ARBOREAL STROMATOLITES FROM THE TRIASSIC AND JURASSIC OF EASTERN NORTH AMERICA: IMPLICATIONS FOR ENVIRONMENTAL CHANGE

Arboreal stromatolites from cyclical lacustrine deposits of the Triassic-Jurassic Newark Supergroup of eastern North American chronicle a specific array of changing environmental conditions. Stromatolite-encasing trees have been recovered from the Lockatong (~220 Ma), Passaic (~210), and Towaco (~200) formations of the Newark basin, and from the East Berlin Fm. in Connecticut and the Scots Bay Fm. in Nova Scotia, both around 200 Ma. In such cyclical environments, as the climate became increasingly humid, arid lands grew lush with plants. Tree trunks, branches, and roots were colonized by stromatolites as previously desiccated flats were inundated by the expanding lake. Eventually, stromatolite-encrusted trees were buried beneath the photic zone, either directly by deeper-water muds, or after degradation and transport. Superficially similar to one another in gross form, in detail the internal structure of the tufas vary dramatically from smooth laminations (Passaic Fm.) to complex, cauliflower-like layering (Lockatong and Scots Bay fms.), suggesting different microbial assemblages were responsible. Similar stromatolites occur in Eocene, Miocene (Whiteside, 2004, in Lowman, and Rinker, Forest Canopies, 2nd edition. Academic Press, 147-149), and modern lacustrine environments (Winsborough et al., 2004, in Bertrand-Sarfati and Monty, Phanerozoic Stromatolites II, Kluwer, p. 71-100). Within the Newark Supergroup, these stromatolites occur in ~20 ky climatic precession cycles in either the wetter phases, or the transition between the wetter phases and the overlying drier phases of the 405 ky eccentricity cycles, in sequences formed by lakes with high but not saline solute levels. They thus existed at a hierarchy of levels along strong temporal gradients. Although there are Newark Triassic examples, these stromatolites are especially numerous in Early Jurassic strata less than 500 ky after the Triassic-Jurassic mass extinction suggesting a possible connection between arboreal stromatolites and reduced grazing in trophically depauperate ecosystems.