Enigmatic spheres from the Upper Triassic Lockatong Formation, Newark Basin of eastern Pennsylvania: evidence for microbial activity in marginal-lacustrine strandline deposits

Abstract

Microbes within sediments often create films or thick mats that interact with mobile sediment, producing microbially induced sedimentary structures (MISS). Preserved microbes in these sedimentary features are difficult to find, especially in oxygen-rich environments. The study of recent discoveries in the Upper Triassic Norian Lockatong Formation, a dominantly lacustrine facies of the Newark Supergroup strata, has revealed rarely reported structures that are assignable as MISS, namely, sand-cored spheres encapsulated in thin mudstone which range in diameter from the sub-millimetre to millimetre scale. These spheres are embedded in a thin layer of very fine silt and mudstone developed in an interpreted marginal-lacustrine shoreline setting. The cores of the spheres consist of randomly oriented, graded, fine-grained sandstone to siltstone; this eliminates previously proposed physical and biological origins for spheres, leaving a microbial origin. The microbial bound sand balls are localised and generated from a graded bed bound by microbial mats both above and below. The bounded bed was eroded and shaped into spheres during transport. The mats added cohesion by providing extrapolymeric substances which prevented the breakdown of the spheres into individual grains. The mats were rolled into spheres during transportation, thus preserving the microbial bound core. These sand balls add to the catalog of microbial diversity that can be used to increase our understanding of biological systems in lacustrine settings.