John G. McPherson

Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages

ABSTRACT Contrary to common contemporary usage, alluvial fans are a naturally unique phenomenon readily distinguishable from other sedimentary environments, including gravel-bed rivers, on the basis of morphology, hydraulic processes, sedimentologic processes, and facies assemblages. The piedmont setting of alluvial fans where the feeder channel of an upland drainage basin intersects the mountain front assures that catastrophic fluid gravity flows and sediment gravity flows, including sheetfloods, rock falls, rock slides, rock avalanches, and debris flows, are major constructional processes, regardless of climate. The unconfinement of these flows at the mountain front gives rise to the high-sloping, semiconical form that typifies fans. The plano-convex cross-profile geometry inherent in this f rm is the inverse of the troughlike cross-sectional form of river systems, and precludes the development of floodplains that characterize rivers. The relatively high slope of alluvial fans creates unique hydraulic conditions where passing fluid gravity flows attain high capacity, high competency, and upper flow regime, resulting in sheetfloods that deposit low-angle antidune or surface-parallel planar-stratified sequences. These waterlaid facies contrast with the typically lower-flow-regime thick-bedded, cross-bedded, and lenticular channel facies, and associated floodplain sequences, of rivers. The unconfinement of flows on fans causes a swift decrease in velocity, competency, and capacity as they attenuate, inducing rapid deposition that leads to the angular, poorly sorted textures and short radii typical of fans. This condition is markedly different than for rivers, where sediment gravity flows are rare and water flows remain confined by channel walls or spill into floodplains, and increase in depth downstream. The distinctive processes that construct alluvial fans, coupled with the secondary surficial reworking of their deposits, yield unique facies assemblages that permit the easy differentiation of fan sequences even where the geomorphic context has been lost, including in the rock record. The fault-proximal piedmont setting critical for their preservation makes properly identified alluvial-fan deposits in the rock record an invaluable tool for reconstructing and interpreting the tectonic and stratigraphic evolution of ancient sedimentary basins and their contained register of Earth history.

Fan-deltas and braid deltas: Varieties of coarse-grained deltas

Two types of coarse-grained deltas are recognized: fan-deltas and braid deltas. Fan-deltas are gravel-rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland. They occupy a space between the highland (usually a fault-bounded margin) and the standing body of water. In contrast, braid deltas (here introduced) are gravel-rich deltas that form where a braided fluvial system progrades into a standing body of water. Braid deltas have no necessary relationship with alluvial fans, as exemplified by fluvioglacial braid deltas. Braid deltas have previously been classified as fan-deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different. Braid deltas are a common present-day geomorphic feature and are abundant in the geological record. Fan-deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems; the shoreline and subaqueous components of both are similar. Fan-delta sequences have a subaerial component that is an alluvial-fan facies comprising interbedded sheetflood, debris-flow, and braided-channel deposits. Fan-deltas produce small (a few tens of square kilometres), wedge-shaped bodies of sediment, commonly displaying high variability in paleocurrent patterns and abrupt changes in facies. The deposits are generally very coarse grained (with large out-sized clasts), very poorly sorted, matrix-rich, polymictic, heterolithic, partially cemented by penecontemporaneous carbonate, and have low porosity and permeability. Braid-deltas, in contrast, have a subaerial component consisting entirely of braided-river or braidplain facies. Their deposits display better sorting, roundness, and clast orientation than do fan-delta sediments; they lack a muddy matrix; they display size grading and bar migration; they commonly have a sheet geometry with high lateral continuity (tens to hundreds of square kilometres); and they exhibit moderate to high porosity and permeability. Valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse-grained deltaic systems are not differentiated.

Alluvial Fan Processes and Forms

Alluvial fans are a prominent landform type commonly present where a channel emerges from mountainous uplands to an adjoining valley. Although occurring in perhaps all global climatic regimes, fans in deserts traditionally have been the most studied due to their excellent exposure and ease of access. This chapter attempts to (a) provide an up-to-date synthesis of the literature on alluvial fans in desert settings, and (b) introduce a framework for their understanding based on concepts that have emerged during the last 120 years of scientific research. This synthesis emphasizes recent developments in fan studies as well as published and unpublished results from our own work in the south-western United States. The conceptual framework developed in this chapter, despite being exemplified by fans from deserts, is also applicable to fans forming under other climatic conditions.

Processes and Forms of Alluvial Fans

Alluvial fans are a conspicuous conical landform commonly developed where a channel emerges from a mountainous catchment to an adjoining valley (Figs. 14.1 and 14.2). Although present in perhaps all global climates, fans in deserts have been the most studied due to their excellent exposure and ease of access. Drew (1873), working in the upper reaches of the Indus River valley in the western Himalaya of India, provided the earliest illustrations and scientific description of desert alluvial fans (pp. 445–447):

Comparison of turbidite facies associations in modern passive-margin Mississippi fan with ancient active-margin fans

Abstract Our comparison of the modern passive-margin Mississippi fan (DSDP Leg 96) with ancient active-margin fans (e.g. Eocene Hecho Group, Spain) reveals major differences in turbidite facies associations (Mutti and Ricci Lucchi scheme) and in seismic characteristics in the lower fan area. The lower (outer) Mississippi fan is composed of channel (Facies B and F) and non-channel facies (C? and D), whereas ancient fans are characterized by non-channelized, thickening-upward, depositional lobe facies (C and D). An absence of depositional lobes in the lower Mississippi fan is also suggested by a lack of convex-upward (mounded) seismic reflections with bidirectional downlap. Continuous seismic reflections of the lower Mississippi fan may represent “sheet sands”, but not those of true depositional lobes with mounded character. Extensive channelization in modern passive-margin fans appears to be a product of the lateral shifting of a major sinuous distributary system, developed as a consequence of low gradients and the transport of sediment with a relatively low sand/mud ratio. In contrast, channels in active-margin fans are short and of low sinuosity as a result of high gradients and the transport of sediment with a relatively high sand/mud ratio. The turbidite facies association scheme, which was developed exclusively from ancient active-margin fans, should be applied to mature passive-margin fans with qualifications because of the differences in spatial distribution of turbidite facies and their associations.

Historical adjustments by Walker River to lake-level fall over a tectonically tilted half-graben floor, Walker Lake Basin, Nevada

Abstract Historical records of lake and river adjustments in the tectonically active, north—south elongated, Walker Lake extensional basin of west-central Nevada provide important insight to the style and rate that rivers react to tectonic tilt in half grabens. The northern part of this basin, historically containing Walker Lake, is now occupied by the south-flowing Walker River. Walker Lake, presently restricted to the central basin sector, is a perennial water body 30 m deep sustained by, and forming the terminus of, Walker River. Both the river and lake have a strongly asymmetric distribution, located preferentially near the active Walker Lake fault bounding the western basin margin. Walker Lake has withdrawn from the northern basin sector since 1882 due to a 45 m drop in lake level caused by human diversion of the river upstream. This withdrawal has forced Walker River to incrementally lengthen, and to sequentially reposition its delta along the retreating northern lake margin. Twelve deltas have been deposited since 1882 in response to these changes. The initiation of eight of the twelve post-1882 deltas was associated with channel avulsion, and four with channel lengthening. Ten of the twelve channel and delta relocation events entailed significant westward lateral movements toward the Walker Lake fault, illustrating the dominating influence of the basin-floor tectonic tilt on river adjustments. The two eastward shifts were triggered intrinsically by avulsions resulting from erosion of the outer bank of channel meanders. The three phases of progressive westward shifting of the river towards the Walker Lake fault occurred over periods of 18, 6, and 16 years, respectively, documenting the rapid response time of Walker River to basin-floor tectonic tilt despite the countering effects of the intrinsic avulsions, and hindrances to lateral migration caused by the muddy substrate of the former lake bottom deposits within which the river is incised.

Fluviodeltaic Reservoir, South Belridge Field, San Joaquin Valley, California

A high percentage of the oil produced from sandstone reservoirs comes from fluviodeltaic sandstones. The abundance of high-quality reservoir sand, the ideal location relative to downdip source beds, and the high potential for stratigraphie trap development all make this depositional setting a favored target for explorationists. However, the sand-body architecture and reservoir quality of fluviodeltaic sandstones commonly display extreme variability over short vertical and lateral distances due to depositional controls. The fluvial-dominated delta system described in this chapter displays a wide spectrum of depositional settings from sandy braided fluvial to muddy delta front. This case study presents an opportunity to compare and contrast the reservoir characteristics of distal-bar, mouth-bar, meandering fluvial, and braided fluvial sands from the same depositional system in a single field, with well spacing close enough to make correlations relatively certain.

Three-Dimensional Geometry of Fluvial Reservoir Sands: Steam-Drive Case Study: ABSTRACT

The three-dimensional geometry of fluvial sands in South Belridge heavy oil field was investigated as part of an Enhanced Oil Recovery study. It was shown that only close-spaced well data are sufficient to define the sand-body geometries and heterogeneities of multichannelled fluvial systems. Reservoir flow-unit patterns cannot necessarily be correctly delineated by isolated vertical sequence analysis. Wireline logs from 19 wells and conventional cores from seven wells in a 10-ac (660 ft x 660 ft) pattern were correlated in detail, using additional input from sedimentology, steam-flow patterns, and reservoir flow-unit continuity.