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The Yager formation of Humboldt County, California, comprises well-bedded mudrock, sandstone, and conglomerate of Paleogene age. These strata are much less deformed than coeval broken formations and melange of the Coastal Belt Franciscan. We infer that deposition occurred within slope, slope-channel, and trench-slope-basin environments, in an overall subduction or transpressional tectonic regime.
The following observations suggest that Yager basins were both restricted in size and elongate in shape: (1) feeder-channel
and mid-fan facies associations are common within the Yager; however, outer-fan depositional-lobe sequences are both rare and poorly developed, and basin-plain deposits are absent. Sandy mid-fan deposits typically grade into thin-bedded and poorly cyclic fan-fringe turbidites. It is likely that "normal" distal-fan facies associations failed to develop because of restricted basin geometries. (2) The regional distribution of turbidite facies and facies associations indicates that basin-fill sequences generally maintain good continuity along strike (northwest-southeast); the facies changes occur along transverse sections, and are, at least in part, temporally controlled. Thus, the basin-fill sequences appear to be elongate in a northwest-southeast direction, parallel with the dominant st uctural grain. (3) Thick sections of complexly folded mudrock are common within the Yager. These fine-grained strata, which include both hemipelagic shales and silty or muddy turbidites, are interpreted as slope deposits and are typically cut by lenses of coarse-grained, thick-bedded channel fill. The slope and channel sequences are nearly as prevalent as the sandy basin-fill sequences, which suggests that the Yager basins were not only restricted in size, but probably perched on an inclined, mud-covered slope.
Analogs for the Yager formation can be found along many modern subduction zones, where small, elongate basins typically form on the lower trench slope behind thrust-bounded or anticlinal ridges. Uplift of the ridges causes sediment-transport conduits (submarine canyons and slope channels) to become blocked; coarse detritus is thus trapped behind the ridges in a manner somewhat comparable to the salt diapirs which confine intraslope basins in the Gulf of Mexico.
Mean vitrinite-reflectance values for Yager shales near Garberville, California, are as high as 0.79%. Burial depths of as much as 6,000 m (19,700 ft) are indicated, using a geothermal gradient of 2°C/100 m. However, the depositional overburden associated with Neogene shallow-marine sediments of the Wildcat Group is estimated to be only about one-third the amount required. The additional overburden apparently resulted from mid-Tertiary thrusting along the Garberville thrust, a fault which marks the tectonic contact between Yager strata and melange of the Franciscan central belt terrane.
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