Book Cliffs Research Articles

Re-evaluating the sedimentology and sequence stratigraphy of classic Book Cliffs outcrops at Tusher and Thompson canyons, eastern Utah, USA: Applications to correlation, modelling, and prediction in similar nearshore terrestrial to shallow marine subsurface settings worldwide

Abstract The Campanian Book Cliffs strata of eastern Utah and western Colorado has been used to develop, test and refine sedimentological and stratigraphic ideas and models over the years, including the principles and concepts of sequence stratigraphy. Book Cliffs data sets are often used as outcrop analogues for similar subsurface settings worldwide. Thousands of geologists have visited the Book Cliffs, including the archetypal Desert Member-to-Lower Castlegate Sandstone outcrops at Tusher and Thompson canyons, which are the focus of this study. These localities regularly serve as representative field stops for teaching sequence stratigraphy. Recent work has begun to cast doubt on certain aspects of the conventional sequence stratigraphic interpretation of the Desert-Castlegate interval, hence re-evaluation of these classic outcrops is warranted. A progradational stack of shallow marine to nearshore terrestrial facies are observed at Tusher Canyon. From bottom to top these include shelf mudstones, lower shoreface-to-inner shelf heterolithics, foreshore-to-upper shoreface sandstones cut by distributary and tidal-estuarine channels, coal-bearing coastal plain mudstones with single-storey fluvial sandstones, amalgamated multi-storey fluvial channel-fills, and a capping coastal plain mudstone package overlain by the marine mudstones of the Buck Tongue of the Mancos Shale. A similar progradationally-stacked facies succession is observed 30 km basinwards at Thompson Canyon, with the exception of the proximal amalgamated fluvial sandstone package, which is absent. Five of eleven Desert Member parasequences, D1 to D5, are noted at Tusher Canyon, while all eleven Desert parasequences, D1 to D11 and two of six Castlegate parasequences, C1 and C2, are identified at Thompson Canyon. Although parasequences and their flooding surfaces can be correlated between Tusher and Thompson canyons, the discrete shoreface-incised channels are only restricted to the proximal portions of each parasequence. These channels mostly cut foreshore-shoreface sandstones, and are not continuously correlatable between Tusher and Thompson canyons. Re-examination of these Desert-Castlegate outcrops exemplifies the misfit with the conventional sequence stratigraphic model, specifically the miscorrelation of the D5 and D11 shoreface-incised channels. Over 60 m of deposits separate the D5 and D11 horizons along 30 km of dip-oriented outcrop between Tusher and Thompson canyons. Contrary to earlier interpretations, the shoreface-incised channels in the Desert-Castlegate are not nested within larger incised valley-fills. An alternative model showing temporal and spatial linkage between the nearshore terrestrial and laterally adjoining shallow marine facies is the best fit with the Desert-Castlegate field observations. Selecting the appropriate correlation style has significant implications for the prediction of sand body continuity, connectivity and distribution, especially applications to subsurface data sets where visualization is limited and correlation uncertainties are high.

Rethinking the Incised-Valley Fill Paradigm For Campanian Book Cliffs Strata, Utah–Colorado, U.S.A.: Evidence For Discrete Parasequence-Scale, Shoreface-Incised Channel Fills

Rethinking the Incised-Valley Fill Paradigm For Campanian Book Cliffs Strata, Utah–Colorado, U.S.A.: Evidence For Discrete Parasequence-Scale, Shoreface-Incised Channel Fills

Using classic outcrops to revise sequence stratigraphic models: Reevaluating the Campanian Desert Member (Blackhawk Formation) to lower Castlegate Sandstone interval, Book Cliffs, Utah and Colorado, USA

Using classic outcrops to revise sequence stratigraphic models: Reevaluating the Campanian Desert Member (Blackhawk Formation) to lower Castlegate Sandstone interval, Book Cliffs, Utah and Colorado, USA

Tectonic controls on Late Cretaceous sediment provenance and stratigraphic architecture in the Book Cliffs, Utah

Tectonic controls on Late Cretaceous sediment provenance and stratigraphic architecture in the Book Cliffs, Utah

Reservoir lithology classification based on seismic inversion results by Hidden Markov Models: Applying prior geological information

Abstract Hidden Markov Models (HMMs) have been applied to predict reservoir lithologies using seismic inversion results as inputs. This approach takes into account the conditional probabilities between different lithologies, i.e. the vertical transitions in sedimentary sequences. These properties are used as prior geological information. In order to relate the seismic inversion results to the true well-log data, HMMs need to be trained based on the Expectation-Maximization theory. Application of the resulting model on a synthetic example from the Book Cliffs (Utah, USA) showed that most lithologies are classified correctly, even for some thin layers. A comparison with point-wise methods in which data samples are treated independently from each other, such as k-means and fuzzy logic classifiers, leads to the conclusion that the spatial correlation in HMMs allows better lithological predictions because the prior information accounts for the geological depositional processes. A real case study with data from the Vienna Basin (Austria) is performed, in which lithologies in a 3D cube are obtained based on properties from seismic inversions, via trained HMMs. While the vertical sequences are shown to be reasonably well predicted, the horizontal continuities are not. This indicates that the future research should focus on the lateral geological relationships.

Geometry, distribution and fill of erosional scours in a heterolithic, distal lower shoreface sandstone reservoir analogue: Grassy Member, Blackhawk Formation, Book Cliffs, Utah, USA

AbstractMany shoreface sandstone reservoirs host significant hydrocarbon volumes within distal intervals of interbedded sandstones and mudstones. Hydrocarbon production from these reservoir intervals depends on the abundance and proportion of sandstone beds that are connected by erosional scours, and on the lateral extent and continuity of interbedded mudstones. Cliff‐face exposures of the Campanian ‘G2’ parasequence, Grassy Member, Blackhawk Formation in the Book Cliffs of east‐central Utah, USA, allow detailed characterization of 128 erosional scours within such interbedded sandstones and mudstones in a volume of 148 m length, 94 m width and 15 m height. The erosional scours have depths of up to 1·1 m, apparent widths of up to 15·1 m and steep sides (up to 35°) that strike approximately perpendicular (N099 ± 36°) to the local north–south palaeoshoreline trend. The scours have limited lateral continuity along strike and down dip, and a relatively narrow range of apparent aspect ratio (apparent width/depth), implying that their three‐dimensional geometry is similar to non‐channelized pot casts. There is no systematic variation in scour dimensions, but ‘scour density’ is greater in amalgamated (conjoined) sandstone beds over 0·5 m thick, and increases upward within vertical successions of upward‐thickening conjoined sandstone beds. There is no apparent organization of the overall lateral distribution of scours, although localized clustering implies that some scours were re‐occupied during multiple erosional events. Scour occurrence is also associated with locally increased amplitude and laminaset thickness of hummocky cross‐stratification in sandstone beds. The geometry, distribution and infill character of the scours imply that they were formed by storm‐generated currents coincident with riverine sediment influx (‘storm floods’). The erosional scours increase the vertical and lateral connectivity of conjoined sandstone beds in the upper part of upward‐thickening sandstone bed successions, resulting in increased effective vertical and horizontal permeability of such intervals.

Recognition and significance of bayhead delta deposits in the rock record: A comparison of modern and ancient systems

AbstractDespite advances in estuarine facies models, ancient bayhead delta deposits are not widely recognized or utilized, and their facies characteristics are poorly documented. Sedimentology of three well‐exposed, bayhead delta deposits within the well‐known stratigraphic framework of the Book Cliffs, Utah, and comparison to modern bayhead deltas provide insight into their variability, and a model for reservoir prediction. Bayhead deltas develop at the innermost part of bays within wave‐dominated and mixed‐energy estuaries on transgressive coastlines. The deposits of these deltas are valuable because they record the high‐frequency turnaround from transgression to regression; in turn, this delineates the long‐term transgression trajectory and informs reservoir prediction. This study uses outcrop data and synthesis of modern bayhead delta characteristics to define criteria for bayhead delta recognition, develop a detailed facies model and highlight their utility in a regional context. The following criteria for bayhead delta identification are proposed herein: (i) overall coarsening‐up pattern; (ii) small‐scale clinoform (5 to 15 m height; 100 to 1000 m length) showing a down‐clinoform decrease in grain size and palaeocurrent energy; (iii) increasing abundance of mud interbeds towards the clinoform toe; (iv) basinward‐directed palaeocurrents generated mainly by fluvial input with strong tidal overprint; (v) greatest tidal influence in the intertidal zone of the inactive delta front and better preservation of turbidites at clinoform toes; (vi) brackish trace and body fossils; and (vii) position within an overall backstepping stratigraphic succession. Ancient bayhead deltas described in this study are situated within a moderately rising to flatly rising transgressive interval over a 300 km transect. Steeply rising transgressive trajectories sequester sandy, thicker, better connected transgressive deposits in the inner part of the estuary where bayhead deltas tend to stack vertically. Flatter transgressive trajectories sequester sand in disparate, disconnected estuarine sandstone bodies with partial preservation of bayhead deltas. Proposed criteria aid recognition of bayhead delta deposits in other basins to reconstruct transgressive shoreline trajectories that inform reservoir models and volumetrics.

Sedimentologic and sequence-stratigraphic characteristics of wave-dominated deltas

Wave-dominated deltaic strata form prolific hydrocarbon plays in many mature basins across the world. Examples include the Jurassic Brent Group play in the North Sea, offshore United Kingdom and Norway (e.g., Husmo et al., 2003); Eocene Jackson Group and Oligocene Frio Formation plays, Texas, onshore United States (e.g., Fisher et al., 1970; Galloway and Morton, 1989); and Tertiary plays in the Niger Delta province, offshore Nigeria (e.g., Evamy et al., 1978), the Baram Delta province, offshore Brunei (e.g., Rijks, 1981), and the Columbus Basin, offshore Trinidad and Tobago (e.g., Sydow et al., 2003). Wave-dominated deltas also form key exploration targets in frontier basins (e.g., Triassic Snadd Formation, Barents Sea, offshore Norway; Klausen et al., 2014, 2016). The overall architecture of many wave-dominated deltaic reservoirs is determined by their sequence-stratigraphic framework together with their structural configuration. Sequence-stratigraphic frameworks contain elements such as shoreface–shelf parasequences bounded by flooding surfaces and fluvio-estuarine complexes that fill incised coastal valleys developed at sequence boundaries. The internal facies distributions and stacking patterns of such elements are ordered, which aids prediction of reservoir character and behavior. Outcrop analogs of wave-dominated deltaic reservoirs display aspects of ordered stratigraphic architecture across a range of spatial scales that are equivalent to those observed in core, well-log, and seismic data and can therefore be used to support interpretation of their subsurface counterparts. The Cretaceous Blackhawk Formation contains wave-dominated deltaic strata that are superbly exposed in the Book Cliffs of eastcentral Utah (Figure 1A). The extensive, continuous exposures of the Book Cliffs outcrop belt have allowed the wave-dominated deltas and related deposits to be mapped for tens of kilometers (e.g., Young, 1955; Balsley, 1980), and, consequently, these strata were the “birthplace” of high-resolution sequence stratigraphy using outcrop data (Van Wagoner et al., 1990). The Blackhawk Formation and related …

Obtaining a high-resolution geological and petrophysical model from the results of reservoir-orientated elastic wave-equation-based seismic inversion

A previous geological and petrophysical model of the fluvio-deltaic Book Cliffs outcrops contained eight lithotypes, within each of which a number of lithologies were grouped. While this model was an adequate representation of the overall depositional architecture, for reservoir-geological purposes the potential reservoir and non-reservoir lithologies needed to be separated. Here, a new and more detailed geological model is presented in which more differentiation is put on the potential reservoir lithologies. This new model contains 12 lithologies with layers down to 1 m in thickness. Assuming a burial depth of 3 km and an average clay content, representative rock physical properties are assigned to lithologies based on published data. After the model thickness has been stretched by a factor of 4 in order to represent a more realistic reservoir, a full-waveform forward seismic response is modelled. These data are used as inputs into an iterative elastic wave-equation-based inversion scheme, with the goal to retrieve the rock properties and layer geometries. The results of this conceptual study show that sandstone units in the shoreface and distributary channels, which are potential reservoirs, are successfully identified. The recovery of medium parameters has a high resolution because the non-linear relationship between rock properties and the seismic data has been exploited.

Lessons Learned from Bison Restoration Efforts in Utah on Western Rangelands

On the Ground Bison are considered the keystone species of the Great Plains but widespread slaughter led to their near extinction. Utah has two wild, free-ranging herds on public lands managed as wildlife though hunting. Both herds are descended from animals reintroduced to the Henry Mountains in the 1940s and more recently the Book Cliffs in 2008. Key elements for the successful ecological restoration of bison include: Legal designation of bison as wildlife in the state Genetically-pure, disease-free source Large expanses of habitat-they take a lot of room Potential conflicts must be identified and addressed in a transparent manner Mutual purpose and trust with all affected stakeholders is essential; i.e., ask, How can we have both sustainable livestock grazing and a viable bison herd on the unit? Active management to address changing situations and maintain herd size at a sustainable level

Towards a sequence stratigraphic solution set for autogenic processes and allogenic controls: Upper Cretaceous strata, Book Cliffs, Utah, USA

Upper Cretaceous strata exposed in the Book Cliffs of east–central Utah are widely used as an archetype for the sequence stratigraphy of marginal-marine and shallow-marine deposits. Their stratal architectures are classically interpreted in terms of accommodation controls that were external to the sediment routing system (allogenic), and that forced the formation of flooding surfaces, sequence boundaries, and parasequence and parasequence-set stacking patterns. Processes internal to the sediment routing system (autogenic) and allogenic sediment supply controls provide alternatives that can plausibly explain aspects of the stratal architecture, including the following: (1) switching of wave-dominated delta lobes, expressed by the internal architecture of parasequences; (2) river avulsion, expressed by the internal architecture of multistorey fluvial sandbodies and related deposits; (3) avulsion-generated clustering of fluvial sandbodies in delta plain strata; (4) ‘autoretreat’ owing to increasing sediment storage on the delta plain as it lengthened during progradation, expressed by progradational-to-aggradational stacking of parasequences; (5) sediment supply control on the stacking of, and sediment grain-size fractionation within, parasequence sets. The various potential allogenic controls and autogenic processes are combined to form a sequence stratigraphic solution set. This approach avoids anchoring of sequence stratigraphic interpretations on a specific control and acknowledges the non-unique origin of stratal architectures.

Identifying Ordered Strata: Evidence, Methods, and Meaning

Abstract Identifying order or pattern in strata on the basis of qualitative interpretation forms the basis for much current sedimentological and sequence stratigraphic analysis. Order can be usefully defined as some arrangement of facies or unit thickness that has a discernable trend or pattern that is unlikely to occur by chance because it requires some particular systematic process to form. Coarsening, fining, thickening, or thinning-upward trends, and arrangement of strata into cycles are examples of order. Qualitative interpretations of order often demonstrate little more than an implicit assumption of order. This paper defines a robust yet simple-to-apply quantitative method to identify order in strata and to indicate when order cannot be reliably demonstrated. The method is based on two calculated metrics, the Markov metric m derived from analysis of a vertical facies succession, and the runs metric r derived from analysis of observed thicknesses of stratal units. Most importantly, both metrics can be compared with equivalent metrics calculated for disordered strata composed of many randomly shuffled versions of the same lithological units. Probability values can then be calculated from the comparison between observed and randomly shuffled cases, and these p values indicate the degree of evidence present for order in the observed strata. Several test examples using synthetic strata show that the m and r values can define and identify different degrees and types of stratal order, and that the metrics are robust for both stationary and non-stationary successions with a range of different lengths and numbers of distinct facies. Analysis of four outcrop examples, two siliciclastic and two carbonate, demonstrate that ordered facies successions and thickness trends may be less common than typically assumed; none of the four examples analyzed show trends in thickness, and only the examples from the Book Cliffs, which represent a bedset scale composite of observations, show evidence for facies order. The examples demonstrate how a quantitative analysis can lead to better understanding of strata, either ordered or disordered, and can provide better insight into the validity of current stratigraphic interpretations and models. Absence of order in many of the analyzed 1D vertical successions may also indicate that we need to focus more on longer-term trends and analysis of 2D and 3D stratal geometries.

Coal Clinker Site in the Late Cretaceous Blackhawk Formation, Castle Gate, Utah, USA

When sedimentary host rock is heated as a result of coal seam fires, the host rock is pyro-metamorphosed (high temperature/ low pressure) resulting in the formation of paralava, a low-grade metamorphic rock formed adjacent to coal seams. This paper describes an outcrop locality of clinker (paralava) in the Upper Cretaceous Blackhawk Formation within the Book Cliffs coalfields of central Utah, which formed as the result of a coal seam fire.

Along-strike sequence stratigraphy across the Cretaceous shallow marine to coastal-plain transition, Wasatch Plateau, Utah, U.S.A.

Current sequence stratigraphic models deal with stratal packages almost exclusively along depositional-dip. A high-resolution outcrop example detailing sequence stratigraphic architecture along depositional-strike is largely lacking. This outcrop study provides a detailed sequence stratigraphic correlation along depositional-strike for a length of ~50km. Moreover, this study focuses on the facies and stratigraphic relationships particularly at the very transition between coeval marine and nonmarine strata, which is relatively underdocumented.Within the Mesaverde Group of central and eastern Utah, the Upper Cretaceous shallow-marine Star Point Sandstone and paralic Blackhawk Formation are relatively well studied along depositional-dip in the Book Cliffs, but the nature of their spatio-temporal transition particularly along depositional-strike in the NNE-SSW trending Wasatch Plateau remains poorly documented. Facies-scale to stratigraphic-scale data were gathered from ten outcrop “windows” along the eastern margin of the Wasatch Plateau. The vertical and lateral transition between marine and nonmarine strata varies in complexity within the study area. In the southern part of the study area near the central Wasatch Plateau, the shallow-marine Star Point Sandstone passes stratigraphically upward into the nonmarine Blackhawk Formation in one simple transition without any intercalation of marine and nonmarine strata. In the northern part of the study area, however, where the Star Point Sandstone to Blackhawk Formation transition is complex, aggradationally stacked shallow-marine sandbodies (i.e., parasequences) taper and completely pinch-out within coastal-plain mudstones in a paleo-landward direction over short distances (<1km), representing potential stratigraphic traps. These marine sandbodies are intercalated with coeval nonmarine strata within the Blackhawk Formation. Fourth-order sequence boundaries and flooding surfaces were correlated along the lower Campanian depositional-strike.The results reveal along-strike undulations in the early Campanian depositional topography, which was characterized by alternating incised valleys, interfluvial topographic highs with mature soil development, and shoreline embayments. In particular, the variable along-strike transition-complexity from shallow-marine to coastal-plain strata is the result of differential tectonic subsidence, where the transition-complexity increases and the stacking pattern is more aggradational towards an early Campanian depocenter in the north of the study area. This study reveals that differential subsidence is the key in controlling along-strike variability of sequence stratigraphy and that correlation of sequence stratigraphic surfaces are far more challenging in nonmarine strata than in adjacent marine strata.

Tectonically Controlled Nearshore Deposition: Cozzette Sandstone, Book Cliffs, Colorado, U.S.A

Abstract The Book Cliffs of eastern Utah and western Colorado have been pivotal in the development of outcrop-based sequence stratigraphic concepts for nonmarine to shallow marine siliciclastic depositional settings. Prior studies in this area, and more generally in the Cretaceous western interior foreland basin of North America, have concluded that nearshore accumulation is controlled for the most part by the interaction between oscillatory eustatic change and longer-term regional patterns of flexural subsidence. New outcrop and subsurface evidence reported here from the eastern Book Cliffs suggests that three-dimensional tectonic tilting at length scales of up to ∼ 50 km (31 mi) and timescales of less than ∼ 200 kyr also strongly influenced sedimentation. Continental ice sheets are thought to have been small at the time. Documented patterns of accumulation are inconsistent with those expected from interactions of eustasy and regional flexure alone. The upper Campanian Cozzette Sandstone Member of the Mount Garfield Formation consists of twelve lithofacies arranged into six lithofacies assemblages, inferred to have been deposited in shallow marine, marginal marine, and nonmarine depositional environments. Shallow marine facies are organized into six wedge-shaped units < 40 m thick, bounded by flooding surfaces, and separated into three larger-scale cycles, which display along-strike and time-equivalent aggradational and progradational stacking. These successions are interpreted as shoreface–foreshore–swamp parasequences, and are erosionally overlain by fluvial and estuarine deposits. Both fluvial and estuarine accumulations are underlain by composite erosional surfaces, belonging either to two distinct incised-valley fills or to one composite fill. The proposed interpretation is based on high-resolution correlations made from digital video and continuous photographs acquired during a helicopter survey of the member. This interpretation differs significantly from published cross sections of the Cozzette, in which the sandstone is inferred to consist of at least two sheet-like shallow marine parasequences, truncated by sequence boundaries. Facies variations, stratigraphic thickness trends, and geometrical relationships reveal that three basinward–landward cycles of syndepositional to postdepositional tectonic tilting in the southern Piceance basin controlled accumulation in the Cozzette Sandstone. Differential subsidence to the southeast led to the development of a northeast-trending clinoform rollover. Subsequent tilting to the north resulted in renewed accommodation creation, and reorientation of the rollover to an eastward trend. Following deposition of six shallow marine parasequences, tilting towards the northeast resulted in bypass and the development of a sequence boundary. The main conclusion of our study, that nearshore sedimentation was modulated by local tectonism at timescales of less than hundreds of thousands of years, casts doubt on the generally accepted eustatic paradigm at a location that was among the most important in the development of sequence stratigraphic concepts.

What does Ophiomorpha irregulaire really look like?

Ophiomorpha irregulaire is a surprisingly poorly characterised ichnotaxon given its common occurrence in hydrocarbon reservoir facies. Current debate surrounds the ichnotaxobases suitable for ichnospecific-level identification and also the palaeogeographic distribution of the taxon. O. irregulaire is conventionally characterised in core by the presence of “spikey”, fine-grained, wall-lining pellets, since the horizontal “meander maze” that is also part of the ichnospecific diagnosis cannot normally be demonstrated. To resolve ichnotaxonomic issues concerning the validity of pellet morphology as a primary ichnotaxobase, material from the type locality is re-described (Cretaceous Book Cliffs, Utah), with an emphasis on burrow wall morphology. Comparative neoichnological studies using the callianassid crustacean Neotrypaea californiensis were additionally conducted to understand the behaviour of modern taxa that produce burrows closely resembling O. irregulaire. High-resolution, three-dimensional morphological models were created from specimens of Ophiomorpha from the type locality of O. irregulaire in Utah, USA, and from Eocene deep marine turbidites of the Juncal Formation, California. Comparison of the morphological features from these two localities, as well as specimens observed in core from offshore Newfoundland, demonstrate conclusively that O. irregulaire is not restricted to the Cretaceous Western Interior Seaway. O. irregulaire may have a stratigraphic range from Jurassic to Recent, and occur in palaeoenvironmental settings ranging from shallow marine to continental slope settings. The flame-like pellet morphology is considered characteristic of the type material, and is a valid criterion for identifying O. irregulaire in core.

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

Facies models for wave-dominated shorelines include an ‘offshore transition zone’ between shelfal mudstones and nearshore shoreface sandstones. Offshore transition-zone deposits are commonly tabular sandstone beds interbedded with continuous mudstone beds. However, observations from the Blackhawk Formation show that the offshore transition zone locally consists of erosive-based sandstone beds with ‘pinch-and-swell’ geometries containing steep-walled gutter casts, in areas larger than 6 × 2 km along strike and dip. This increases the amount of sand-on-sand contacts, and leads to improved vertical permeability. Predicting the distribution of erosive offshore transition within the subsurface is therefore desirable. In this study, offshore transition-zone deposits have been studied using virtual outcrops. Tabular offshore transition-zone deposits have continuous sandstone and mudstone beds much longer than 500 m, and erosive offshore transition-zone deposits have discontinuous shales, on average, 60 m long. Reservoir modelling shows a 10- to two-fold increase in vertical permeability in erosive compared to tabular offshore transition deposits, the magnitude decreasing with increasing fraction of shale. Erosive offshore transition deposits occur near distributary channels, subaqueous channels and abrupt bathymetric breaks. A regional study shows that erosive offshore transition-zone deposits are mainly developed where parasequences prograde into deeper water offshore the platform break of the preceding parasequence, are commonly associated with basinal turbidites and may be related to erosion by bypassing turbidity currents.

Facies Relationships and Stratigraphic Architecture of Distal, Mixed Tide- and Wave-Influenced Deltaic Deposits: Lower Sego Sandstone, Western Colorado, U.S.A

Abstract The facies characteristics and stratigraphic architectures of ancient mixed, wave- and tide-influenced deltas are poorly documented, despite the abundance of analogous modern shorelines. This paper presents the first facies analysis and stratigraphic-architectural characterization of the distal part of the Cretaceous lower Sego Sandstone (Book Cliffs, Colorado, U.S.A.), whose correlative proximal part is arguably the most comprehensively documented example of ancient deltaic strata that exhibit evidence for the activity of tides and waves. The distal lower Sego Sandstone consists of tide-dominated and wave-dominated deposits arranged within four regressive–transgressive tongues bounded by flooding surfaces. The proximal part of each regressive–transgressive tongue contains tide- dominated sandstones that sharply to erosionally overlie, and laterally interfinger with, wave-dominated deposits in the distal part of the tongue. Tide-dominated sandstones are fine to medium grained, and comprise wavy bedded and cross-bedded sandstones with abundant mud drapes, reactivation surfaces, and bidirectional, ebb- and flood-tide-directed paleocurrents. They define active tidal distributary channel-fill and nonchannelized tidal-bar deposits of similar lithologic character, which represent the proximal part of the delta front. Abandoned tidal distributary channel fills comprise intensely bioturbated mudstones. The outer, paleoseaward parts of tidal bars contain direct evidence for mixed tide and wave influence in the form of hummocky cross-stratified, very fine-grained sandstone beds intercalated with cross-bedded, fine- to medium-grained sandstones containing tidal indicators. These deposits locally pass paleoseaward into low-angle and hummocky cross-stratified sandstones with grain size and texture similar to tidal-bar deposits, implying reworking by storm waves. Wave-dominated deposits consist of coarsening-upward successions of mudstone and hummocky cross-stratified sandstone beds that represent the distal part of the delta front. Localized rhythmic variations in the wavelength of hummocky cross-stratification in some deposits may record tidal modulation of water depth and storm-wave velocities. These facies components and their architectural arrangement suggest that tides and waves operated together throughout deposition of the “lower Sego Delta,” but that tide-dominated and wave-dominated deposits were spatially partitioned into the proximal and distal parts of each regressive–transgressive tongue. Thus, tides and waves were effective in transporting and reworking sediment in different water-depth ranges and at different geographic locations. Mixed tide- and wave-influenced deposits are generally preserved only at the down-dip pinchout of regressive–transgressive tongues, because they are removed by tidal scour during delta progradation in more proximal locations. The spatial partitioning of time-equivalent tide-dominated and wave-dominated deposits is readily explained in a single depositional model, which requires the interpretation of neither sequence boundaries nor temporal changes in process regime across the sharp-to-erosional juxtaposition of tide-dominated sandstones above wave-dominated deposits in a regressive–transgressive tongue.

Distribution of discontinuous mudstone beds within wave-dominated shallow-marine deposits: Star Point Sandstone and Blackhawk Formation, Eastern Utah

Deposits of wave-dominated shorelines are typically considered to act as relatively simple hydrocarbon reservoirs and are commonly modeled as “tanks of sand.” However, important heterogeneities that can act as barriers to fluid flow occur at the parasequence, bedset, and bed scales, especially in viscous oil or low-permeability oil fields. Heterogeneities at the parasequence and bedset scales have been well studied, but discontinuous mudstone beds occurring within the shoreface have received little attention. The Book Cliffs and Wasatch Plateau are among the best-exposed and best-studied deposits of wave-dominated shallow-marine systems in the world. Two parasequences within these outcrops have been studied in detail to investigate the distributions of intrashoreface shales and to propose models for the controls on their distribution. A data set consisting of 30 km (18.6 mi) of virtual outcrops derived from oblique helicopter-mounted light detection and ranging (LIDAR) scanning with supporting stratigraphic sections makes it possible to collect a large quantity of accurate geometric data of depositional elements from inaccessible cliffs. Nine-hundred and twenty-one discontinuous mudstone beds were measured. These occur as ellipses with long axes oriented normal to the paleoshoreline. Lengths and widths of these mudstone beds exhibit a lognormal distribution, with means of 21.9 and 13.8 m (71.9 and 45.3 ft), respectively. Within the shoreface succession, the number of mudstone beds increases downward whereas size does not vary significantly with stratigraphic height. An average of 100 m (328 ft) cumulative length of shale exists per 100 m (328 ft) of horizontal outcrop; this increases threefold near both wave-dominated deltas and bedset boundaries that reflect minor sea-level fluctuations during progradation.

The Emptiness of the Stratigraphic Record: A Preliminary Evaluation of Missing Time In the Mesaverde Group, Book Cliffs, Utah, U.S.A

Abstract Observations of modern sedimentation rates in nonmarine and shallow-marine clastic environments indicate that deposits formed in comparable settings in the ancient record could accumulate in a fraction of the time that would appear to be available according to conventional chronostratigraphic dating methods. Typically as little as 10% of elapsed time is represented by sediment, the remainder by breaks in sedimentation, many of which are inconspicuous. An examination of the chronostratigraphy and sedimentary history of the Mesaverde Group of the Book Cliffs, Utah, U.S.A., confirms this pattern. Shoreface, deltaic, and nonmarine successions are considered to be particularly fragmentary. Ravinement surfaces may each account for up to 105 years of missing time. In the sedimentary record of shallow-marine and nonmarine deposits stratigraphic continuity is not to be expected, and calculation of time-related issues, such as mass-balance transport rates and the trajectories of shoreline transgression and regression, must take this into account.