John M. Holbrook¹

(1) Southeast Missouri State University, Cape Girardeau, MO

ABSTRACT: Sequence Stratigraphy, Allostratigraphy, and Architectural-Element Analysis as a Genetic Continuum: An Illustration from the Albian Muddy Sandstone, Southeastern Colorado

Both upper and lower fluvial sandstone members of the Muddy Sandstone reveal six orders of "nested" and genetically interrelated channel-form bounding surfaces that are traditionally considered separately. The lowest three orders (nested-channel scours, channel scours, and channel-belt boundaries) are traditionally addressed by architectural-element analysis, and reflect localized channel scouring in response to variations in river discharge and sediment load. Scour tied to these elements collectively carve intravalley allomember boundaries, which in turn carve valley-fill allomember boundaries. These allomembers mostly record local scour-and-fill in response to climatic and tectonic forces operating during falling and lowstand phases. Composite scour of allomembers, or elements, results in sequence boundaries which here reflect regional scour in response to sea-level change.

Muddy valley-form sequence boundaries tend to record composite surfaces formed of multiple intravalley allomember scours. Smooth sequence boundaries, however, tend to reflect composites of multilateral architectural elements or valley-fill allomembers. These observations, coupled with previous studies, support a generalized eight-category classification of fluvial-based sequences that encompasses architectural, allostratigraphic, and sequence stratigraphic considerations. These include four classes for dominantly valley-form sequence boundaries (simple valley, complex valley, compound valley, and compound-complex valley) and four classes for smooth sequence boundaries (channel sheet, stacked channel sheet, multivalley sheet, and stacked multivalley sheet).

Presence of intravalley and valley-fill allomembers in Muddy strata, and their abundance in modern deposits, suggests that such allounits are far more common in the ancient than previously reported. They now need to be considered when assessing ratios of accommodation versus sediment supply, regional permeability trends, and up-dip fate of sequence-bounding unconformities.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado