Barnes and Morin reported in 1988 that a gravity survey of McCarthy's Marsh (sometimes referred to as the Fish River Flats) was completed during August 1977 and included 80 measurements. The resulting contoured data indicate a small sedimentary basin localized in the northeastern corner of the topographic depression which forms the marsh.
Faults probably border both the northern and eastern edges of the structural basin, which was probably filled by sediments eroded from the adjacent
mountains. These mountains include plutons rich in radioactive minerals, which may have been locally concentrated in the sedimentary rocks within the basin; and a potentially commercial uranium deposit has now been discovered in another valley on the opposite side of the Darby Mountains. Geophysical
modeling suggests a minimal sedimentary thickness of about 1 km and possibly up to 2 or even 3 km depending on the density of the sediments filling the
basin.
Geomorphic evidence indicates recent or active faulting at one basin boundary and might thus suggest a young age, low sediment density, and small total thickness. However, coal specimens collected near the edge of the basin may be Tertiary in age and thus suggest an older basin age, probably higher sediment density, and higher total thickness.
The authors concluded that the gravity data are summarized in the contour map of Plate 1, which shows an [unusual?] triangular shaped low in the northeast corner of the topographic basin. This low probably represents the deepest part of the structural basin. The closely-spaced linear contours on the north and east sides of the gravity low are interpreted to be caused by high-angle faults. The gradient on the north side is approximately centered on the scarp trace of the Bendeleben fault. Geologic mapping (Miller and others, 1972, and Till and others, 1986) south of the gravity survey suggested another fault on trend with the belt of parallel contours on the east side of the basin, although the fault trace was not actually located.
Plate 2 shows aeromagnetic contours from the same area which are taken from surveys at 1,000 ft elevation and with 1-mile flight-line separation flown
by Lockwood, Kessler, and Bartlett, Incorporated for the Department of Natural Resources, Alaska Division of Geological Survey (1971). Closely spaced
magnetic contours on the edge of the gravity low are interpreted as resulting from volcanic flows within the sedimentary section. Such flows crop out on
the south side of the Bendeleben fault. The remaining magnetic contours are consistent with a thick sedimentary section.
The calculated depth of the basin depends on the density of the material within it, which is probably controlled in part by the sedimentary history of
the structure. Figure 1 is a computer plot of profile AB (located on Plate 1), and of one possible basin cross section calculated by a simple two-dimensional iterative computer program using the method of Bott (1960) on a profile from which a regional gradient of 15 mGal in 40 km has been subtracted.
The vertical scale is enlarged eight times, exaggerating the Vlike result of the calculation, which is only a preliminary approximation for a profile so close to the corner of the basin. The sediment density used in the model was 1.67 g/cm (a density contrast of 1.00 g/cm), which is close to a minimum for uncompacted water-saturated sediments, so the calculated depth of more than 1 km might be considered a minimum depth. However, if the basin was formed and filled primarily in Quaternary time by clay and silt sediments, the thickness of permafrost and ice lenses could be large, and the depth might
be much smaller. However, the presence of volcanics within the section tends to diminish the possibility of a thick ice section. Figure 2 is another model of the same profile using a sediment density of 2.17 g/cc (density contrast 0.50 g/cc). Such a density would be more typical of mid-Tertiary sedimentary
rocks, and the depth (2.5 km) might be considered more reasonable for a structure this large. Allowance for the inaccuracy of the two-dimensional
model would also tend to increase the calculated depth.
Figure 5 is another profile which is located closer to the center of the basin, but which is supported by fewer gravity measurements. Along this line
the two-dimensional gravity model is probably more valid, and the 2.00 g/cc (0.67 g/cc density contrast) sediment density of the model may be a more
probable average. This profile suggests that the most probable depth is slightly less than 1.4 km, which would be increased if older sediments were
assumed or if allowance were made for inaccuracies in the modeling technique.
See Plates:
Plate I: https://i2mconsultants.com/downloads/corp/McCarthyBasin1988reportPLATE1.pdf
Plate 2: https://i2mconsultants.com/downloads/corp/McCarthyBasin1988reportPLATE2.pdf
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