The Lockatong Formation is a Late Triassic geological formation in the Newark Basin of Pennsylvania, New Jersey, and New York. It is named after the Lockatong Creek in Hunterdon County, New Jersey.
Description The Lockatong is defined as a light to dark gray, greenish-gray, and black very fine grained sandstone, silty argillite, and laminated mudstone. In New Jersey, the cyclic nature of the formation is noted with hornfels near diabase and basalt flows.Orndorff, R.C., et al., (1998). Bedrock Geologic Map of Central and Southern New Jersey. United States Geological Survey, Scale 1:100,000.
Depositional environment The Lockatong is often described as lake or litoral sediments. The interfingering nature of the sediments with the surrounding Stockton Formation and Passaic Formation suggests that these litoral environments shifted as climate or as the dynamic terrane of the area developed.Faill, R.T., (2004). The Birdsboro Basin. Pennsylvania Geology V. 34 n. 4. The deposition of calcitic sediments is indicative of a climate with high evaporation rates.
More recent studies suggest that the Lockatong was a highly turbulent environment, akin to the modern Salton Sea, that was frequently subjected to high wind events that overturned the lake waters, which depleted the dissolved oxygen in the water or made the lake more toxic. These environmental shifts led to massive fish kills that accumulated on the shoreline, where they decomposed or were scavenged by phytosaurs, leading to the formation's distinctive deposits of numerous disarticulated fish parts.
Paleobiota Invertebrate burrows are the most common fossils in the Lockatong Formation.
Tetrapods Tetrapods of the Lockatong Formation Genus Species Locales known from Material Notes ImagesEupelorE. durusPhoenixville?Shoulder girdle elementsAn dubious amphibian, potentially a metoposauridGwyneddosaurusG. ericiGwyneddPartial skeleton (Gastric ejection)A dubious reptile, likely a chimera consisting of coelacanth and Tanytrachelos fossils. Has also been considered a synonym of Tanytrachelos.HypuronectorH. limnaiosGranton Quarry (Ewing Creek Member?), Weehawken Quarry (Nursery Member), Edgewater QuarrySeveral partial skeletons A drepanosaur, originally known as "the deep-tailed swimmer".IcarosaurusI. siefkeriGranton Quarry (Ewing Creek Member?)A partial skeleton A kuehneosaurid.RhabdopelixR. longispinisGwyneddIsolated vertebrae and other remains, now lostA dubious reptile, likely a chimera consisting of Tanytrachelos, Icarosaurus, and/or fish fossils.RutiodonR. carolinensisGranton Quarry, Princeton, PhoenixvilleSkull, teeth, other fragmentsA phytosaur.TanytrachelosT. ahynisGranton Quarry, Haines & Kibblehouse Quarry (Skunk Hollow-Tohickon Members), Weehawken Quarry, other Palisades area outcrops. A large number of partial skeletons A small and fairly common tanystropheid.
Fish Fish of the Lockatong Formation Genus Species Locales known from Material Notes ImagesCarinacanthusC. jepseniGwyneddA partial skeletonA very rare hybodont sharkCionichthysC. sp.Gwynedd, Weehawken QuarryPartial skeletonsA rare redfieldiid, originally called Redfieldius obrai.DiplurusD. longicaudatus Granton Quarry, Gwynedd, Princeton A partial skull and scales A rare coelacanthD. newarki Granton Quarry, Gwynedd, Princeton, Weehawken Quarry and other Palisades area outcrops, Arcola. Numerous skeletons An abundant coelacanth, sometimes given its own genus (Osteopleurus). Includes "Osteopleurus milleri", from Granton Quarry, which is indistinguishable apart from its larger size.LysorocephalusL. gwynnedensisGwyneddA partial skullA dubious fish likely synonymous with Turseodus. Originally misidentified as a lysorophian amphibian.Pariostegus? P. sp. Arcola, Granton Quarry Isolated fossils A rare but very large coelacanth which may be synonymous with Diplurus.RabdiolepisR. gwyneddensisGwyneddA partial skeletonA dubious coelacanth, likely synonymous with Diplurus or Pariostegus.Semionotus "S. brauni group" Weehawken Quarry and other Palisades area outcrops, Arcola. Numerous skeletons A locally common semionotiform. Likely encompasses two species.Synorichthys S. cf. S. stewarti Granton Quarry, Weehawken Quarry, Arcola. Numerous skeletons A locally common redfieldiid.Turseodus T. acutus Granton Quarry, Princeton, Phoenixville, Weehawken Quarry and other Palisades area outcrops, Arcola, Gwynedd, Miller's Quarry., Haines & Kibblehouse Quarry. Numerous skeletons An abundant palaeonisciform which may include multiple species.
Ichnofossils Ichnofossils of the Lockatong Formation Ichnogenus Ichnospecies Locales known from NotesAnchisauripusA. gwyneddensisGwyneddPossible junior synonym of Atreipus milfordensisApatopusA. lineatusSchuylkill outcropsPhytosaur tracksAtreipusA. milfordensisSouderton (Tumble Falls Member), Gwynedd, Arcola, Schuylkill outcrops?Tracks from quadrupedal ornithischian dinosaurs or silesaurid dinosauromorphs.BrachychirotheriumB. parvum Arcola, Schuylkill outcropsAetosaur tracks? May include B. eyermani, a junior synonym.GrallatorG. sp.Theropod dinosaur tracks.GwyneddichniumG. majore Gwynedd, Souderton, numerous other localities.Tracks from small tanystropheids such as Tanytrachelos.G. minoreGwyneddNomen dubium: indeterminate or synonymous with G. majore.G. elongatumGwyneddNomen dubium: indeterminate or synonymous with G. majore.PlatypternaP. lockatong Gwynedd Dubious ornithischian tracks.RhynchosauroidesR. brunswickiiArcola? Gwynedd?Originally known as Kintneria brunswickii. Sphenodontian or lizard tracks.R. hyperbatesArcola, Schuylkill outcrops.Sphenodontian or lizard tracks.R. sp.Souderton, Phoenixville.Indeterminate Rhynchosauroides specimens.
Geological facies Laminated mudstone Grey to black laminated mudstone (shale) was deposited in long-lasting perennial lakes. Some of the laminae are graded due to their sediment settling out after a turbidity flow. Others have a lenticular or "pinch-and-swell" shape (with alternating narrow and elliptical cross-sections), which results from the motion of waves at the water surface. Small burrows and minor sediment deformation are also known to occur. However, these shales and their layers become finer, flatter, and less disturbed as the lakes deepen. They also lose oxygen and acquire higher concentrations of dark organic material and carbonate.
The finest shales formed in the deepest parts of the largest lakes. Their layers are very thin and consist of perfectly even, continuous bands of organic material alternating with carbonate (limestone) or clay. Sometimes these shales can be up to 8% organic material by weight. The different layers may be due to seasonal variation in sediment deposition, chemical conditions, and/or algal growth. A complete lack of influence from waves or bioturbating animals indicates that the lakes were very deep and anoxic at their lowest extents. Fossils such as well-preserved fish skeletons are common in the absence of decomposing organisms. The minimum depth necessary to maintain this environment has been estimated to range from 60 meters to up to 80 or 100 meters.
Thin-bedded mudstone Red to grey thin-bedded mudstones are lake or lakeshore sediments intermediate in layer width between shale and massive mudstone. Graded and "pinch-and-swell" layers are common in these facies. This indicates that these shallower sediments experienced disturbances like floods or increased wave action during storms. Other thin-bedded mudstone is so heavily bioturbated by burrows that it has a "shredded" appearance, where it is difficult to distinguish the individual layers. The higher-energy shallow water environment means that the sediments which make up thin-bedded mudstone are coarser than those of shale. Most of the shallow mudstone layers are interbedded with siltstone and/or sandstone. In some cases, cross-bedding, ripple marks, or other sedimentary structures can be observed in coarse layers. Though small and rare, stromatolite structures made of micrite are known to occur around some lakeshore sediments.
Thin-bedded mudstones with deep mudcracks develop in environments where the lake mud is frequently exposed to dry air. These mudcracks can be simple crevices or more complex multi-branched structures. After a flood, the mudcracks are refilled with mud or other sediments. In some areas tiny circular or elliptical vesicles (air bubbles) are preserved within the refilled mudcracks. Vesicles most commonly form in narrow layers of drying fresh mud deposited on top of older, tougher mud-cracked lake sediments. The mud layers which contain vesicles often have a scalloped appearance where their edges peel upwards above the mudcracks.
Massive mudstone Red to grey massive mudstone has no discernable layers. Almost all massive mudstone develops mudcracks, indicating dry conditions above water most of the time. Brecciated massive mudstone is heavily cracked in multiple directions. It has been described as a "breccia fabric": a patchwork of angular mud fragments held together by a carbonate or silicate matrix. This type of mudstone formed on dry lakes (playas) which constantly redeveloped mudcracks as they were hydrated and dried out multiple times. Vesicles can occur in breccia fabric which dried quickly enough to trap air bubbles without collapsing. Vesicular massive mudstone is more chaotic in structure, dominated by numerous vesicles and thin, jagged cracks. The playas responsible for vesicular massive mudstone were much drier than their brecciated equivalent.
Peloidal or efflorescent massive mudstone is similar to brecciated massive mudstone, but its mud fragments are small, rounded clumps. This clumping pattern resembles that of modern salty playas, where dissolved salt gives the lakebed mud a crumbly powdered texture. Traces of mudcracks are still present, but have been heavily deformed by the textural changes. The wettest type of massive mudstone is burrowed massive mudstone, which is thin-bedded mudstone that has been completely homogenized by bioturbation.
Crystal structures Saline minerals (typically calcite) are common in the mudstones of the Lockatong Formation. Calcite crystals may be present in the form of hexagonal pseudomorphs. The original hexagonal crystal (now replaced by calcite) was probably pirssonite or a similar mineral, which settled on the lakebed after crystallizing near the water surface. In laminated mudstone, some laminae may consist entirely of sheets of hexagonal calcite crystals. They act similar to sand grains and can occur in graded, "pinch-and-swell", and continuous laminae of lake mudstone.
Crystal clasts are a different kind of crystal structure occurring in the Lockatong Formation. They involve blade-like cavities (perhaps originally from sodium carbonate) which have been filled with crystals of various other minerals. Calcite, analcime, albite, dolomite, and potassium feldspar are all known to occur within Lockatong crystal clasts. Although gypsum is abundant in the crystal clasts of the overlying Passaic Formation, it is absent in the Lockatong Formation. Some crystal clasts grow perpendicular to the layers, often sending out multiple branches as they radiate from a bedding plane. These radiating crystal clasts typically form in transgressing shallow lake mudstones, as saline waters penetrate the lakebed and promote crystal growth.
Most crystal clasts are more random in orientation and distribution. Random crystal clasts are typically graded, growing larger and more euhedral the deeper they occur within a given layer. They most commonly occur in peloidal massive mudstone, as brine sinks into a saline mudflat and crystallizes. Periodic rains dissolve crystals closer to the surface, explaining why crystals higher in a sequence are smaller and more irregular in shape.
Sandstone and conglomerate facies