3D modelling of grain size distribution in Quaternary deltaic deposits (Llobregat Delta, NE Spain).

Abstract

The Llobregat delta constitutes a Pliocene-Quaternary sequence recording progradation. An 8 meter-thick, delta-plain succession temporarily accessible in the subaerial delta plain was sub-divided into four units (A to D), which constitute a shallowing upwards sequence recording sedimentation from shoreface at the bottom (Unit A) to lacustrine settings at the top (Unit D). Unit B basically comprises gravelly, coarse-grained sandy lenticular bodies (up to 20 m wide, up to a few decimetres thick), within a homogenous, medium-grained sandy background, interpreted as prograding foreshore bars. 3D facies models, built by using the sequential conditional indicator simulation method, reproduced the sedimentary heterogeneities identified in Unit B, using as input data the sedimentary body geometry and its correlation with sandstone grain size values, with the grain size modes taken as a continuous, quantitative proxy for facies distribution. The simulations not only reproduced satisfactorily the significant sedimentary heterogeneities generated by the facies arrangement, but also predicted and described the spatial relation and the three-dimensional shape of sedimentary bodies accurately. Quality of the models should be ensured by a consistent prior analysis of the spatial structure of the grain size mode based on semivariogram models.