Mixed Storm and Tidal Facies Successions of Mixed Grain Size, Shallow Marine Sediment in Foreland Basin Units: the Albian, Viking Formation, Townships 45 to 54, Ranges 10W4 to 21W4, Alberta, Canada

Abstract

<p>The siliciclastic, Lower Cretaceous, Albian, Viking Formation was deposited in the foreland basin of eastern central Alberta. Sediments in the area studied were formerly interpreted as "offshore bars" or lowstand shorefaces. It is shown in the first extensive, detailed core and well log study o the basinal portion of the formation that the sediments are sheet sand deposits laid down by a mixture of storm and tidal flows. The sediments divide into twenty facies ranging from grey shale to conglomerate but mostly consisting of bioturbated or crossbedded muddy sandstones and siltstones.</p> <p>The facies assemble into six units: Mu, Epsilon, Delta, Gamma, Beta and Alpha, in stratigraphically ascending order. Unit boundaries are at points of rapid facies changes. There are no extensive discontinuities. Units Epsilon, Delta and Gamma are gradationally coarsening upward from basal bioturbated marine siltstones to heterolithic, sometimes glauconitic crossbedded or bioturbated marine sandstones. Unit Beta is consistently line grained showing a slight coarsening upwards from bioturbated siltstones to crossbedded siltstones.</p> <p>Datum to unit boundary isopachs and unit isopachs indicate post unit Mu, westward subsidence leading to accommodation of units Epsilon through Gamma. During deposition of units AIpha and Beta, subsidence returned to a more even basinwide pattern.</p> <p>Unit isopachs of Epsilon, Delta and Gamma reveal prograding and aggrading "shoreface" wedges in the southwest of the study region. Details of the isopachs reflect fingers of irregular sand sheets. The facies of these sand sheets are interpreted to be finally deposited by storm enhanced flows in a tidally dominated basin. Unit Beta provides evidence for a facies summary of a distal storm/wave dominated shallow shelf. In all cases, lateral facies changes appear random.</p> <p>The units do not fit common sea level curve interpretations because the onset of transgression is located within the upper, coarser portion of the units rather than at the unit boundaries. Neither the units nor the formation can be correlated precisely to existing sea level curves. Combined with the evidence for tectonic control of unit geometries, this suggests periodic activity such as thrust loading of the cordillera west may control unit development in foreland basins.</p>