Variations in Alluvial Architecture Across The Oligo-Miocene Huesca Fluvial System, Ebro Basin, Spain
J.P.P. Hirst, 1991. "Variations in Alluvial Architecture Across The Oligo-Miocene Huesca Fluvial System, Ebro Basin, Spain", The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and its Implications for Hydrocarbon Discovery and Recovery, Andrew D. Miall, Noel Tyler
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The Huesca System is interpreted as a terminal, distributive fluvial system. Flow across this ‘fluvial fan’ was largely through a network of well-defined channels and the deposits typically consist of channel-sandstone bodies enclosed within floodplain fines. Amalgamation of the sandstone units varies depending on the location within the system. Lateral profiles are used to show that there are systematic and broadly quantifiable variations in the alluvial architecture across the system.
A precise apex for this distributive system cannot be determined, and thus a datum line has been constructed within the proximal region. This datum has enabled plots to be constructed showing medial to distal variations. The results indicate 1. the proportion of in-channel sediment decreased distally; 2. the sandstone bodies thin distally and thus the paleochannels were becoming shallower, and 3. sheet sandstones deposited by laterally unstable channels are prevalent medially, whereas more stable channels, indicated by ribbon sandstones, were more common distally. The decrease in channel depth and in-channel component are probably the result of evaporative water loss, channel bifurcation and gradient decrease. The increased channel stability distally is interpreted to be the result of reduced ability to erode the banks, more frequent avulsion and ephemeral flow.
The alluvial architecture of the ‘peripheral areas’ of the system, adjacent to the thrust front that defines the northern margin of the basin, differs slightly from the main part of the system. The main difference is the higher frequency of ribbon sandstones in the peripheral areas. This may be due to the tectonic instability of the area near the thrust front and periodic inundation by the marginal alluvial fans. Also, these marginal fans would have increased the flow paths of streams into the peripheral parts of the main system.
The study has applications to hydrocarbon exploration in analogous sequences. Radial variations in reservoir potential are indicated and the extents of sandstone bodies in the subsurface may be estimated from the width-thickness relationship determined from the medial part of the system.