Steep-fronted Gilbert-type deltas are common features of tectonically active settings, as well as of physiographic settings where accommodation is dictated by landforms with steeply inclined margins, such as incised valleys, fjords, and proglacial lakes. Existing facies models for Gilbert-type deltas are largely qualitative; this study presents a quantitative analysis of the variability in facies architectures of such deltas. A database approach is used to characterize the preserved sedimentary architecture of 62 Gilbert-type deltas of Cretaceous to Holocene ages developed in various basin settings worldwide. Data on 706 architectural elements and 12,872 facies units are used to develop quantitative facies models that describe the variability in architecture and facies of Gilbert-type deltas at multiple scales of observation, and to account for the possible controls exerted by allogenic and autogenic factors.
The analysed data reveal high variability in the geometry and facies of Gilbert-type deltas. The thickness of the examined deltas varies from 2 to 650 m, yet positive scaling between delta thickness and length is consistently recognized across the studied examples, which is interpreted in terms of relationships between accommodation, sediment supply and delta lifespan. Based on their facies character, the deltas are classified into gravel- and sand-dominated types, with contrasting facies organizations of topset, forest and bottomset elements, and by different relationships between facies and dimensions; yet, both types exhibit significant spatial variability in the distribution of sediments linked to debris flows or turbidity currents, and in vertical stratal trends. Changes in allogenic (e.g., changes in base-level or, rate of sediment influx) and autogenic mechanisms (e.g., channel avulsion) are inferred as causes for significant differences in facies organization, both across distinct deltas and within individual deltaic edifices.
The study highlights the marked variety of architectural and sedimentological (e.g., grain size, depositional processes) properties of Gilbert-type deltas. Findings allow the relation of outcrop observations to a general template and the quantitative determination of potential analogues with which to assist the prediction of the dimensions and facies of deltaic sedimentary bodies in the subsurface. Information on facies relationships and basinward variability of Gilbert-type deltas is valuable for the recognition and correlation of deltaic bodies in the subsurface.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)