Lockatong Formation

The Triassic Lockatong Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and New York. It is named after the Lockatong Creek in Hunterdon County, New Jersey, silty argillite, and laminated mudstone. In New Jersey, the cyclic nature of the formation is noted with hornfels near diabase and basalt flows.

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.

Paleobiota

Invertebrate burrows are the most common fossils in the Lockatong Formation.

Tetrapods

Fish

Ichnofossils

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

While most Lockatong sediments are mudstones associated with lakes or lakebeds, river or stream deposits can also occur. These deposits form in areas equivalent to the edge of the Newark basin, and typically consist of sandstone and conglomerate. Since the Lockatong Formation is primarily exposed in the center of the Newark basin, basin-margin facies are rare. The overlying Passaic Formation has more extensive exposures near the basin margin, and thus a higher prevalence of sandstone and conglomerate.

Thin beds of rippling sandstone are termed wave-dominated sandstone. This type of sandstone formed in the sandy shallows of lakes with low-angled lakebeds. Their characteristic wavy layering represents ripple marks formed during storms and other disruptive events. Slightly thicker sandstone foreset beds (preserved sandbars) are often associated with wave-dominated sandstone. Sandstone beds overlying mudstone become coarser the further up one goes in the sequence. Mudcrack-like structures can also develop when the sand is exposed to the air.

Some rivers flowing into the basin create deltas along the edges of lakes. Lockatong deltas produced sandstone beds with climbing ripple cross-bedding, a specialized sedimentary structure indicative of decelerating water. Some deltaic sandstone bedding is similar to the foreset beds of wave-dominated sandstone. However, the layers are more bowl-shaped and clinoform (i.e. stacked at much steeper angles). Upward-coarsening trends are still abundant, and root casts are sometimes found as well. Clinoform deltaic sandstone formed in Gilbert deltas, which involve coarse riverbed sediments abruptly being deposited onto a lakebed. Extensive stacks of this sandstone type indicate rising and falling lake water levels, inducing the deltas to shift and overlap older sediments. Sheet-like deltaic sandstones also have climbing ripples, but their layers are at a much lower angle than clinoform deltaic sandstone. They are often interbedded with mudcracked mudstone, typically vesicle-rich thin-bedded mudstone. The deltas which form these kinds of sandstone were low-relief temporary deltas that manifested during sheet flood events. Both the sheet deltas and the ephemeral lakes supplied by them would have dried up shortly afterwards. Deformation is common within the sheet delta sandstone layers due to repeated sheet floods through time.

Close to the Ramapo Fault, limestone, gneiss, granite, quartzite, and older Devonian conglomerate from the surrounding mountains. At their maximum size, conglomerate clasts are boulders up to half a meter across, but most clasts are much smaller. Large pebbles and cobbles are typically supported in distinct sandstone lenses with convex upper margins and flat lower margins. The largest clasts are found near the upper margin of the lenses. These facies are termed matrix-supported conglomerate, corresponding to debris flows on an alluvial fan. Some conglomerate involves bands of smaller pebbles interbedded with laminated sandstone in lenses with flat upper margins and concave lower margins. This type of conglomerate is known as clast-supported conglomerate, which was deposited in ephemeral stream channels on an alluvial fan. Away from the fault, sandstone becomes more common and conglomerate becomes more rare, indicating that the alluvial fans flatten into dry sandy plains. Root casts and burrows are abundant in deposits corresponding to the lower portions of an alluvial fan, where porous sediments and a high water table occur simultaneously.

The southwest and northeast corners of the Newark basin have another type of sandstone and conglomerate facies: axial facies. The most visible layers in these areas are graded conglomerate beds with large-scale and easily visible cross-bedding. Between the conglomerate layers lie sequences of interbedded mudstone and sandstone, with sandstone beds becoming thicker the higher one goes in a sequence. Unlike the cross-bedded conglomerate, the mudstone and sandstone layers have rare or absent sedimentary structures. Instead, they are heavily bioturbated by burrows and roots. Axial facies are riverbed and overbank deposits from braided rivers flowing down to supply the basin with water and sediment. The southwestern river system which flowed into the Newark basin is likely the same as that which formed the Hammer Creek Formation further west.

Age

Relative age dating of the Lockatong places it in the Upper Triassic, being deposited between 237 and 207 (±5) million years ago. It rests unconformably below many different formations of the Atlantic Coastal Plain. It interfingers with both the Stockton Formation and Passaic Formation. There are numerous diabase intrusions and basalt into the Stockton with local contact metamorphic rocks.Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geological Survey, Harrisburg, Pennsylvania.

Economic uses

See also

*Geology of Pennsylvania
*Geology of New Jersey

References

{{Geology of the Newark Basin

Norian Stage
Mudstone formations
Sandstone formations of the United States
Shale formations of the United States
Triassic geology of New Jersey
Triassic geology of New York (state)
Triassic geology of Pennsylvania
Geologic formations of New Jersey
Geologic formations of New York (state)
Geologic formations of Pennsylvania
Paleontology in New Jersey
Paleontology in New York (state)
Paleontology in Pennsylvania