Geoscience Reference
In-Depth Information
Short-term or intrinsic variations in sedimentary pro-
cesses produce facies assemblages of these simple se-
quences, but major climatic or tectonic changes or the
long-term progressive erosion of the source area ('age-
ing') may cause a progressive vertical trend in sedimen-
tary style. Such changes have been identified in Quater-
nary fan sequences in Australia (Williams, 1973; Wasson,
1979), Nevada (Bluck, 1964) and Spain (Harvey, 1978,
1984b, 1990). In the Spanish case many fans show early
dominantly debris-flow deposition, followed later by sheet
and channel gravels and finally by trenching or dissection
becoming dominant over aggradation (Figure 14.4(g)). A
similar sequence has been observed at Zzyzx, California,
but with a strong climatic signal (Harvey and Wells, 1994;
Harvey, Wigand and Wells, 1999). Late Pleistocene fan
aggradation, dominantly by debris flows, was followed by
Holocene fanhead trenching and fan progradation under
fluvial processes (Figure 14.6). The timing of these phases
is constrained by the interaction of the fan system with
two dated shorelines of pluvial Lake Mojave (Harvey and
Wells, 2003).
Ancient alluvial fan sequences often show progres-
sive changes in sedimentary environment, often involv-
ing upwards coarsening of megasequences (Steel, 1974;
Steel
et al.
, 1977; Rust, 1979; Nilsen, 1982; Mack and
Rasmussen, 1984). In most cases the progressive changes
are seen largely as tectonically or 'ageing' controlled. The
coarsening-up model may be appropriate for distal loca-
tions on fans that are undergoing fanhead trenching, but
the proximal parts of an aggrading fan would be subject
to a fining-up tendency as backfilling progressively buries
the bedrock topography and any one site becomes progres-
sively further from bedrock outcrop and thus effectively
more distal (Figure 14.5(a)). It may be that on extant
Quaternary fans, where sections in proximal sediments
are commonly exposed in fanhead trenches, fining-up
trends are common (Harvey, 1990), but from the ancient
record where preservation potential may favour more dis-
tal environments a coarsening-upward trend may be more
common.
Over geological timescales, phases of fan sedimenta-
tion may only be temporary stages in the evolution of
sedimentary basins associated with basin uplift and inver-
sion (Mather, 1993). In the Neogene basins of southeast
Spain a phase of Plio-Pleistocene alluvial fan sedimen-
tation followed earlier marine and then low-energy ter-
restrial environments. Basinwide deposition ceased fol-
lowing uplift and dissection of the basins (Mather, 1993;
Mather and Harvey, 1995) and the conversion of the fans
to through drainage (Harvey, 2006; Maher, 2006; Stokes,
Figure 14.6
Summary sequence of late Pleistocene and Holocene alluvial fan erosion and deposition in Zzyzx, California: 1,
Mid Pleistocene heavily calcreted fan deposits; 2, main phase of Late Pleistocene aggradation: debris flow in proximal areas and
from small tributary fans, fluvial deposits in main distal fan; 3, Late Pleistocene shoreline of pluvial Lake Mojave; 4, hillslope
debris flows, contemporaneous with phases 2 and 5; 5, latest Pleistocene fan deposits resulting from limited fanhead trenching
and distal progradation; 6, Early Holocene shoreline of pluvial Lake Mojave; 7, lake shoreline sediments relating to phases 3
and 6; 8, hillslopes now stabilised during the Holocene; 9, Mid Holocene fan sedimentation, limited deposition at fan apex,
fanhead trenching in midfan and distal progradation by fluvial deposition; 10, further fanhead trenching and distal progradation
by processes during the Late Holocene; 11, active fan sediments; 12, modern salt flats of Soda Lake (modified after Harvey and
