The Markley gorge channel, an ancestral Sacramento river channel, is in the central part of the Sacramento Valley of California. This fossil channel was cut subaerially during late Eocene time and filled in a marine to marginal marine environment during late Eocene and Oligocene times. Before the late Eocene transgression, an early mature stage of stream erosion was reached. Terraces, islands, meanders, and tributaries can be delineated by subsurface methods. The slope along the channel banks ranges from 6 to 36°. Variations of the slope along the channel banks depend on the existing faults, structures, and drainage configuration. Because the gorge fill is unconformably overlain by Miocene continental sediments, the original relief is not known, but it at least excee s 3,000 ft. This channel has been traced in the subsurface more than 80 mi and has a maximum width of 10 mi. The north end of the gorge is the Wheatland Formation cropping out at the foot of the Sierra Nevada Mountains near the town of Marysville. The south end of the gorge is in the Montezuma basin where gorge sediments merge with deeper water sediments of the Sidney Shale and shallower water sediments of the Kirker Formation.

The gorge fill consists predominately of shale with minor amounts of sandstone and conglomerate which have considerable lateral and vertical lithologic variation.

Truncation of the underlying Eocene, Paleocene, and Upper Cretaceous formations by the Markley gorge combine with local structure to form commercial gas accumulations. Examples are: Maine Prairie (75 Tcf, 1,020 BTU); Liberty Island (24 Tcf, 988-996 BTU); Millar (18 Tcf, 980 BTU); Todhunters Lake (57 Tcf, 890-897 BTU); and Greens Lake (8 Tcf, 830-850 BTU). Along the north part of the Markley gorge, in the proximity of the city of Sacramento, gas traps exist in westerly dipping Upper Cretaceous sandstone reservoirs. These reservoirs occur in reentrants and islands within the predominantly shale-filled gorge. These spurlike reentrants were created by easterly trending tributaries which intersected the main southerly trending Markley channel.

Techniques in locating Upper Cretaceous reentrants and islands are well control, drainage pattern analysis, gravity profiling and mapping, seismic profiling and mapping, and differential compaction features.

An understanding of the geologic history of the gorge and of the truncated sediments is also necessary to locating potential future gas fields along the gorge.

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