"ELKO CRATER FIELD" REVISITED - RECONNAISSANCE REPORT

MF-1168 is entitled "Map Showing the Elko Crater Field". This paper reports on a subsequent field survey of this "Elko Crater Field". This survey was conducted during April/May of 2001, and was intended to be only a reconnaissance effort for the purposes of securing evidence of ejecta or meteoritic material, and to examine the stratigraphy of the craters. This survey did not result in any finds of ejecta or meteoritic material. A close inspection of the stratigraphy of the "crater rims" did not reveal any evidence of disordered layering. All of the above is considered as negative evidence for an impact origin for the Elko Crater Field. An exposure that is nearby a cluster of "craters" in the Suzie Creek area revealed evidence, which affords an interpretation for a terrestrial geologic origin for all of these depressions. A thin (~1m) veneer of colluvium overlies a section of volcaniclastic sediment that can be divided into an upper and lower unit. The upper unit (~20m) is a light brown to tan-colored coarse sand that is interbedded with thinly layered, fine-grained tuff. The lower unit is predominantly a greenish-gray bentonitic (expansive) clay. The base of the lower unit is not exposed in the Suzie Creek area, but where the tributaries come in contact with this unit, a flat-floored streambed is produced. The "craters" only occur within exposures of this formation. In addition, many small landslides and soil slumps are in evidence throughout the Suzie Creek area. The Elko Craters are interpreted by this writer as being a subsidence feature within the "upper" volcaniclastic unit, the result of groundwater sapping of the "lower" bentonite-rich unit. Typically, these depressions are rimmed by the uppermost "veneer" of cobble-rich colluvium which (in a process that is still not clear) armors the rim against erosion, giving the appearance that the rim is "raised". Regardless of whether this interpretation is valid, the Elko Crater Field is a unique and geologically striking feature that requires additional investigation. Should this interpretation have some merit, and given the ease these features can be discerned in satellite imagery, there is the implication (if similar conditions can exist on Mars) that features originally interpreted as being craters may actually be depressions produced through subsidence by groundwater sapping.