EVIDENCE FOR A TWO-STAGE ERUPTION AT TRIMBLE KNOB, AN EOCENE VOLCANIC PLUG IN HIGHLAND COUNTY, VA
At the peak and on the western side of the exposed neck, poor- to moderately-developed columnar jointing is present. The basalt contains clinopyroxene and olivine phenocrysts in a fine-grained groundmass. The columnar basalt transitions to massive basalt along the southeast edge of the hilltop. A debris flow scarp has exposed the bedrock on the eastern slope of Trimble Knob. A transition zone of basalt with a hackly texture extends about 12 m down the eastern side of the neck and includes sparse angular black shale xenoliths several cm across. In thin sections, autolithic basalt fragments are cemented with carbonate minerals. Below the transition zone, a diatreme breccia containing rounded to subrounded xenoliths of shale, tan sandstone, and gray limestone is observed. In thin section, carbonate cement, clinopyroxene and olivine phenocrysts, and autobrecciated basalt clasts are observed. Based on field observations and petrographic characteristics, the diatreme xenoliths are identified as the Milboro Shale, the Ridgely Sandstone, and limestone from the Helderberg Group. The diatreme breccia was created by an explosive eruption that ripped out clasts of the country rock in a chaotic fashion. We propose that there was a single, two-stage eruption at Trimble Knob. An initial explosive eruption produced the diatreme, followed by a less violent flow of magma through the center of the plug.