The Archer City Formation is a geological formation in north-central Texas, preserving fossils from the Asselian and early Sakmarian stages of the Permian period. It is the earliest component of the Texas red beds, introducing a tropical ecosystem which will persist in the area through the rest of the Early Permian. The Archer City Formation is preceded by the cool Carboniferous swamp sediments of the Markley Formation, and succeeded by the equally fossiliferous red beds of the Nocona Formation. The Archer City Formation was not named as a unique geological unit until the late 1980s. Older studies generally labelled its outcrops as the Moran or Putnam formations, which are age-equivalent marine units to the southwest.
The sediments of the Archer City Formation reconstruct a coastal floodplain crossed by small meandering rivers and isolated ponds. The climate was hot and monsoonal, preserving a variety of paleosols and plant assemblages ranging from dry upland forests to moist riverside galleries. "Seed ferns" (Sphenopteris, Autunia, Odontopteris, etc.) and early conifers (Walchia, etc.) occupied drier soils, while marattialean ferns (Pecopteris, Asterotheca, etc.) and horsetails (Sphenophyllum, Annularia, Calamites, etc.) were prominent in more humid areas.
The Archer City Formation preserves a diverse fauna of fossil vertebrates in pond bonebeds across Archer and Clay counties. Many fossils are rare fragments of species which are better-preserved in younger parts of the red beds. Nevertheless, complete skeletons and important type specimens are also known from the formation. Some notable synapsids include Apsisaurus witteri, Edaphosaurus boanerges, Ophiacodon uniformis, and Dimetrodon milleri. Early reptiles such as Protorothyris archeri and Romeria spp. make up a small component of the terrestrial fauna. Amphibians are common in every niche, with examples such as Phonerpeton pricei, Neldasaurus wrightei, Edops craigi, Eryops megacephalus, and Pantylus cordatus, just to name a few. The rivers were roamed by Orthacanthus texensis (a xenacanth "shark") and Ectosteorhachis nitidus (a megalichthyid fish).
History and stratigraphy Fossil bonebeds in the vicinity of Archer City were first investigated in detail by Alfred Romer and Llewellyn Price, starting in the late 1920s. At the time, these bonebeds were assigned to the Pueblo, Moran, or Putnam formations of the Cisco Group. The Archer City Formation was first named in a 1987 geological map seeking to update the stratigraphy of North Texas. The formation and its stratigraphic placement were formalized in text a year later.
The formation outcrops in an arc from Montague County in the east, through Clay, Archer, and Young counties, as far as Throckmorton County in the west. It is most broadly exposed in Clay and Archer counties, including under its namesake of Archer City.
The Archer City Formation is a unit of the Bowie Group, lying above the Markley Formation and below the Nocona Formation of the Wichita Group. Some publications have labelled the Nocona Formation as the "Upper Archer City Formation", thus considering the Archer City Formation to be a unit of the Wichita Group. This proposal is not widely agreed upon. The Cisco Group and its constituent formations have been restricted to marine strata further south and west. Of these strata, the Archer City Formation is roughly equivalent in time to the uppermost Pueblo Formation, the Moran Formation, the Sedwick Formation, and the Santa Anna Branch shale (= Putnam Formation), from oldest to youngest.
Age Based on correlations with coastal strata further west, the Carboniferous-Permian boundary is positioned near the top of the Markley Formation. This suggests that the Archer City Formation occupies most of the Asselian (299–293 Ma), the first global stage of the Permian Period. The plant fossils of the Archer City Formation also resemble those of Asselian Europe. The upper part of the Archer City Formation may extend into the succeeding Sakmarian stage (293–290 Ma). The Santa Anna Branch Shale and overlying Coleman Junction Limestone both preserve fossils of Sweetognathus merrilli, a conodont index fossil of the early Sakmarian. The Asselian and Sakmarian global stages are equivalent to the early-middle parts of the Wolfcampian regional stage.
Paleoenvironment As with the rest of the Texas red beds, the Archer City Formation was deposited on a coastal plain between an inland marine basin in the west and patches of steadily eroding mountains in the north and east. Some of the mountains are still standing in the present day, such as the Wichita Mountains and Arbuckle Mountains, while others have been fully eroded or buried, such as the Texan portion of the Ouachita Mountains. The marine basin is formally known as the Midland Basin, a major component of the Permian Basin oil field. Rising sea levels through the Early Permian allowed the coastline to gradually transgress northward and eastward. The major bonebed sites of the Archer City Formation were probably away from the coast.
During the Permian, the area was near the equator, with a latitude of 0° to 5° N. Temperature estimates can be inferred for the Archer City Formation via δ18O records for hematite and phyllosilicate minerals in paleosols. These data points suggest soil temperatures of , significantly warmer than the cool and wet conditions of the Markley Formation. This fits with climate evidence from rapid sedimentary changes between the formations, as well as an increasingly patchy distribution of high-humidity flora. The Archer City Formation would have had a hot monsoonal climate, with moist or swampy areas shrinking to the vicinity of riverbanks and ponds during the dry season.
Geology In terms of their lithologies, the Archer City and Nocona formations are nearly indistinguishable, with equivalent sets of fluvial and floodplain sediments. The vast majority of each formation is homogeneous red mudstone with small calcareous nodules. Fluvial sandstone beds and paleosols are also common. Fossils and other organic debris are concentrated into rare greyish claystone bonebeds, which correspond to perennial ponds. Most of the pond bonebeds are too small to continuously support an ecosystem of large aquatic vertebrates, suggesting that the carcasses may have been washed in from larger bodies of water.
Sandstone beds By volume, the most prevalent sandstone beds are upwards-fining stacks of point bar, scroll-bar, and coarser channel fill deposits. These are all hallmarks of small meandering rivers, with channels only a few meters wide. Other common sandstone features include subdued, fine-grained crevasse splay deposits. The crevasse splays are often associated with plant fossils which have been remineralized by copper minerals.
Some sandstone beds are isolated straight channel fills, without corresponding riverbank deposits. These straight beds may reach 30 meters in width (though most are much narrower) and several kilometers in length. Coarser beds preserve sinuous megaripples while finer channels develop small climbing ripples towards the top. Straight channels would have formed during high-energy erosion events, but the climate was probably too moist for frequent flash floods.
Paleosols Four different types of paleosol (labelled types E through H) have been identified in the Archer City Formation. Three (F, G, and H) first appear in this formation, while type E paleosols are retained from the underlying Markley Formation. All four continue to persist through the Texas Lower Permian up until an aridification event in the mid-Clear Fork Group. The paleosols are generally dark red or brown, with only intermittent gleying in the type E variety. The Archer City Formation mostly lacks the high-humidity paleosol varieties of the Markley Formation (types A through D).
Type E paleosols are silty or fine sandy soils with persistent mottling, hematite nodules, and few distinct horizons. They are a type of entisol, young soils corresponding to isolated, frequently disturbed areas with a high water table, such as riverbanks. Type F paleosols are alfisols, patterned with deep root casts and numerous horizons, including intermediate layers with calcareous nodules and smectite clay. They develop in stable forested uplands with a low water table and a semi-arid to subhumid climate (~70 cm of annual precipitation). Type G paleosols are similar, with sandy vertical cracks and abundant calcareous nodules. They qualify as vertisols, inundated and dried at a seasonal frequency along the margins of a floodplain. Type H paleosols have an even greater proportion of calcareous minerals, forming blocky crusts interspersed among chlorite- and mica-rich clay. They could be termed aridisols or (more broadly) inceptisols, deposited in dry upland areas free from the influence of groundwater.
Paleobiota
Synapsids Synapsids of the Archer City FormationGenusSpeciesMaterialNotesImagesApsisaurusA. witteriA partial skeletonA varanopid, originally identified as a diapsid reptile.DimetrodonD. milleriNearly complete skeletonA small sail-backed sphenacodontidEdaphosaurus E. boanergesIsolated neural spines and at least one partial skeletonA common medium-sized edaphosaurid, often confused with Edaphosaurus cruciger.LupeosaurusL. kayiLimb and hip bones and vertebraeA basal edaphosauridOphiacodonO. retroversusRare fragmentsA large ophiacodontid, also described under the names Theropleura and Diopeus.O. uniformisA partial skeleton with a well-preserved skullA medium-sized ophiacodontid, also described under the name Poliosaurus.StereophallodonS. ciscoensisSkull fragments and vertebraeA large ophiacodontid
Reptiles Reptiles of the Archer City FormationGenusSpeciesMaterialNotesImagesBolosaurusB. striatusA jawA bolosaurid parareptileProtorothyrisP. archeriFive skulls with associated postcranial materialA protorothyridid eureptileRomeriaR. primaA skullA captorhinid eureptileR. texanaA partial skull and partial skeleton
Temnospondyls Temnospondyls of the Archer City FormationGenusSpeciesMaterialNotesImagesAchelomaA. cumminsiPartial skull and postcraniaA trematopidAspidosaurusA. sp.Neural spine armor fragmentsA dissorophidBroiliellusB. brevisTwo skulls, one with a partial skeletonA dissorophidBrevidorsumB. profundumPartial skull and postcraniaA dissorophidDiploseiraD. angustaA partial skeletonA dissorophid, formerly considered a species of Dissorophus.EdopsE. craigiSeveral skulls and postcraniaAn edopoidEryopsE. megacephalusJaws and other fragmentsA common eryopidNeldasaurusN. wrightaeFour partial skulls and postcaniaA trimerorhachid dvinosaur"Parioxys""P." bolliA partial skeletonA temnospondyl of uncertain affinities. Parioxys ferricolus, the more complete type species of Parioxys, has variably been interpreted as an eryopoid or a dissorophid."Parioxys" bolli is based on more fragmentary fossils and may belong to an unrelated genus, perhaps a trematopid related to Ecolsonia cutlerensis.Pasawioops P. cf. mayiA skullA micropholid amphibamiformPhonerpetonP. priceiAt least seven skulls with associated postcranial fragmentsA trematopid, formerly considered a species of Acheloma.ReiszerpetonR. renascentisA skullA dissorophidTersomiusT. texensisA skullA micropholid amphibamiformTrimerorhachisT. sp.A trimerorhachid dvinosaur. Reports from the Archer City Formation are unsubstantiated, though it is known from time-equivalent strata in New Mexico.ZatrachysZ. serratusSkull fragmentsA zatracheid
Other amphibians