedsets or multi-story channel sandstones, bounded by relatively mature paleosols or erosionally top-truncated. They represent multiple small-scale depositional events that span not more than 14 ka followed by relatively long (100 a to < 1 ka) hiati, characterized by subaerial exposure and soil development, which suggest a relationship to longer-term floodplain abandonment, due to either longer-term avulsion or fluvial entrenchment during base-level falls. Fluvial aggradational cycle sets are often capped by coals and carbonaceous strata. Marine trace fossils and dinocysts above coal beds suggest marine transgression due to compactional subsidence or sea-level rise. The scale of cyclicity of the fluvial aggradation cycle sets (14 ka) is similar to underlying marine parasequences (10 to 15 ka) and suggests that nonmarine and marine cycles have a similar control. Fluvial sequences are aggregates of fluvial aggradational cycle sets that are bounded by unconformities or their correlative conformities that represent longer periods (1 to 5 ka) of subaerial exposure and pedogenic development. Cyclicity on the sequence scale in this succession is attributed to high-frequency Milankovitch-scale (∼ 20 ka, 40 ka) rises and falls in sea level.

Evaluating along‐strike variation using thin‐bedded facies analysis, Upper Cretaceous Ferron Notom Delta, Utah

AbstractThin‐bedded delta‐front and prodelta facies of the Upper Cretaceous Ferron Notom Delta Complex near Hanksville in southern Utah, USA, show significant along‐strike facies variability. Primary initiation processes that form these thin beds include surge‐type turbidity currents, hyperpycnal flows and storm surges. The relative proportion of sedimentary structures generated by each of these depositional processes/events has been calculated from a series of measured sedimentological sections within a single parasequence (PS6–1) which is exposed continuously along depositional strike. For each measured section, sedimentological data including grain size, lithology, bedding thickness, sedimentary structures and ichnological suites have been documented. Parasequence 6–1 shows a strong along‐strike variation with a wave‐dominated environment in the north, passing abruptly into a fluvial‐dominated area, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave‐dominated environment further to the south‐east. The lateral facies variations integrated with palaeocurrent data indicate that parasequence 6–1 is deposited as a storm‐dominated symmetrical delta with a large river‐dominated bayhead system linked to an updip fluvial feeder valley. This article indicates that it is practical to quantify the relative importance of depositional processes and determine the along‐strike variation within an ancient delta system using thin‐bedded facies analysis. The wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents (i.e. surge‐type turbidity currents and hyperpycnal flows) and storm surges, and shows that there are significant interactions and linkages of these often paired processes.

Enhanced bioturbation on the down‐drift flank of a Turonian asymmetrical delta: Implications for seaway circulation, river nutrients and facies models

AbstractExtant models predict delta front environments down‐drift of river mouths as unfavourable for organisms because of the physico‐chemical stresses caused by sediment and fresh water influx. This study, however, finds evidence for near‐optimal living conditions down‐drift of contemporaneous mouth bars and distributary channels, as well as at the tops of abandoned lobes, in part of the asymmetrical ‘Notom Delta’ complex of the Ferron Sandstone (Turonian, south‐eastern Utah, USA). Presented herein is a sedimentological and ichnological model using thirty‐two detailed measured sections along a 16 km transect through two continuously exposed, ca 10 m thick allomembers containing delta front, mouth bar and distributary channel facies. Azimuths from sedimentary structures show south‐eastward deflection of near‐shore palaeocurrents relative to the inferred north–south shoreline, as well as minor reversal of flow. Two end‐member trace fossil suites are recognized in delta front sandstones: (i) a stressed suite of low abundance, low diversity, diminutive traces reflecting mobile deposit feeding, resting and locomotion behaviours; and (ii) a comparatively unstressed, high abundance, moderate diversity suite with a regular, heterogeneous distribution of deep, vertical or U‐shaped suspension‐feeding burrows which, in places, thoroughly homogenize the sandstones. The down‐drift side of the delta was colonized by suspension feeders during seasonal reversal of the seaway gyre when mud plumes were swept northward or when river‐derived nutrients were sufficiently concentrated relative to fresh water and sediment input. During normal seaway circulation, very high sedimentation rates and mud‐laden, wave‐dampened waters down‐drift of the river mouths heightened the preservation potential of the pervasively bioturbated facies. Up‐drift of the river mouths, these bioturbated facies were either not preserved or not developed until the lobe was abandoned. This alternative model for delta planform asymmetry contributes to the refinement of facies models for deltaic systems and provides a framework for predicting the distribution of bioturbation‐enhanced porosity and permeability in lobe‐deflected deltas.

Paleo-channel reconstruction and grain size variability in fluvial deposits, Ferron Sandstone, Notom Delta, Hanksville, Utah

Planform meanderbelt exposures display the evolution of the channel which leads to better understanding of three dimensional architecture of the fluvial deposit. Due to the paucity of extensive plan-view exposures in rock record, reconstruction of paleo channels from plan form exposures has been hardly studied. This study combines closely spaced paleocurrent and grain size data from plan-view exposures of point bar deposits in the delta plain deposits of the Cretaceous Ferron Sandstone near Hanksville, Utah, USA. Airborne Light Detection and Ranging (LiDAR) was used to create hillshade images in which numerous crescent shaped sandstone ridges were identified. These are dominated by unidirectional paleocurrent and thus they are interpreted as scroll bars associated with four distinct meander loops. The loops show a combination of growth by lateral expansion and downstream translation. Cross-cutting relationships allow us to decipher the order of loop formation. Paleocurrent directions closely follow the shape of individual scroll bars. Plan-view grain size distributions show a coarsening trend toward the bend apex on individual scroll bars. However, at the scale of an entire meander loop, this trend is less prominent. Paleohydraulic estimates show that these channels have low sinuosity (1.3–1.7) and have an average discharge ranging from 42.1 to 66.4m3/s. Comparison with larger sandbodies in underlying valley systems suggests that the studied channels represent the upper delta plain distributary channels and indicate an overall backstep with respect to channels in the immediately underlying incised valley.

Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A

Abstract Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges–Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth. Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a single-story body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.

Paleohydrology and 3D Facies Architecture of Ancient Point Bars, Ferron Sandstone, Notom Delta, South-Central Utah, U.S.A

Abstract Most outcrops of fluvial deposits consist of a series of cliff exposures, either natural or manmade (e.g., roadcuts). Predictions, and especially observation of plan-form geometry, such as might be made with numerical experiments or from studies of modern rivers, are challenging to test in most ancient outcrops. This study examines ancient exhumed channel belts from the Cretaceous Notom Delta of the Ferron Sandstone Member in south-central Utah. Extensive plan-view exposures with local vertical cliff exposures allowed documentation of channel plan-form, channel-belt dimension, bar migration patterns (translation versus expansion), and cross-sectional facies architecture. Channel-fill thickness and bedding structure, documented from the cliff exposures, were used in paleohydraulic reconstructions. Approximately 270 paleocurrent directions were integrated with grain-size measurements to reconstruct the 3D facies architecture. Paleocurrent measurements are consistent within specific facies architectural units (such as unit bars) and show systematic variation at the channel-belt scale that can be used to infer channel and bar migration patterns. In this example, the migration pattern of the single-thread channel bend was interpreted to change from expansion to translation with a corresponding bend sinuosity that increased from 1.01 to 1.44. Inclined large-scale foresets are interpreted to be indicative of unit bars. Empirical equations result in estimated average channel depths from 1.7 m to 3.6 m with corresponding widths of 23 m to 89 m respectively. These empirical estimates match the dimensions measured in the field. For example, channel widths of around 50 m were measured from abandoned channel fills. Bar thickness, measured from vertical outcrops, ranges from 5.4 m to 6.3 m, which yields a narrower estimate of channel width ranging from 47 to 59 m. Integration of sediment size, bedforms type, and channel depth were used to estimate the discharge of the river, which is on the order of < 400 m3/s. This suggests that the Ferron deltas were characterized by small, steep-gradient “dirty” rivers, which is consistent with the hyperpycnal nature of linked downstream deltaic systems.

Anatomy of falling‐stage deltas in the Turonian Ferron Sandstone of the western Henry Mountains Syncline, Utah: Growth faults, slope failures and mass transport complexes

AbstractFalling‐stage deltas are predicted by sequence stratigraphic models, yet few reliable criteria are available to diagnose falling‐stage deltaic systems in surface exposures. Recent work on the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains Syncline of south‐central Utah has established its environment of deposition as a series of modest‐sized (5 to 20 km wide), probably asymmetrical, mixed‐influence deltas (‘Ferron Notom Delta’) that dispersed sediment eastwards from the rising Sevier orogenic hinterland into the Western Cordilleran Foreland Basin. Analysis of sandstone body stacking patterns in a 67 km long, depositional strike‐parallel (north–south) transect indicates that the growth of successive deltas was strongly forced by synsedimentary growth of a long wavelength (ca 100 km), 50 m amplitude fold structure. Herein, two discrete areas within this transect, superbly exposed in three dimensions, are documented in order to determine the details of stratal stacking patterns in the depositional dip direction, and thereby to assess the stratigraphic context of the Ferron Notom Delta. In the two study areas, dip transects expose facies representing river mouth bar to distal delta front environments over distances of 2 to 4 km. Key stratal packages are clinothems that offlap, downlap, and describe descending regressive trajectories with respective to basal and top datums; they are interpreted as the product of relative sea‐level fall. The vertical extent of clinoforms suggests that deltas prograded into &lt;30 m of water. Furthermore, these deltaic successions preserve abundant evidence of delta front slope failure, growth faulting, and incision and filling of deep (&lt;15 m) slope gullies. Gully fills are composed of chaotic intraformational breccia and/or massive sandstone, and constitute linear, ‘shoestring’ sandbodies in the distal portions of individual palaeodelta systems. They are interpreted to have been cut and filled during the late falling‐stage and lowstand of relative sea‐level cycles. The north–south distribution of the stratal style described above seems to be focused on the flanks of the growth anticline, and so the numerous falling‐stage systems tracts preserved within the Ferron Notom Delta probably owe their origin to synsedimentary structural growth, and the unstable fluid pressure regime that this growth imposed on the sea floor and shallow subsurface.

High-frequency and low-amplitude relative sea-level changes in the Turonian Ferron Notom Delta, Henry Mountains region Utah, USA: implications for sequence stratigraphy and hydrocarbon exploration

The Turonian Notom Delta is one of the Ferron fluvial-deltaic wedges deposited in the foreland basins of the Cretaceous Seaway of North America. The wedge is exposed three dimensionally in the Henry Mountains region Utah, USA. High-resolution sequence stratigraphic analysis along a 35 km dip-oriented outcrop belt shows that the wedge consists of six depositional sequences. Ammonite and Inoceramid biostratigraphy and 40Ar/39Ar age dating of various bentonites show that the wedge was deposited from 91.25 Ma to 90.63 Ma, an interval of merely 0.62 Ma. Assuming each sequence is formed over a similar time span, each sequence, thus, represents about 105 yr, suggesting high-frequency depositional sequences. Amplitude of relative sea-level changes, built on shoreline trajectory, is 2–61 m, with an average of only 20 m. Fluvial incision during high-frequency and low-amplitude relative sea-level fall is mainly around the highstand prism. Incision diminishes rapidly up dip and down dip, and the resultant incised valleys are narrow and shallow. Such sea-level fluctuations also result in small, stratigraphically complex shoreline sandstones that need special attention during hydrocarbon exploration.

Facies-Architecture Study of A Stepped, Forced Regressvie Compound Incised Valley In the Ferron Notom Delta, Southern Central Utah, U.S.A

Abstract Detailed facies-architecture study of a portion of a > 10-kilometer-wide compound incised valley in outcrops of the Turonian Ferron Notom Delta shows three simple incised valleys filled with different facies associations. The valley-fill deposits were mapped with 30 detailed measured sections, photomosaics, and bedding diagrams. The oldest, incised valley 3, forms a terrace deposit that erodes directly into hummocky-cross-bedded lower-shoreface deposits. It is filled with fine- to medium-grained tidally influenced deposits, characterized by abundant mud-draped cross beds, sparse burrowing, and a bimodal paleocurrent-direction pattern. The younger incised valley 2 locally cuts through the valley 3 terrace and also into the lower-shoreface deposits. The valley 2 fill comprises multi-story medium-grained fluvial deposits with minor finer-grained tidally influenced fluvial deposits in the upper 10%. The youngest, incised valley 1, also locally cuts into the lower-shoreface deposits, removing the terraces formed by valleys 2 and 3. Valley 1 is filled entirely with medium-grained fluvial deposits. The composite valley fill records generally increasing fluvial dominance and decreasing tidal influence during successive cut-and-fill episodes associated with each simple valley fill. It is hypothesized to correlate with a longer-term, stepped relative fall of sea level, punctuated by standstills, or minor rises of sea level.

Milankovitch-Scale Sequence Stratigraphy and Stepped Forced Regressions of the Turonian Ferron Notom Deltaic Complex, South-Central Utah, U.S.A

Abstract Laterally extensive outcrops in south-central Utah of the Turonian Ferron Notom Delta Complex and its bounding shales contain numerous chronometrically age-dated bentonite beds. The chronometry enables estimation of rates of accommodation and accumulation change. Integration of 56 measured sections and walking and tracing of beds allowed rigorous application of the concept of shoreline trajectory, using the lower bentonites as stratigraphic datums. Geometric stratal relationships along with facies dislocations were used to define high-frequency sequences and systems tracts. Forty-three parasequences were grouped into 18 parasequence sets, five simple sequences, and one sequence set. Parasequence sets show repeated, aggradational–progradational, degradational, progradational–aggradational, and retrogradational stacking patterns, which match the recently developed accommodation succession model. The chronometric analysis shows that the high-frequency sequences are Milankovitch-scale (approximately 100,000 yr), and record sea-level falls of up to 50 meters. The most complete sequence set, sequence set 3, shows clear evidence of a prolonged and stepped forced regression, with two additional higher-frequency sequences (possibly 40 kyr cycles), forming a forced regressive sequence set. This matches previous observations that valley systems, associated with the sequence boundaries, are compound, and also record higher-frequency cut-and-fill cycles. Among the various controlling factors causing cyclicity, such as shoreline autoretreat, delta-lobe switching, and allogenic controls such as climate, tectonics, and eustasy, we hypothesize that glacio-eustasy is the most likely control on the cyclicity of Ferron deposition, overprinted on a tectonically active basin. Gradual build-up of glaciers are hypothesized as a possible explanation of the punctuated downsteps observed in the forced regressive sequence set 3.

Stratigraphic uncertainty in sparse versus rich data sets in a fluvial-deltaic outcrop analog: Ferron Notom delta in the Henry Mountains region, southern Utah

The Turonian Notom delta is one of the Ferron fluvial-deltaic wedges deposited in the Cretaceous foreland basin of North America and is well exposed in the Henry Mountains region, Utah. This article uses variable sampling of measured sections of the fluvial-deltaic wedge along a 19-mi (30-km) outcrop belt to address correlation uncertainties in sparse data sets, such as are common in subsurface studies. A high-resolution sequence-stratigraphic analysis based on 58 measured sections, extensive walking out of beds, and field photomosaics shows that the wedge is composed of 43 parasequences, 18 parasequence sets, and six depositional sequences. Resampling the outcrop belt with 13 sections provides a sparse data set. Correlations using the data set show considerable uncertainties. Fluvial complexity is greatly underestimated, and in the extreme case, only 40% of the fluvial bodies are identified. Overcorrelation results in as much as 400% exaggeration of sandstone width and thickness. Some of the uncertainties can be reduced by incorporating nonmarine sequence-stratigraphic models and various techniques that allow a more reasonable evaluation of fluvial body geometry. With sparse data, uncertainties also arise in marine successions. A key challenge is to identify and correlate the small-scale units, particularly forced regressive deposits. Many of their diagnostic features that can be easily recognized from outcrops and cores may not be as easily detected from well logs and on seismic data. In subsurface studies where sparse data are observed, integrating detailed facies analysis cores and sequence-stratigraphic concepts is critical to predict the occurrence and understand the architecture of such deposits.