Landsat Image

As at BP, Oasis is seemingly composed of three rings: a well-defined, prominent ring (referred to as the middle ring), a narrow, partly exposed central ring, and an outer "ring" composed of numerous discontinuous, concentric low hills. The Landsat image (Fig. 6 and color plate 2) indicates an average diameter of about 15.7 km for disturbed strata, and other sources (e.g., radar: Fig. 7) indicate almost 19 km. The limit of deformation on the north side is well constrained in some of the exposures at 8.8-8.9 km from the center (Figs. 6 and color plate 2), and to the northeast at a similar distance where the regional structural trends are undisturbed. Elsewhere, the limit is not well defined.

Fig. 11. Ground-based magnetic profile at BP crater, measured with an Envi-Mag proton magnetometer at 10 m spacing. Data were corrected using local base station data, measured at about hourly intervals. The position of the crater rim is indicated.

Inner Ring: The inner ring is ~1100-1300 m in diameter. It is defined by a bedrock arc from the northwest to west and southwest; the northern and northeastern part of the ring are missing or not exposed at the surface. The ring is about 200-250 m wide. It is separated from the middle ring by a region that appears like an alluvial surface or low hilly region. On the southern side, the bedrock exposures of the inner ring seemingly merge with the wide area of bedrock exposures of the middle ring. The distance between the inner edge of the middle ring and the outer edge of the inner ring is about 855 m.

Middle Ring: The bedrock exposures of the middle ring have a 5.5 to 6.2 km diameter. The diameter of the middle ring is variable with azimuth. It is widest and most continuous in the northwest (about 1300 m across). From the north around to the east and south, it is composed of closely spaced hills. Within the northwest part of the ring, the bedrock exposure is composed of concentric topographic highs and lows with the ridges spaced 2.6 to 3.7 km apart. The southwest part of the middle ring contains a large basin 1450 x 1740 m across.

Oasis Structure Central Ring

Fig. 12. (a, top) Approaching Oasis impact structure from the north; overview of the central ca. 5 km diameter ring of hills. (b, bottom) Oasis inner ring as seen from the south.

A major drainage feature occurs in the south-southwest. Here a channel extends from the center of the structure into the surrounding plains. It also has a linear dune associated with it. The drainage on the west-southwest side seems to cuts through the middle ring and extend into the hills between the middle and inner rings.

Outer Ring: The outer ring consists of concentric, discontinuous hills that extend from the middle ring (~ 5700 m in diameter) out to a distance of as much as 9 km from the center. Individual hills extend only short distances and are separated by alluvial/aeolian material. The hills appear to have little topographic expression vis-à-vis the middle ring. They are more obvious on the north and east sides, and more subdued to the southwest.

Fig. 13. Oasis impact structure. Strongly deformed (folded) section of inner ring of hills (on the eastern part of the structure).

In the western quadrant (azimuth 262°) of the outer ring there is an elliptical hill about 370 x 1140 m in size, at 4902 m from the center. It is topographically asymmetric, with a high eastern wall and a low western wall. A series of northwest trending (N61°W - N53°W) linear ridges cut across the region and cut the southern and western part of the Oasis structure.

Various spectral classification techniques (e.g., IsoData, Unsupervised, Principal Component) were used to determine the extent of the spectral variation within the rocks exposed at the surface. The different techniques produce essentially the same result, namely that within the constraints of spatial and spectral resolution all of the bedrock units are the same. The spectral classification of the bedrock associated with the Oasis structure is similar to that of the bedrock exposed to the north. The alluvial units exhibit some spectral variation. This variation probably results from different combinations of material across the alluvial surfaces and may be related to different fluvial systems.

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