Stratigraphic Architecture of a Prograding Shelf-Margin Delta System in Outcrop: Sobrarbe Fm., Ainsa Basin, Spain

Moss-Russell, Amy C.¹; Pyles, David ¹; Anderson, Donna S.²; Bracken, Bryan R.³; Bouroullec, Renaud ¹; Silalahi, Henri S.^1
1 Chevron Center of Research Excellence, Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
² Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
³ Energy Technology Company, Chevron Corp., San Ramon, CA.

The Sobrarbe Formation, Ainsa Basin, Spain, contains multiple, prograding, condensed-section bound, stratigraphic cycles of a linked, shelf-slope-basinal positioned system. The Sobrarbe is an outcrop analog to prograding, shallow-marine to deepwater systems such as the West Siberian Basin, NW slope Australia, and Sakhalin Island. Because of the extent of the exposure, the physiographic profile is apparent, as is the position of the delta at the shelf edge. The goal of this study is to 1) use observation in outcrop to reduce risk in subsurface interpretation and 2) constrain the timing of deepwater sedimentation in prograding systems.

This study focuses on fluvial-deltaic strata from one condensed section bound cycle/parasequence. Data consists of measured sections, paleocurrents, strike and dips, mapped key surfaces and sand bodies, and sand body dimensions.

Fluvial strata consists of high aspect ratio channel belts with associated crevasse splays and well developed paleosols with few wood fragments and no coal seams. Grain size inside the channels range from fine sand to cobbles, whereas outside channel strata is composed of clay and silt. The channel belts are offset and rarely cannibalize one another, except crevasse splays. Deltaic deposits are physiographically located at the shelf margin. These strata contain mouth-bar deposits consisting of inclined bedding of bioturbated, very-fine to fine-grained sand. These bars contain erosional surfaces lined by debrites. In the upper-slope system, there is a dense occurrence of highly-amalgamated, vertically-stacked, turbidite channels that downcut into mouth-bar foresets. Siltstone forms background slope strata outside the channels, while the dominant grain size in the channels is very-coarse sand in association with mudclast conglomerate occurring locally at the base of channel stories.

The transition from entirely fluvial to entirely deltaic at the shelf break occurs over a distance of 3 km, reflecting the length of the very narrow shelf. Over the same distance, grain-size reduces from conglomerates in the fluvial point bars to dominantly very fine to fine sand in the mouth bars. The intra-mouth bar erosional surfaces are highly continuous within one parasequence and connect fluvial channels to coeval slope channels. These slope channels in turn feed base-of-slope fans. This relationship is interpreted to reflect coeval deposition across the physiographic profile as the parasequence prograded basinward.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009