Graded and Massive Beds: A 3-D Mosaic of Coalesced, Rapidly-Deposited “Pod”-Like Elements?

Bill Arnott
Earth Sciences, University of Ottawa, Ottawa, ON, Canada

Although complete turbidites adorned with their traction-deposited sedimentary structures grab the limelight, it is the seemingly mundane graded and less common ungraded strata that make up the vast majority of the sand-rich deep-marine sedimentary record. Although classification schemes devised to classify these strata are numerous, central to most is the notion that deposition is directly from suspension. However little discussion has been paid to the details of that deposition - is it slow or fast, incremental or as one big slug?

A common observation in the sedimentary record is that graded and ungraded beds commonly show lateral variations, in places abruptly, of grain size and sorting made visible by local variations in cementation and/or weathering characteristics. Although it could be argued that these changes reflect local textural variations within the depositing turbidity current, might a more consistent mechanism be the lateral merging of discrete, independently- and rapidly-deposited “pods”. Typically pods merge imperceptibly (in three-dimensions) as their inherently loosely-packed, water-saturated sediments fuse along their margins. In other cases, previously deposited and coalesced “pods” had sufficiently dewatered and consolidated, but later were scoured, leaving a discernible erosion surface overlain by a new “pod”. In this scenario deposition and related amalgamation occurs during a single event, and not, as might be assumed, the result of two separate turbidity currents. Suspension deposition was succeeded, at least commonly, and probably after a short hiatus, by bed-load transport and deposition that extended across the general bed surface and deposited planar- and/or cross-stratified sediment.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas