| Hermit Formation | |
|---|---|
| Stratigraphic range: Permian, EarlyLeonardian[1]275–270 Ma | |
.jpg) (Isis Temple, central Grand Canyon) The 3 top geologic units visible: 3)-(white-cliffs-prominence)-Coconino Sandstone, 2)-(slope-forming-Deep brnsh-red)-Hermit Formation, 1)-(red-uppermost-vertical-cliff & resistant)-Esplanade Sandstone (top unit of 4–memberSupai Group) (Note: DistinctiveRedwall Limestone upperhorizontal-platform.) | |
| Type | Geological formation |
| Underlies | Coconino Sandstone |
| Overlies | Supai Group |
| Thickness | 900 feet (270 m), at maximum |
| Lithology | |
| Primary | sandstone andmudstone |
| Other | siltstone and intraformationalconglomerate |
| Location | |
| Region |  Arizona-(North & Central Arizona) California-(southeast) and Utah-(southern) |
| Country |  United States-(Southwestern United States) |
| Type section | |
| Named for | Hermit basin,Coconino County, Arizona[2] |
| Named by | Noble (1922)[2][3] |
ThePermianHermit Formation, also known as theHermit Shale, is a nonresistant unit that is composed ofslope-forming reddish brownsiltstone,mudstone, and very fine-grainedsandstone. Within theGrand Canyon region, the upper part of the Hermit Formation contains red and white, massive, calcareous sandstone and siltstone beds that exhibit low-anglecross-bedding. Beds of dark red crumbly siltstone fill shallowpaleochannels that are quite common in this formation. The siltstone beds often contain poorly preserved plant fossils. It outcrops across northwestern Arizona from the Sedona area, and itoutcrops in the Grand Canyon and the westernMogollon Rim, into theAubrey Cliffs. It forms steep slopes that are typically mostly covered by debris andcolluvium derived from the overlyingsedimentarystrata.[1]
As summarized by McKee in 1982,[3] the sedimentary strata assigned to the Supai Group and Hermit Formation have a long and complicated nomenclature. However, out of numerous publications, three major papers have been responsible for the defining the Hermit Formation as currently mapped in the Grand Canyon region[1] First in 1910, Darton[4] proposed the name of Supai Formation for Pennsylvanian-Permianredbeds exposured at its type section in the vicinity ofSupai Village inCataract Canyon, central Grand Canyon. As defined by Darton, the Supai formation included all Pennsylvanian-Permian strata, mostly red beds, lying between the Redwall Limestone and the Coconino Sandstone in the Grand Canyon region. In 1922, Noble[2] recognized and separated the strata now comprising the Hermit Shale from the top of Darton's Supai. In 1975, McKee[5] changed the Supai Formation to Supai Group and subdivided it into four formations within the Supai Group: the Watahomigi, Manakacha, and Wescogame formations and the Esplanade Sandstone. Given it heterogenous nature and abuandnce of sandstone and siltstone within it, the Hermit Shale was renamed the Hermit Formation. Subsequence publications of the last 35 years have used the largely used the termHermit Formation.[1]
The Hermit Formation consists of a poorly exposed, slope-forming, heterogeneous assemblage of interbedded red-to-brown mudstone, siltstone, and very fine sandstone. Northward along theHurricane Cliffs and into adjacentUtah, the sandstone content increases as it laterally interfingers and merges with the Queantoweap Sandstone. The Hermit Formation varies in thickness from about 30 m (98 ft) in the eastern portion of the Grand Canyon and nearSeligman to over 270 m (890 ft) in the areas ofToroweap overlook andShivwits Plateau. The Hermit Formation can be traced in the western Mogollon Rim region east of Seligman to the Sedona region, where it is about 90 m (300 ft) thick.[1]
Silty sandstone and sandy mudstone comprise most of the lithology of the Hermit Formation. At most outcrops, sandstone predominates near the base of this formation and decreases as mudstone increases in abundance upward. The sandstone and mudstonebeds are typically rhythmicallyinterbedded with 15 or more cycles exposed in most locations. The silty sandstones are structureless toripple laminated to trough cross-stratified. The structureless sandstone layers consist of ledge-forming beds that are around 1 m (3.3 ft) in thickness and may or may not contain limy, nodularconcretions. The ripple-laminated sandstone beds exhibit subaqueous, faint-to-prominent ripple cross-lamination. The troughs exhibited by the trough cross-stratified sandstone up to several meters across. Near the base of the Hermit Formation at many localities within Grand Canyon and in theSedona, Arizona areas, rare trough to planar-tabular sets of cross-stratified sandstone, fine-grained and well-sorted, with climbing translatent strata, occur. Because of poor exposures, both the extent and geometry of individual sandstone beds are uncertain. Commonly, the mudstone is featureless. However, clean rock outcrops display fine ripple lamination and calcareous nodular concretions.[1]
Although a minor component overall, thin intraformationalconglomerates are locally present within the Hermit Formation. Thepebbles are typically locally derived from the adjacent carbonate concretions found within the intercalated sandstone and mudstone. A few of these pebbles also consist of carbonate-cemented, fine-grained sandstone and siltstone. These conglomerates occur both as individual beds and incorporate into sandstone beds. They are common abundant in the area of Sedona, Arizona. They decrease in abundance all directions from there.[1]
In parts of central and eastern Grand Canyon region, the lower contact of the Hermit Formation consists of paleovalleys of considerable depth cut into the Esplanade Sandstone. These paleovalleys are filled by deposits of the Hermit Formation. However, in the western Grand Canyon and other parts of western and northern Arizona, little evidence of an erosional break can be found between the Hermit Formation and the Esplanade Sandstone. In these areas, a recognizable interruption in the accumulation of sediment between the Esplanade Sandstone is lacking and the Hermit Formation is only separated by an arbitrary boundary from the Esplanade Sandstone.[1][3]
Throughout Grand Canyon and into the Aubrey Cliffs regions, the upper contact of the Hermit Formation with the overlying Coconino Sandstone is a sharp, flat, lithologic disconformity. This disconformity lacks agradation of any kind. Dessication cracks that extend 6 m (20 ft) deep or more from the top of the Hermit. They frequently are filled with the overlying sandstone. These cracks are not seen where the Coconino is absent in northwestern Arizona. In northwestern Arizona where the Coconino is absent, the contact between the Hermit Formation and the overlying Toroweap Formation is a sharp erosional unconformity with relief up to 11 m (36 ft) or more.[1][6]
The fossils ofinvertebrates are typically uncommon and poorly preserved in the Hermit Formation. This is due to the unfavorable conditions of fossil preveration associated with the arid fluvial environments and grain size of the Hermit Formation. Two fossil insect wings have been described from the Hermit Formtion of the Grand Canyon region. Both aremeganeurids, members of an extinctclade resembling modern dragonflies, and date the Hermit Formation to the earlyPermian Period.[7] Other reported invertebrate fossils include a partial wing of anodonate and a forewing of an unnamedblattoid orcockroach. Also, a poorly preserved external mold of aeurypterid,Hastimima spp., has been collected from the Hermit Formation. Finally, The Hermit Formation contains the unstudied trackways, burrows, and resting and feeding traces of invertebrates along with the more commonrootbioturbation and microbial structures.[8][9]
The Hermit Formation has yielded what is certainly best-preserved and most diverse assemblage ofvertebratetrace fossils, including tracks and trackways, known from the Grand Canyon region. It contains theichnotaxaAmphisauropus kablikae,Amphisauropus,Batrachichnus salamandroides,Dimetropus isp.,Dromopus lacertoides,Erpetopus isp.,Hyloidichnus bifurcatus, andIchniotherium cottae. The presence ofDimetropus andIchniotherium suggests a lateArtinskian toKungurian age for the Hermit Formation because the it is overlain by the late Kungurian Coconino Sandstone. This assemblage of vertebrate trace fossils is an abundant and relatively diverse collection ofreptile andanamniote tracks and rarersynapsid tracks.[10]
In 1929, David White[8] published a monograph about the fossilflora is of the Hermit Formation. In it, he reported 29species of fossil plants and identified an additional ten additional species only identified to genus level, or of uncertain identification. However, it is likely that there are only half that many actual species in the Hermit Formation because many fossils are poorly preserved and doubtfully described. The major plant groups reported from the Hermit Formation includeseed plants, includingseed ferns,conifers,horsetails,Ginkgoopsida, andalgae.[11][12]
Based on fossils and it sedimentology, the Hermit Formation is interpreted as being deposited by fluvial processes on a seasonally arid coastal plain. Locally, eolian processes deposited loess and created scattered sand dunes, which left local accumulations of eolian sand deposits.[1][8]