Abstract

Holocene fan-delta systems form along narrow coastal plains in close proximity to high-relief, relatively young mountains. Fan morphology is best developed along microtidal coastal zones characterized by abundant rainfall. Fan-delta deposits are fault-bounded on their proximal margins and consist of gravel and gravelly sands that are usually immature to submature texturally and mineralogically.

Fan-deltas are common Holocene depositional systems along divergent plate margins, convergent plate margins, and strike-slip margins. Ancient fan-delta deposits are generally found along the margins of tectonically active basins analogous to Holocene settings as well as along the margins of some intracratonic basins.

The differentiation of fluvial and beach conglomerates, conglomeratic sandstones, and sandstones is crucial to the recognition and paleogeographic reconstruction of ancient fan-delta systems. A series of criteria useful in core and outcrop have been developed for this purpose.

In general, distinguishing features of channel deposits are poor sorting, less well segregated/more lenticular bedding, erosional contacts, small-scale fining-upward sequences, clayey and coaly laminae, gravelly sandstones, coarser clast size with higher sphericity, lower roundness and upcurrent dipping imbrication. Horizontal beds and swash laminae are common in beach deposits, and large-scale trough and/or planar crossbeds and horizontal beds are common in braided fluvial deposits. Also important is the differentiation of fan-delta and submarine fan deposits. In fan-delta deposits conglomerates are usually more abundant, nongraded, and clast supported. Nonmarine fan-delta deposits are more poorly sorted, have less well developed bedding continuity and regularity, and lack graded beds, Bouma sequences, and slump fold strata.

Models for recognition of fan-delta deposits are designed to differentiate slope, shelf, and Gilbert-type sequences. Slope-type fan-deltas are truncated by the shelf-slope break and have poorlydeveloped coarsening-upward trends. Shelf-type fan deltas are more fully developed and display well developed coarsening-upward trends. Gilbert-type fan-deltas have gravelly foreset beds and are known only from some ancient intracratonic basins.

The importance of fan-delta deposits as hydrocarbon reservoirs is just being realized. Productive reservoirs in fan-delta deposits have variable porosity and permeability and may be difficult to evaluate with conventional wireline logging tools. They are found in divergent plate tectonic and foreland basin settings where combination structural-stratigraphic hydrocarbon traps are common.