GEOLOGICAL MAPPING OF THE SPROUL CRATER, A SPATTER CONE IN THE SAN FRANCISCO VOLCANIC FIELD

The Sproul is a ~20 ka volcano located in the eastern portion of the monogenetic San Francisco Volcanic Field (SFVF). This volcanic field contains over 600 scoria cones and stratovolcanoes that range in age from 6 Ma to 900 ybp. The volcano is unique to the volcanic field as it is the only documented spatter cone. The cone is also interesting because it formed around a fissure, creating an elongated shape. To better understand this cone, geological mapping was done in tandem with facies descriptions.

The morphology of the cone changes significantly from west to east likely in response to changing eruptive conditions. On the west end a step-like “horseshoe” feature is present that diminishes to the east, following the projected migration of the fissure vent. The facies display spatter being dominant for the middle horseshoe. The inner horseshoe gradually slopes into the basin of the crater which extends ~1 km to the eastern end. Facies towards the southeast the inner horseshoe are welded-cinder dominant. Around the northern shoulder of the cone this step-like feature continues to be present but far more irregular than the northwest steps. This continues on to the central part of the cone, where abrupt cliffs at the lower northern steps expose continuous cinder. To the northeast, a breach is present that gives way to a lava flow. Within the basin, clusters of tumuli extend along the line of the projected fissure vent. Lava flow wraps around the inner base of the north facies near the tumuli, suggesting a nearby vent. The facies become younger to the southeast, correlating to the migrating vent. The southeast facies, the highest point of the Sproul, reveals spatter more frequently than the northwest end of the Sproul. This high point suggests rapid spatter deposition which created the steepest point of the cone. Along the southern ridge, which wraps from the west to the east end, a consistent single rim is present.