ALBERT

Description: A bstract :  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.

Description: :  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.

Description: A bstract :  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 m 3 /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.

Description: The Upper Cretaceous Ferron Sandstone Member of the Mancos Shale Formation in Utah includes coal and gas deposits and is an important outcrop analogue to study reservoir characterisation of fluvial–deltaic petroleum systems. Numerous sedimentological and sequence stratigraphic studies of the Notom fluvial–deltaic wedge have been conducted recently; however, palynological analyses had not previously been undertaken. Here, we present palynological data from 128 samples collected in the Notom wedge of the Ferron Sandstone Member outcropping in south-central Utah. The purpose of this study is to use palynological analysis to refine the broader depositional environments, evaluate the climatic setting, and to build a biostratigraphic palynological framework. The dominance of terrestrial palynomorphs, especially the high yield of moisture-loving cryptogam spores, indicates a primarily ever-wet depositional environment characteristic of hydromorphic floodplain palaeosols formed in subtropical to tropical climates. Although dinoflagellates are rare, four intervals with occurrences of marine cysts indicate periods of increased marine/tidal influence associated with previously identified flooding surfaces within Milankovitch-scale parasequences of the largely non-marine stratal succession. These flooding surfaces confirm correlations from regional high-resolution sequence stratigraphic studies and allow correlative marine parasequences and systems tracts to be extended within floodplain-dominated stratal successions. The presence of Nyssapollenites albertensis pollen places the interval studied within the Nyssapollenites albertensis Interval Zone ( Nichols 1994 ), constraining the age of the Ferron Sandstone Member to the latter part of the Cenomanian and the early Coniacian. This largely agrees with the bentonite- and ammonite-derived Turonian age proposed in previous studies.

Description: A bstract : 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.

Description: This study evaluates the proportion, length, and effective properties of thin (0.003–0.7 m [0.01–2.3 ft]) shale beds and drapes in tidally influenced channels within a compound valley fill with a focus on estimating geologically based effective rock properties. The Cretaceous Ferron Sandstone is an outcrop analog for fluvial–tidal systems with primary reservoirs being deposited as tidally influenced valley filling point bars. The study outcrops expose three valley systems in Neilson Wash of Utah. Light detection and ranging–derived digital outcrop models have been used to characterize shale length, width, thickness, and frequency of each valley fill succession. Long, uncommon, and anisotropic shales in valley 1 (V1) were deposited in a braided setting with little tidal influence. In contrast, shales in valley 2 (V2) were abundant, short, common, and equidimensional, suggesting deposition by more tidally influenced meandering rivers. Short, frequent, and equidimensional shales in valley 3 (V3) were deposited in single-thread meandering rivers with less tidal influence. A sandstone–shale model was used to estimate the effects of shales on vertical to horizontal permeability ratio ( \[{k}_{v}/{k}_{h} \] ). The unique character of each depositional unit was reflected in resultant \[{k}_{v}/{k}_{h} \] distributions. The valley fill deposits, V1, V2, and V3, had average \[{k}_{v}/{k}_{h} \] ratios of 0.11, 0.09, and 0.17, respectively. More tidally influenced reservoirs such as the studied V2 had short but frequent shales, which resulted in low \[{k}_{v}/{k}_{h} \] estimates. Estimates of \[{k}_{v}/{k}_{h} \] for valleys that predominantly contained fluvial point bar deposits with lesser tidal influence (V1 and V3) were higher. The results of this study highlight the link between shale heterogeneity, reservoir architecture, and inferred flow parameters.

Description: A bstract :  Facies architectural analysis of a river-dominated deltaic parasequence in the Turonian Ferron Sandstone Member, near Factory Butte, Utah, was conducted to determine the hierarchical spatial arrangement of mouth bars and their role in constructing the delta. Five architectural elements were identified using 21 measured sections, paleocurrent measurements, and bedding diagrams. They are: (1) prodelta (PF), (2) frontal splay (FS), (3) detached distal bars (DBs), (4) distributary mouth bars (DMBs), and (5) terminal distributary channels (TDCs). Lower in the parasequence, detached bars consist of proximally located cross-bedded and quasi-parallel laminated subaqueous bars that onlap the delta-front slope and grade distally into finer-grained distal delta front splays. These bars translate downstream in a progradational–aggradational–degradational trajectory and are overlapped by adjacent bars, forming a bar assemblage. These bars are interpreted to have been deposited by descending hyperpycnal flows as a decelerating axial jet. Deposition of these bars occurs at the delta-front to prodelta chokepoint, presumably due to flow deceleration. The distal bar assemblage is capped by an extensive but discontinuous shale drape, suggesting local abandonment. Upper facies assemblages show a distinct switch to predominantly cross-bedded distributary mouth bars, corresponding to a decrease in water depth, an increase in friction, and a likely transition into planar versus axial jets. This is indicated by aggrading cross-bedded distributary mouth bars that overlie the distal bars. Laterally, the mouth-bar deposits are interstratified with terminal distributary-channel deposits with prominent scour surfaces at the base. Separate distributary channels and their bars coalesce, ultimately to build the entire delta lobe. The detached distal bars prograde locally west to northwest and lack fair-weather wave-formed sedimentary structures, suggesting a local embayment along the regionally northeast prograding shoreline. Low levels of ichnofaunal diversity indicate a stressed, river-dominated setting. At the base of the section, frontal splays and detached bars, dominated by storm-driven river-flood deposits, are deposited over muddy prodelta facies. Detached bars are likely initiated about 500 meters distal of the distributary channel, in water depths of ~ 10–30 meters, based on the thickness of the parasequence. Once this subaqueous platform is constructed by the distal bar assemblage, more proximal distributary mouth bars fill the remaining shallow-water accommodation. As the system continues to shallow, distributary channels override and scour the bars, reflecting reduced accommodation.