Geoscience Reference
In-Depth Information
Figure 14.17
Segmented fan surface: Grotto Canyon fan, Death Valley, California. Note the older dissected basin-fill sediments
(left foreground), three phases of fan aggradation, represented by the three fan surfaces, differentiated by the degree of dissection
and colour as influenced by varnish development. Separating each aggradational phase was a dissectional phase. As a whole the
fan is a telescopic, prograding fan.
14.4.2.2
Northern Nevada
to the effects of Quaternary climatic change, modified lo-
cally by fan setting and catchment characteristics (Harvey,
1990). Aggradation dominates the Quaternary cold stages,
which in this area were cold and dry (Harvey, 2002b,
2003), and some form of fan dissection, fanhead trench-
ing, midfan trenching or throughfan dissection, charac-
terised the milder semi-arid Mediterranean interglacials.
In zones of local tectonic and base-level stability the se-
quence is a simple climatically induced one. In some areas
the climatic signal is modified by a tectonic signal (e.g.
Carrascoy fans: Harvey, 1988; Silva
et al
., 1992) or by
a differential, tectonically induced base-level effect (e.g.
Tabernas fans: Harvey
et al
., 2003).
On the Cabo de Gata fans (Harvey
et al.
, 1999) the
same climatic signal can be identified, but on those fans
facing the coast it is complicated by the effects of base-
level change following the Quaternary sea-level changes
of the Mediterranean Sea. High interglacial sea levels
caused coastal erosion of the fan toes, which foreshort-
ened the fan profiles, triggering incision, which cut back
through the fans (see above, Section 14.3.2.3). During
interglacials reduced sediment supply had resulted in fan-
head trenching at the same time as the basal-induced distal
incision occurred (Figure 14.18(a)), resulting in through-
fan trenching (Harvey, 2002b, 2002d, 2003). During the
intervening glacials proximal fan aggradation resumed
and as sea level fell, exposing only a low-gradient sea-
floor sediment simply prograded on to the exposed sea
On the Stillwater fans in northern Nevada, major periods
of sedimentation appear to have coincided with periods
of aridity, and during the colder periods of the Late Pleis-
tocene the fans appeared to have been receiving very little
sediment (Harvey, Wigand and Wells, 1999). At those
times lake levels of pluvial Lake Lahontan lapped on to
the fan toes. As lake levels fell at the end of the Pleistocene
the steep slope of the foreshore allowed distal fan incision
to take place, but it was only during the increasing aridity
of the Mid Holocene that the fans prograded substantially
on to the exposed lake shore (Figure 14.18(b)), resulting
in extensive lobes of sediment extending on to the former
lake floor (Harvey, 2002b, 2003, 2005).
14.4.2.3
Musandam, UAE and Oman
The Quaternary fans of the Musandam peninsula can
be grouped into two, mountain front and tributary junc-
tion, fans (Al Farraj and Harvey, 2000, 2005). Base-level
change has had little or no influence on the dynamics of
the mountain front fans, except in the very northern part
of the UAE where they are lapped by the Arabian Gulf
coastline (Al Farraj and Harvey, 2004). On the other hand,
the tributary junction fans in Wadi Al Bih, the major west-
coast drainage system, have been subjected to major local
base-level changes, related to wadi-floor aggradation and
