A thick succession of fossiliferous Lower Cretaceous clastic rocks crops out near Ono, California. Although these rocks have been studied by California geologists and paleontologists for almost 100 years, no detailed maps or sections have ever been published.

This district is the type area for the stratal unit previously called the Horsetown “group” or “formation,” here redefined as the Horsetown stage. Eight biostratigraphic zones, ranging in age from Hauterivian to late Albian in the European time scale, are defined within this stage. The upper and lower boundaries of the stage are defined by the bases of the zones of Peninquieria hdenana and of Neocraspediles aguila, respectively.

Two new formations are recognized within the mapped area; the Rector formation, a thin sand-stone and conglomerate unit, is markedly transgressive and lies on the igneous and metamorphic rocks of the Klamath Mountains; the Ono formation, a thick mudstone unit with two coarse clastic tongues, lies on the Rector formation and underlies unnamed conglomerate units of probable late Albian age. The rocks of the Ono formation are predominantly mudstone with nodular limestone; graywacke and conglomerate comprise about 30 per cent of the section. The boundaries of the lithologic units are not coincident with those of the Horsetown stage.

A southeast-dipping homocline complicated by numerous small and several larger transcurrent faults is the main structural feature of this area.

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