EVIDENCE FOR PERVASIVE K-METASOMATISM IN CLASTS IN THE EARLY CRETACEOUS POTOMAC FORMATION, COASTAL PLAIN, VIRGINIA

The oldest stratigraphic unit in the Coastal Plain in Virginia is the Early Cretaceous Potomac Formation. At its thin western edge, it was deposited primarily in fluvial-deltaic environments, and is composed of quartzofeldspathic sands and organic-rich clays along with clasts of diverse size and composition. Conglomeratic portions of this unit are particularly well-exposed in the active Vulcan Materials sand and gravel quarry in Hopewell, VA (Puddledock plant), which was the primary location for our study. Most cobble-sized clasts here are quartz-rich (vein quartz or quartzite), but a smaller percentage of other rock types is also present. The initial goal of this project was to use these other clast types (primarily felsic volcanic rocks) to gain insights into what was exposed during the Cretaceous. In particular, we planned to compare clast compositions to distinctive quartz keratophyres found in the peri-Gondwanan Roanoke Rapids terrane immediately to the west. We sampled two basic cobble types in the quarry: 1) porphyritic felsic volcanic rocks; and 2) apparent felsic pyroclastic rocks. The second type contains “streaky” gray lenses that superficially resemble flattened pumice fragments, within a finer-grained, lighter-colored matrix. This type, alternatively, represents heterogeneously deformed mylonites (an interpretation that can be debated at the meeting!), in which the gray lenses are highly deformed quartz ribbons. The latter interpretation is supported by petrographic observations: the lenses consist almost solely of quartz, now recrystallized to fine-grained polygonal aggregates. We obtained whole-rock compositions of ten samples, which range from 70 to 83 wt% SiO₂. An unexpected result is that all samples are highly enriched in K₂O (~6-11 wt%), whereas MgO, CaO, and Na₂O are strongly depleted (below detection in many cases). Thus, all clasts, regardless of starting composition, experienced pervasive K-metasomatism. An enrichment in K-feldspar in the fine-grained matrix of all samples was confirmed by staining a subset of thin sections. The timing of this metasomatism was most likely post-depositional, reflecting extensive interaction with seawater. This interpretation is consistent with post-Potomac sea-level rise, reflected in the immediately overlying marine Aquia Formation.