THE HUDSPETH FORMATION AND
CONDITIONS OF DEPOSITION
The Hudspeth Formation was named by Oles, (Wilkinson and Oles,1968), and mapped by Oles and Enlows (1971) to represent thick and extensive Cretaceous mudstones (Khm) exposed in the vicinity of Mitchell. The mudstones are interlayered with beds of conglomerate (Kgc) that were numbered 1 to 9 (base to top), resulting in mudstones numbered 1 to 11. Consequently, the mudstone presents a layer-cake assembly in which the unit below the lowest conglomerate is known as the “main Hudspeth” and has been measured to be 3000 feet thick.
The lowermost unit of the Hudspeth Formation is a light-tan sandstone that has been named the Basal Member (Khb).
It rests directly upon precretaceous basement rocks and represents a cretaceous sea advancing upon a complex shoreline made of metavolcanics, metacarbonates, phyllites, bedded cherts, and serpentinite. The Basal Member contains fossils of plant fragments, small pelecypods, and grains of the basement rocks.
Hudspeth mudstones above the Basal Member are thin-bedded and black (as most cretaceous mudstones of Western North America seem to be) with thin lenses of laminated siltstone.
They contain fossil belemnites, ammonites (and their juvenile equivalents), shells of gastropods, pelecypods, and echinoid spines.
Clastic accumulations of broken pelecypod and ammonite shells may be of thanatocoenosis origin.
Calcareous concretions are common and are elongate parallel to bedding. They sometimes yield fish teeth and parts of crab shells.
Also common in the mudstones are ferruginous concretions and septarian nodules.
Very rare fossils of ichthyosaurs and pteranodons have been found in the Hudspeth mudstones.
Sandstone beds occur throughout the Hudspeth mudstone sections and in some places become the dominant lithology.
These are identified on the geologic maps that I have included here in which Hudspeth mudstone (Khm) and sandstone (Khs) are represented separately. Many important characteristics of these sedimentary rocks are as follows:
Turbidite sandstones with classic “A” through “C” subdivisions are common in the Hudspeth Formation and often contain mudstone rip-ups.
The Hudspeth sandstones are especially remarkable for the abundance of sole markings, including flute casts (A, B, C, D, E, ) which provide evidence of the direction of current transport.
These directions have been reported by many investigators but when localities have been given, contradictions appear. My own observations (also on the maps) provide a consistent indication of depositional currents moving from NE to SW.
Also abundant are load casts (A, B, C) of almost unlimited variety. Current lineations, tool marks, and syneresis cracks are common.
Trace fossils are rare but can be found in these sandstones. Ripplemarks occur at some sandstone horizons.
Soft-sediment deformation occurred in many parts of the Hudspeth Formation:
Examples are common in which overlying conglomerates subsided into poorly consolidated muds.
Elsewhere, weak conglomerates have been invaided by bodies of sandstone.
Some layers of mud slipped down inclined slopes, forming wave-like crenulations before solidifying.