Terrain-based map of mélange and intact rock at the junction of Coastal and Central Belt rocks of the Franciscan Complex between Gualala and Ukiah, northern California Coast Ranges

Abstract

A terrain-based map of a large area of Franciscan Complex between the San Andreas and Maacama faults east of Point Arena, California, provides much new detail compared to prior mapping. The new map reveals an unusual, structurally disrupted area that is defined by northwest-trending patches of erosionally resistant topography interspersed in subdued topography. These two terrain types, here termed Hard and Soft, are distinguished using differences in the detailed shapes of individual topographic elements that in these rocks are indicative of relatively intact rock and severely sheared mélange. The disrupted area lies at the junction between rocks of two different parts of the Franciscan Complex: sandstone and argillite of the Coastal belt to the northwest and mélange and variously metamorphosed sandstone and other rocks of the Central belt to the southeast. The rocks in the disrupted area seem to consist largely of Coastal belt sandstone interspersed in Central belt mélange. They are separated from areally continuous Coastal belt to the west by the newly defined Cedars fault and from continuous mélange of the Central belt to the east by the newly defined Boonville fault. A plausible history to account for Coastal belt rocks and Central belt mélange now interspersed at the same structural level in the disrupted area is westward thrusting of Central belt over Coastal belt on the Coastal belt thrust, located mostly above the present level of exposure, followed by extensional dip-slip faulting involving both the disrupted area and its bounding faults. Most of the study area has been mapped on the ground only in reconnaissance. The terrain map was prepared largely by interpretation of small-scale aerial photographs, with distinctions in the steepness of slopes and curvature of ridge and side-hill crests made from the photographs. These distinctions were then confirmed by differences in their areal abundance determined through quantitative mapping of slope and curvature. Most of the remainder of the map was prepared in the same fashion directly from the slope and curvature maps. Geologic identities are assigned to as much of the area as possible, but some resistant parts, mostly in the disturbed area, remain identified solely as Hard terrain. Some units, mostly along the southern edge of the map, are compiled from published sources. This report serves as a description and explanation of the associated spatial database (Wentworth, 2025), which consists of attributed line and polygon shape files and metadata describing them.