QUANTIFICATION OF STRAIN IN THE NORTHERN NARRAGANSETT BASIN WITH IMPLICATIONS FOR THE RETRODEFORMATION OF CARBONIFEROUS FOOTPRINTS AND TRACKWAYS

In the Masslite Quarry in Plainville, Massachusetts, deformed raindrop impressions commonly occur in association with deformed trace fossils and can therefore be used to retrodeform the tracks on those slabs. Raindrop impressions are excellent strain markers because, as craters, they have an originally (semi) circular rim, lack ductility contrast with their host rock, and are not affected by compaction-related strain. Unfortunately, not all track-bearing slabs contain raindrop impressions, which leads to difficulties with the retrodeforming of those trace fossils.

To quantify the deformation of these rocks, we have analyzed over 350 raindrop impressions on the bedding surfaces of 12 samples from the Rhode Island Formation in Masslite Quarry. Many of these samples resemble strained mudcracks and all contain an anastamosing cleavage with the long axes of the raindrop impressions approximately parallel to the intersection of cleavage on bedding. In these samples, the range in orientation of the intersection lineation varies between ~10° and ~30°, and the mean axial ratio of the raindrop impressions varies between ~1.37 and ~1.95. This range in the mean axial ratio suggests that there is strain heterogeneity on the bedding surfaces of these samples, so a global mean strain value cannot be used for retrodeformation. However, a strong correlation exists between the range in orientation of the intersection lineation and the mean axial ratio, which allows us to retrodeform samples that lack raindrop impressions based on the range in orientation of the intersection lineation.

We are currently working to determine if the range in the mean axial ratio represents true strain heterogeneity or if it is a product of changing angles between bedding and cleavage. We will use these data, along with the relationship between the intersection lineation and the mean axial ratio, to retrodeform footprints and trackways found on slabs from Masslite Quarry. These tracks will then be identified, described, and compared with non-distorted material collected elsewhere.