RETRODEFORMATION OF CARBONIFEROUS TETRAPOD TRACKS IN THE RHODE ISLAND FORMATION, PLAINVILLE, MASSACHUSETTS, USING DEFORMED RAINDROP IMPRESSIONS

In New England, the fossil record of early tetrapods consists primarily of footprints and trackways preserved in sedimentary rocks of the Narragansett Basin. However, deformation during the Alleghanian orogeny has distorted these traces, leading to problems with identification, description, and comparison with non-distorted material collected elsewhere.

In the Masslite Quarry in Plainville, Massachusetts, deformed raindrop impressions commonly occur in association with the trace fossils. Raindrop impressions are excellent strain markers because, as craters, they have an originally (semi) circular rim and lack ductility contrast with their host rock; however, they only record the strain on the bedding surfaces. Our preliminary results come from samples, many of which resemble strained mudcracks, with a cleavage that lies at a high angle to bedding and raindrop impressions that are elongate approximately parallel to the bedding-cleavage intersection lineation. This lineation is difficult to see at the hand-sample scale, so measurements were made using a microscope. The orientation of the intersection lineation varies between ~10° and ~30°, indicating the presence of an anastomosing cleavage. The raindrop impressions were photographed and their orientation and ellipticity were measured using ImageJ. These data were used to construct Rf/Φ plots. In each sample, the ellipticity of the individual impressions varies, as does their orientation. Although this suggests that the raindrop impressions may not have been perfectly circular initially, the variation in their orientation is similar to that of the bedding-cleavage intersection lineation. This implies that the variation in raindrop impression orientation and ellipticity may be a result of strain heterogeneity within a sample.

Since these raindrop impressions are typically found with the deformed trace fossils, their strain data can be applied to the footprints and trackways. Tracks may be an order of magnitude larger than or similar in size to the raindrop impressions. The former can be retrodeformed using the average strain of that sample, whereas the latter should be retrodeformed using the local intersection lineation and strain magnitude.