Bristol Dry Lake, USA

Bristol Lake is a dry lake in the Mojave Desert of San Bernardino County, California, 42 km (26 mi) northeast of Twentynine Palms. Bristol Dry Lake is located southeast of Amboy, California and U.S. Route 66, and is also south of Cadiz, California. Amboy Crater and the Bullion Mountains are to the west, and Old Woman Mountains to the east.
The lake is approximately 23 km (14 mi) long and 20 km (12 mi) at its widest point.

At the surface, the Bristol Dry Lake basin in the Basin and Range province of California exhibits what appears to be a slightly off centre bull’s-eye distribution of surface and near-surface salts (Handford 1982; Rosen 1991). Low-gradient alluvial fans ring the 10-km wide playa, while extensive calcretes and pedogenic calcites, associated with halophyte plants, cement mid-to-distal fan gravels and sands. 

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Bristol Dry Lake, California, view of the saline pans from the highway at a time of subaerial exposure.

There is a 62 km2, ≈2 m thick, roughly circular and level black basaltic lava flow atop the playa surface along the west and northwest edges of the lake depression. It originated at the same time as the Amboy Crater and is likely a Holocene feature. It indicates ongoing hydrothermal circulation, so explaining the CaCl2 brines that typify lake porewaters and the natural, but highly unusual occurrences of antarcticite precipitates in the lake trenches (Salty Matters, May 31 2017).

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Bristol Dry Lake, California. A) Plan view showing main at surface geology. B) Excavated brine trench where antarcticite can form with cooling of halite-saturated brine.

Basinward of the distal fans, in the playa margin facies, is a centripetal gypsiferous sabkha some 300 m wide where celestite precipitation forms large decimetre-size nodules that can coalesce into metre-size patches. The gypsum unit is characterised by a growth-aligned fabric created by competitive alignment of gypsum prisms in a zone of capillary wicking (Rosen and Warren, 1990)  Once these gypsum crystals are buried and converted to aligned anhydrite nodules it would be next to impossible to distinguish them from the much more commonplace subaqueous textures. Basinward of the playa margin facies, halite hoppers (up to 0.5 m in diameter) form displacive intrasediment precipitates in brine-saturated muds of the saline mudflat facies. Finally, in the basin-centre depression, a 0.2-m thick chevron-halite crust (salt pan) is forming by evaporation of ponded ephemeral waters. The lateral facies distribution seen at the surface also stacks throughout the entire length of deep cores collected in the playa (Rosen 1991). Saline pan halite beds alternate with halite-saturated siliciclastic muds in a 500-m core from the basin centre. Playa margin cores exhibit alternating beds of playa margin sediments and saline mudflat (sabkha) units for most of their length. Towards the base of the deepest playa-margin core, distal alluvial fan sediments are also present.

Rosen (op. cit.) notes that a delicate balance between subsidence and mechanical/chemical deposition of minerals was necessary to maintain a vertically-consistent ephemeral brine environment during the deposition of over 500-m of basin fill.I would say this is not really an example of some rather unusual delicate balance preserved in the sedimentary signature, so much as it is evidence of a hydrological/deflation forcing mechanism. It is a response to the same Stokes-surface hydrology that drives sedimentation in the sabkha and pan regions of most Quaternary continental playas and sea margin sabkhas. Accommodation space in the Bristol Dry Lake depression is created by subsidence in a tectonically active region, in what has been a variable but mostly arid climate throughout the basin’s Quaternary history. The basin’s hydrology dictated the contrast in accumulation styles between the salt rich succession in the basin centre and the coarser siliciclastic dominated basin edge.

The volume of sediment accumulating in the basin centre in Bristol Dry Lake can never surpass the ability of the wind to blow any dried sediment out of this depression centre into surrounding areas. That is, the surface of the sedimentation in the basin lowermost parts is maintained by deflation at a position that is always near the watertable. The only types of sediment that can remain in the arid basin centre are in equilibrium with the watertable and the top of the overlying capillary zone. When the watertable was a metre or so below the sedimentation surface, aggrading sediments were held in place by meniscus forces inherent to the capillary fringe and accumulated in saline mudflats as sabkha sediments. When the watertable rose to become a water surface, it created a semipermanent saline brine pan that then deposited and stacked salt pan crusts. 

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Generalized stratigraphy based on Bristol Dry Lake form the alluvial fan margin into the saltflat saline pan centre. (after Handford, 1991; Rosen and Warren, 1990).

Basin edge sedimentation was not hydrologically forced, but built up as volumes of centripetally-supplied sediment accumulated. This is turn facilitated the creation of an aggrading potentiometric head feeding ground and surface waters to the basin centre lows. About the edges of Bristol Dry Lake, where the accumulating clastic sediment is closer to its source, it is coarser-grained and less liable to deflation. Clastics are accumulating in sufficient volumes to build facies belts situated well above the regional watertable, hence the accumulations of alluvial fan, dune and sandflat sediments. At times when the subsidence rate exceeds the rate of sediment supply in this area, its hydrology becomes more like that of the basin centre, and saline mudflat facies come to dominate over much of the playa floor. Such a hydrologically forced system means that when regions of maximum subsidence in a basin change position, it moves the location of the hydrologically forced sediments into the new lowermost areas on the playa floor. Ultimately, once the rate of subsidence decreases to where the rate of clastic sediment supply exceeds the ability of the wind to deflate, the margin sediments will cover the whole basin, and evaporite sedimentation will cease.

Burial anhydrite

Dense saturated bottom brines percolating down through a shallowly buried primary evaporite unit can create diagenetic havoc within the original texture. For example, saturated bottom brines filtering through a buried cumulate halite deposit can convert it into a tightly cemented coarsely crystalline halite mosaic retaining little evidence of the primary texture (Warren, 2016, Chapter 1). Dense warm burial brines flushing through gypsum beds that are initially dominated by capillary gypsum crystals can set up diagenetic conditions where the gypsum is replaced by nodular anhydrite pseudomorphs of the original gypsum. Such changes occur in Bristol Dry lake as gypsum formed in near-surface capillary hydrology transitions in nodular anhydrite in burial.

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Gypsum converts to burial anhydrite in Bristol Dry Lake, California. A) Lenticular gypsum meshwork from a lake floor sediment sample a metre below the lake surface. B) nodular anhydrite still retaining the outline of the gypsum precursor. Sample from a lake core some 140 metres below the lake surface.

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After a storm, freshened waters cover the salt pans of Bristol Dry Lake.


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