Insights from Potential-Field Data on the Structural Framework of the Sacramento Delta, California
Abstract
Potential-field data combined with geologic map and oil and gas well data indicate that a large basin underlies the Sacramento Delta. The basin geometry is important for Delta ground-motion studies. The basin, as defined by a large gravity low, is 100 km by 40 km. The deepest part of this asymmetric basin, as defined by the lowest gravity values, is bounded on the west by the east-dipping Vaca-Kirby Hills fault. A prominent gravity gradient coincides with the fault, as does a short-wavelength magnetic anomaly attributed to the truncated western extent of magnetic Miocene Neroly Sandstone. New gravity and aeromagnetic data indicate that this basin margin south of the confluence of the San Joaquin and Sacramento Rivers is more complex, stepping to the east along N- to NNW-striking structures around the prominent uplift (and 40-mGal gravity high) of Mt. Diablo. The eastern boundary of the gravity-defined basin does not coincide with the Midland fault, but rather with an inflection just west of the gently southwest-dipping basement surface defined by well intercepts. The southern margin of the basin shoals gently to the south to a local gravity high coincident with the upthrown side of the Stockton, Vernalis, and West Tracy reverse faults. The northern basin margin also shoals gently to the north at the latitude of the Dunnigan Hills anticline and fault. The basin axis as defined by lowest gravity values appears to be offset 5 to 10 km east of the Tertiary and Late Cretaceous depocenters as defined by well data, indicating that the gravity low may result from several superposed basins. A 2-D gravity and magnetic model across the basin suggests that the lowest part of the composite basin may be 9-10 km deep. The estimate of basin depth is complicated by low density (2500 kg/m3) volcanic basement intersected at 5 km depth in a well near the southern basin margin, suggesting some gravity variation due to differences in basement density rather than depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMGP43A1226L
- Keywords:
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- 0920 Gravity methods;
- EXPLORATION GEOPHYSICSDE: 0925 Magnetic and electrical methods;
- EXPLORATION GEOPHYSICSDE: 1219 Gravity anomalies and Earth structure;
- GEODESY AND GRAVITYDE: 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISM