Origin, genetic interrelationships, and stratigraphy over the continuum offluvial channel-form bounding surfaces: an illustration from middle Cretaceous strata, southeastern Colorado

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 ov erlooked. An understanding of the range of processes generating these chann el-form surfaces is pivotal for interpreting fluvial architecture and under standing sequence-boundary genesis. This paper examines origin, interrelati onships, and significance for the full range of channel-form bounding surfa ces (channel-fill to sequence boundary) in middle Cretaceous strata of sout heastern Colorado, and assesses the role of such surfaces in sequence and a llostratigraphy. 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 alloformati ons were established based on recognition of three continuous discontinuiti es. The Cucharas Canyon Alloformation is floored by one sequence-bounding u nconformity that is smooth (surface SB3), and is capped by surface SB4. It is mostly a basal sandstone cliff composed of fluvial lithofacies assemblag es, but is characterized commonly by paralic lithofacies assemblages in its upper part. The Huerfano Canyon Alloformation is floored by a locally inci sed sequence boundary SB4, and is capped by a smooth transgressive surface of erosion. It comprises cliff-forming fluvial lithofacies assemblages. Bot h alloformations reveal six orders of channel-form bounding surfaces. Each order is bundled by surfaces of the succeedingly higher orders; thus, the s urfaces can be considered to fit in a hierarchical or 'nested' progression. The surfaces record, in order of ascending rank: nested-channel cuts, chan nel scours, channel-belt boundaries, nested-valley boundaries, valley-fill boundaries, and sequence-bounding unconformities. Though generated by discr ete processes, each of these surfaces is formed as a composite of the lower -order surfaces. These surfaces are thus genetically interrelated into a bo unding-surface continuum. This continuum is best constrained by allostratig raphic techniques, with sequence stratigraphy and architectural-element ana lysis 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 multip le nested-valley scours. Smooth sequence boundaries tend to reflect composi tes of multilateral channel-fill elements or valley-fills. These observatio ns promote a set of predictive relationships between sequence boundaries an d overlying fluvial sandstone that are here categorized into four sequence architectural styles with valley-form. boundaries (simple valley, complex v alley, compound valley, and compound-complex valley) and four with smooth b oundaries (channel sheet, stacked channel sheet, multivalley, and stacked m ultivalley). Valley and nested-valley incisional boundaries in middle Cretaceous allofor mations most likely result from climatic forcing, and do not necessarily re cord regional surfaces. Such surfaces are abundant in modern deposits, and should be far more common in ancient strata than previously reported. Becau se of their potential impact on results, they need to be considered as a fa ctor when estimating ratios of accommodation space versus sediment supply, evaluating regional permeability trends, and assessing up-dip fate of seque nce-bounding unconformities. (C) 2001 Elsevier Science B.V. All rights rese rved.