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.