Sedimentology of the Whiteclay Gravel Beds (Ogallala Group) in northwestern Nebraska, USA: Structurally controlled drainage promoted by Early Miocene uplift of the Black Hills Dome
Abstract The newly recognized Whiteclay Gravel Beds (WGB) of the Miocene Ogallala Group crop out as a narrow, discontinuous ribbon of sands and gravels in Dawes and Sheridan Counties, northwestern Nebraska, USA. The WGB are exposed in a series of municipal gravel quarries and natural exposures that define a linear trench in underlying strata at least 20 m deep and up to 300 m wide, with short, southeast-trending reaches separating generally longer east-trending sections. This gravel-filled trench can be recognized from the Nebraska–South Dakota border near Whiteclay, Nebraska southeastward to east of Gordon, Nebraska, a distance of ∼ 30 km. The outcrop belt of the WGB is coincident in location and trend with the Whiteclay Fault Zone. Where exposed in quarries, the walls of the trench are steep-sided, vertical, or locally overhanging. Polished surfaces, slickensides, and parallel joint sets are common in the walls of the trench near Whiteclay, but uncommon in those to the east. The narrow belt defined by this trench is filled by stratified gravel ( The WGB are interpreted to have formed in response to tectonic upheaval associated with uplift of the Black Hills of South Dakota in Early Miocene times. Fault rupture topography facilitated formation of a steep-sided canyon, or valley, up to 20 m deep, being virtually straight with sharp bends at intervals of several km. An alluvial channel belt developed in the floor of the valley, filling the available accommodation space with coarse sand and gravel via aggradational stacking of the deposits of successive channels and channel belts. Channel belts were probably braided, with individual channels up to 4 m deep and a few tens of metres wide. The multi-storey character of the deposit indicates multiple episodes of cutting and filling. The coarse grain-size of the fill suggests energetic discharge with frequent bankfull flows, even though the system had a relatively low gradient (0.004). An abundance of reworked fossil debris is derived from several stratigraphic units, clasts of which have been identified in the fill. The presence of a contemporary merychippine horse and a primitive species of the oreodont Brachycrus constrain formation of the WGB to a short interval within the Early Miocene (c. 17.5 Ma). The mammal fauna suggests that this stream was a valuable source of water, while fragments of aquatic organisms such as turtles and fish indicate perennial discharge. The WGB provides a crucial window into a pluvial period in the Miocene that is largely unpreserved elsewhere in the basin, facilitated in part by fault rupture topography.
Origin, genetic interrelationships, and stratigraphy over the continuum of fluvial channel-form bounding surfaces: an illustration from middle Cretaceous strata, southeastern Colorado
Abstract A hierarchy of channel-form bounding surfaces generally partitions fluvial strata, each reflecting river scouring processes at a different scale. Low-order (e.g. channel scours, channel-belt boundaries, etc.) and high-order (i.e. valley-form sequence boundaries) surfaces are discussed extensively in the literature. The intermediate-scale boundaries, however, are largely overlooked. An understanding of the range of processes generating these channel-form surfaces is pivotal for interpreting fluvial architecture and understanding sequence-boundary genesis. This paper examines origin, interrelationships, and significance for the full range of channel-form bounding surfaces (channel-fill to sequence boundary) in middle Cretaceous strata of southeastern Colorado, and assesses the role of such surfaces in sequence and allostratigraphy. Middle Cretaceous strata of the Muddy Sandstone were photographed over the entire length (14 km) of the Huerfano Canyon. Lithofacies were then mapped in detail for all Muddy deposits, and bounding surfaces were identified by architectural-element analysis within Muddy fluvial strata. Two alloformations were established based on recognition of three continuous discontinuities. The Cucharas Canyon Alloformation is floored by one sequence-bounding unconformity that is smooth (surface SB3), and is capped by surface SB4. It is mostly a basal sandstone cliff composed of fluvial lithofacies assemblages, but is characterized commonly by paralic lithofacies assemblages in its upper part. The Huerfano Canyon Alloformation is floored by a locally incised sequence boundary SB4, and is capped by a smooth transgressive surface of erosion. It comprises cliff-forming fluvial lithofacies assemblages. Both alloformations reveal six orders of channel-form bounding surfaces. Each order is bundled by surfaces of the succeedingly higher orders; thus, the surfaces can be considered to fit in a hierarchical or ‘nested’ progression. The surfaces record, in order of ascending rank: nested-channel cuts, channel scours, channel-belt boundaries, nested-valley boundaries, valley-fill boundaries, and sequence-bounding unconformities. Though generated by discrete processes, each of these surfaces is formed as a composite of the lower-order surfaces. These surfaces are thus genetically interrelated into a bounding-surface continuum. This continuum is best constrained by allostratigraphic techniques, with sequence stratigraphy and architectural-element analysis treated as approaches under the auspice of allostratigraphy. Middle-Cretaceous strata reveal a close interrelationship between sequence-boundary morphology and architecture of overlying fluvial deposits. Valley-form sequence boundaries tend to record composite surfaces formed of multiple nested-valley scours. Smooth sequence boundaries tend to reflect composites of multilateral channel-fill elements or valley-fills. These observations promote a set of predictive relationships between sequence boundaries and overlying fluvial sandstone that are here categorized into four sequence architectural styles with valley-form boundaries (simple valley, complex valley, compound valley, and compound-complex valley) and four with smooth boundaries (channel sheet, stacked channel sheet, multivalley, and stacked multivalley). Valley and nested-valley incisional boundaries in middle Cretaceous alloformations most likely result from climatic forcing, and do not necessarily record regional surfaces. Such surfaces are abundant in modern deposits, and should be far more common in ancient strata than previously reported. Because of their potential impact on results, they need to be considered as a factor when estimating ratios of accommodation space versus sediment supply, evaluating regional permeability trends, and assessing up-dip fate of sequence-bounding unconformities.
Siliciclastic sequence stratigraphic patterns in foreland, ramp-type basins
Stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns. In active tectonic basin settings, regional subsidence patterns can be very different from passive continental margin settings, resulting in substantial modifications of the basic sequence stratigraphic model. In ramp-type foreland basins, on the tectonically active side, subsidence rates decrease seaward in contrast with passive continental margins, where the opposite subsidence pattern exists. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence, resulting in an interval of relative sea-level fall during a eustatic cycle. On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward off zone A, away from the orogenic belt where subsidence rates are lower. On passive continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B, where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then type 1 sequence boundaries can occur. In foreland basins, these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone A. The pattern of seaward-decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns. When the two zones occur, the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts. A transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas.