Geoscience Reference
In-Depth Information
equilibrium that can occur often lead to a range of signif-
icant and far-reaching and often negative environmental
impacts (Gill, 1996).
many playas and, when they do so, may take thousands
of years to accumulate. Many of the larger salt lakes owe
their extensive evaporite deposits, usually in the form of
a series of mud-salt (Hardie, Smoot and Eugster, 1978) or
protodolomite-gypsarenite couplets (Dutkiewicz and von
der Borch, 1995), to the gradual or repeated desiccation of
larger water bodies by climatic change or tectonic activity,
as in the case of Lake Magadi (Kenya), Lake Bonneville
(USA), the Makgadkgadi Pans (Botswana) and the Dead
Sea (Israel-Jordan). In a number of instances (e.g. playas
in Tunisia and Australia) evaporate preservation is rela-
tively poor, with salt crusts (often 0.1-1 m) often being
partially or completely dissolved by groundwater or sur-
face water inflow (e.g. Bryant
et al.
, 1994a). This can lead
to the net accumulation of clay-silt sediment, which is
only capped by the thin salt crust during desiccation.
15.3
Influences of pan hydrology and
hydrochemistry on surface morphology
Pan surfaces change over time in the course of the pan cy-
cle
(
Lowenstein and Hardie, 1985; Bryant
et al.
, 1994a)
(Figure 15.10). While Stage 1 (flooding) is normally very
rapid, it is worth noting that Stages 2 to 4 can take a
variable period of time (0.5-100 years) depending on the
nature and level of groundwater interaction with inflow.
In all cases, however, influx of surface water halts evap-
orative loss from groundwater, reverses many of the re-
actions in the interstitial zone and sets up new gradients
between the surface and groundwater bodies. It also in-
troduces 'fresh' clastic material from inflow and from the
atmosphere, which settles on the lake bed and provides
an environment for organisms that play a part in over-
all sedimentation (Bryant
et al.
, 1994a). Diatoms, which
store SiO
2
(Neev and Emery, 1967), and algal mats, in-
volved in the precipitation of carbonates and the formation
of kerogen-rich organic layers (Brock, 1979; Grant and
Tindall, 1985), are particularly important in this respect.
Larger organisms, such as the brine shrimp
Artemia
, cause
bioturbation and sediment reworking (Eardley, 1938).
As surface waters evaporate (Stage 2) they become in-
creasingly brackish, leading to precipitation of salts at
the periphery of the water body and on the surface of
the brine as precipitation thresholds are reached. These
crystals, initially held by surface tension, sink to the lake
floor and become nuclei for further, distinctive patterns of
crystal growth (Lowenstein and Hardie, 1985). They may
also be concentrated by wind action (Stage 3) into arcuate
bands known as salt ramps (Millington
et al.
, 1995), which
persist as minor landforms after evaporation of the brine.
The desiccation of the pan surface (Stage 4) will lead
to further interstitial crystal growth and dissolution, and
an eventual return to the groundwater dominated regime.
In instances where these stages (1-4) represent the long-
term drawdown of the groundwater table in response to
climate changes or anthropic intervention, an additional
stage (Stage 5) representing degradation and reworking
of the pan surface through deflation of evaporate minerals
and silt-clay sediment (e.g. lunette formation) and fluvial
reworking can occur (Bowler, 1986).
Given the continual reworking of surface crusts and
sediments within the pan cycle, it is not surprising that
15.3.1
Pan topography
Pan surface morphology is the product of periodic flood-
ing and desiccation, including: (a) rainfall effects, (b)
groundwater depth, brine concentration and associated
crystal growth and dissolution at or near the sediment
surface and (c) aeolian deflation. Given the potential dy-
namism of these three factors, surface features themselves
are among the most ephemeral of geomorphological phe-
nomena, some lasting no longer than the interval between
one rainfall event and the next.
Haloturbation is an important process, usually involv-
ing gypsum or halite. In the saline mudflat and saline
pan environments surface cracking is apparent, leading
to polygon formation. Thin hard crusts of carbonates or
puffy crusts of more soluble minerals may appear on dry-
ing (Hardie, Smoot and Eugster, 1978). Surface flaking,
with gypsum precipitation, is also common.
Salt crusts have smooth surfaces only while above the
level of capillary action, as at Bonneville Flats (Eugster
and Kelts, 1983), or when wet; on drying, crystal expan-
sion leads to the formation of salt blisters and salt poly-
gons, the latter up to 10 m in diameter (Krinsley, 1970)
(Figure 15.11). Plate boundaries become foci for evapo-
ration and precipitation, producing thrust surfaces up to
50 cm above the pan floor and capable of lifting gravel
size material. Extrusion at the plate edges may lead to the
formation of mud and salt pinnacles (Figure 15.12(a)).
Subsequent inundation and desiccation leads to a fresh
cycle of polygon development.
Under artesian conditions groundwater effluents may
be marked by the growth of spring mounds or dissolution
of salt karst chimneys (Last, 1993), known collectively
