The Arroyo Formation is the oldest and most eastern component of the Clear Fork Group. It extends in a Northeasterly direction from Concho County up as far north as Wilbarger County. North of the Red River in Oklahoma, the equivalent formation is the upper
Garber Formation and cave deposits of
Richards Spur (formerly Fort Sill).
Southern area The Arroyo Formation was first named by Beede and Waite (1918). The
type locality was a series of marine
limestone,
shale, and
gypsum deposits cropping out at Los Arroyos (formerly Los Arroyo), a specific
arroyo in
Runnels County a few miles west of
Ballinger. Wrather (1917) observed the same geological sequence in
Taylor County, although he named it the
Abilene Formation, which was a preoccupied name rejected by Beede and Waite (1918). At first, Beede and Waite (1918) tentatively placed the Arroyo Formation in the
Wichita (or Albany) Stage/Group, rather than the overlying Clear Fork Stage/Group. Olson (1989) called the portion of the Arroyo Formation below the
Salt Fork of the Brazos River the "Southern area", contrasting it with the extensively studied "Classic area" further north. The portions of the Arroyo Formation exposed in this area are among the oxidized Permian sediments collectively termed the
Red Beds of Texas and Oklahoma. They are represented by Red
clays,
shales, and
mudstones, with occasional layers of
sandstone or
conglomerate, With the absence of the characteristic Standpipe Limestone of the "southern area", Arroyo red beds in the "classic area" are difficult to differentiate from the overlying Vale Formation on a purely geological basis. A similar issue obscures the boundary between the Vale and Choza Formations, with the absence of the Bullwagon
Dolomite which separates the two further south. As a result,
sedimentologists generally do not distinguish the component formations of the Clear Fork Group, and simply prefer to call these northern red beds the
Clear Fork Formation. Under this system, the Arroyo Formation would be roughly equivalent to the informal
"Lower Clear Fork Formation". Olson (1958) attempted to provide an informal geological boundary between the Arroyo and Vale based on the presence of an even red shale or mudstone layer. This layer is found along a northeastern-oriented line that bisected the Clear Fork area in the western part of Baylor County. This even layer, he argued, was formed by slow, brackish streams in a lowland delta close to sea level. Therefore, their presence may have been a result of the same marine transgression responsible for the Standpipe Limestone further south. Under this hypothesis, the Vale Formation could be found in stratigraphically higher areas west of the line, and the Arroyo formation would be east of the line. Nevertheless, Olson admitted that this boundary was imprecise due to the variable depth of the shales and the varying topography of the surrounding landscape. A large number of sites are known bearing either abundant plant or animal remains. The animal-bearing sites are among the most diverse
Early Permian tetrapod assemblages in the world, with numerous remains of
amphibians,
pelycosaurs (mammal relatives),
chondrichthyans (sharks), and
eureptiles. The last and largest known species of
edaphosaurid,
Edaphosaurus pogonias, is known from the Arroyo, with the family going extinct soon afterwards. Other pelycosaurs, including
Secodontosaurus,
Varanosaurus, and several species of
Dimetrodon, were abundant, though the abundance of
Dimetrodon was retained in the Vale and Choza Formations.
Captorhinids were the most common eureptiles in the Arroyo, represented by basal taxa such as
Captorhinus and
Labidosaurus. Captorhinids experienced a taxonomic turnover at the beginning of the Vale Formation, as advanced taxa like
Labidosaurikos and
Captorhinikos replace or evolve from the more primitive captorhinids soon after the red shale boundary between the two formations. The first specimens of the gracile eureptile
Araeoscelis were discovered at the Craddock Bonebed, one of the most productive Arroyo Formation sites in Baylor County. Aquatic amphibians like
Diplocaulus,
Trimerorhachis, and
Eryops are common. Terrestrial amphibians like
Seymouria,
Diadectes,
microsaurs, and various
dissorophoids (
Acheloma,
Broiliellus,
Aspidosaurus, etc.) were present as well. Many of these terrestrial amphibians did not survive into the Vale Formation. Burrows containing
aestivating Brachydectes,
Gnathorhiza, and
Diplocaulus are common in the middle part of the Arroyo Formation, likely indicating a period with a drier climate than the early or late Arroyo. The most common shark remains belong to
Orthacanthus platypternus, although teeth from
Xenacanthus luederensis are also known from some early Arroyo sites. ==Paleobiota==