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.
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.
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. 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.