Abstract

The Meganos Gorge is a large fossil canyon of late Paleocene-early Eocene age present in the subsurface in portions of Contra Costa, San Joaquin and Sacramento Counties, California. The Meganos Gorge Fill crops out on the north flank of Mount Diablo as Divisions, A, B and C of the Meganos Formation as defined by Clark and Woodford in 1927. Axial length of the preserved portion is 71 kilometers (44 miles) covering an area of approximately 518 square kilometers (200 square miles). Maximum thickness of Gorge Fill is about 762 meters (2500 feet). Volume of the preserved section is approximately 104 cubic kilometers (25 cubic miles). Average slope of the gorge sides varies from 5 to 16 degrees. The Gorge Fill is dominantly shale, entirely marine and contains a foraminiferal fauna of Laiming’s D Zone (early Eocene). An extensive thickness of sand fill at the surface outcrop may represent a lower fan deposit portion of the Gorge Fill.

Sediments beneath the Capay Shale and above the Starkey Sands east of the Midland Fault are predominantly Upper Cretaceous. These sediments have in the past been erroneously designated “Meganos-Martinez undifferentiated” because of paleontological determinations from wells which had penetrated Gorge Fill shale prior to recognition that a gorge was present.

The eastern (inland) extremity of the gorge at Walnut Grove has considerable sand concentrated in the central portion of the Gorge Fill with shale at both sides.

Erosion was subaerial east of the Midland Fault which approximated the eastern limit of the depositional basin at the time of gorge cut. Erosion west of the Midland Fault was submarine. Although the Midland Fault was active before and after the cut and fill of the gorge there is no indication of measurable offset during the time interval of the cut and fill.

The dominant portion of the Gorge Fill is marine shale that has undergone considerable compaction relative to the surrounding sediments into which the gorge was cut. This compaction exerts a major effect on structure and the thickness of overlying formations. Failure to recognize the geologic effects of this compaction leads to serious errors in interpretation, including: 1. False interpretation of seismic closure in areas near the edge of the gorge. 2. Mapping faults that do not exist. 3. Anticipation of anticlinal closure at depth beneath compaction anticlines. 4. Failure to recognize the correct productive limits of fields. 5. Incorrect subsurface structural interpretations.

The Gorge Fill shale provides entrapment of gas in several fields from different types of stratigraphic traps. These fields include many of the best accumulations in the southern Sacramento Valley.