Abstract

The Red Hills is the easternmost range within the Basin-and-Range province at the latitude of Parowan in southwestern Utah. It exposes two important late Mesozoic geologic margins — the westernmost deposits of the Cretaceous Interior Seaway and the easternmost thrusts of the Sevier orogenic belt. Jurassic to Eocene sedimentary strata, overlain by Tertiary pyroclastic deposits, record Sevier contractional deformation and foreland sedimentation overprinted by Miocene to present Basin-and-Range extensional deformation. Three east-vergent Sevier thrusts exposed in the Red Hills juxtapose, from west to east, the Lower Jurassic Navajo Sandstone over isoclinally folded Middle Jurassic Carmel Formation, the Carmel Formation over overturned Late Cretaceous Straight Cliffs Formation, and the Straight Cliffs section over the subhorizontal Late Cretaceous Iron Springs Formation (the “Iron Springs thrust”). The Straight Cliffs section exposed in the Red Hills has previously been mapped as the “Lower Iron Springs Formation”, but is reassigned here to the Straight Cliffs Formation based on its marginal marine character (oyster coquina and a succession of progradational nearshore/shoreface/back-beach parasequences).

The central (Carmel/Straight Cliffs) thrust in the Red Hills is overlapped at an angular unconformity by a subhorizontal, debris-flow conglomerate of probable Late Cretaceous age, here designated the conglomerate of Parowan Gap, which has not previously been recognized as a unit distinct from the disconformably overlying Paleocene(?) Grand Castle Conglomerate. The conglomerate of Parowan Gap is truncated by the Iron Springs thrust, whereas the Grand Castle Conglomerate overlaps that thrust. Thus, the conglomerate of Parowan Gap provides a constraint on the relative ages of Sevier thrusts exposed in the Red Hills. Sevier deformation at this latitude persisted longer than has previously been recognized, as the latest displacement on the Iron Springs thrust penetrated upward through its previous overlap deposits - including the entire Grand Castle Conglomerate and the basal Claron Formation.

In Parowan Gap, several Tertiary ignimbrite welded tuff lithologies are thoroughly jumbled and appear to have slid into place in a megalandslide above a basal unconsolidated ash. The Red Hills are cut by numerous north- to northeast-striking normal faults, which accommodate modern Basin-and-Range extension. Landslides are common in the Red Hills, most originating as slope failures within the Grand Castle Conglomerate.