THE MERRIMACK GROUP IN THE EPPING 7.5-MINUTE QUADRANGLE, SOUTHEAST NEW HAMPSHIRE: NEW FINDINGS

The geologic map of the Epping 7.5-minute Quadrangle in Southeast New Hampshire, completed as part of the USGS EDMAP Program, further subdivides metasedimentary units of the Merrimack Group. These changes require an update to the 1997 New Hampshire state geologic map.

In the southern portion of the quadrangle, the Silurian-Ordovician Eliot Formation is separated from the Silurian-Ordovician Berwick Formation by the Calef Member of the Eliot Formation and the associated Calef Fault. Additionally, a separate late brittle fault marked by zones of silicification has been mapped as a boundary between the Eliot and Berwick Formations in northern sections of the quadrangle. Lithological contacts between these units are gradational and are defined by varying pelite content.

The presence of large scale, tight to isoclinal folds contributes to the map pattern of units. The Calef Member is exposed as the core of a north-plunging anticline within the Eliot Formation and is terminated on the west by the Calef Fault. In the eastern part of the quadrangle, a more quartzitic rock crops out, here shown as a sandier facies of the Eliot Formation.

Detailed mapping has extended members of the Berwick Formation from the adjacent Mt. Pawtuckaway Quadrangle into the Epping Quadrangle. The pelitic Gove and Watson Hill members of the Berwick Formation crop out on the east and west sides of the Flint Hill Fault Zone, respectively, and are possible lithostratigraphic equivalents. The staurolite-out isograd coincides with the Flint Hill Fault Zone, evidenced by a higher-temperature mineral assemblage in the Watson Hill Member (muscovite+quartz+biotite+sillimanite+garnet). The Unnamed Member dominates the western third of the quadrangle and is separated from the Berwick Formation by higher percentage of calc-silicate layers. The presence of epidote in the calc-silicate layers throughout the Berwick Formation suggests low XCO2. Amphibole is present in the Unnamed Member east and west of the Flint Hill Fault Zone, while diopside+garnet are present exclusively west of the fault, showing a westward increase in metamorphic temperature.