PHYTOTURBATION BELOW A LATE DEVONIAN – WESTPHALIAN UNCONFORMITY IN SOUTHEASTERN MASSACHUSETTS

The Norfolk and Narragansett Basins of Massachusetts host Paleozoic shales, sandstones, and conglomerates which were deposited in a tectonically evolving intermontane setting, and have been considered mainly Carboniferous in age. Several paleosol horizons from 30 to 150 cm thick occur within a suspected Late Devonian succession of interbedded alluvial and volcanic strata in the northern Narragansett Basin. The paleosols contain what are interpreted to be phytoturbation structures that preserve features common to plant roots and compare favorably to other rooted horizons known from Devonian intermontane deposits. Most of the structures appear to be carbonate-cemented rhizoliths, and root cast, tubule, replacement and rhizocretion morphologies are all present. Some carbonized compression fossils of roots are visible in association with the more prevalent carbonate-cemented forms. Microscopic investigation of replaced roots show cell size distributions that mimic what would be expected in fossilized parenchyma and vascular tissue in addition to possible preservation of bacteria or fungi. Lateral to downward dichotomous branching habits are common, although occasional upward-branching and unbranched habits appear. All rhizoliths have roughly cylindrical cross-sections with diameters ranging from 0.5–3.5 cm. Sedimentary fabric near the margin of discrete rhizocretions is compressed relative to surrounding matrix suggesting pre-lithification phytoturbation of original sedimentary fabric and probable in-situ calcification of roots. Using rhizolith morphology and intensity of phytoturbation as a guide, multiple modes of soil community development can be recognized, from shallow, monospecific early colonization and stabilization of fresh substrates to long-term inhabitation and deep penetration by multiple root forms.