Deepwater Facies and Architecture of the German Rancho Formation at Salt Point and Gualala Point, Sonoma County, CA

Clark, Julian; Fildani, Andrea; Sullivan, Morgan; McHargue, Tim; Power, Bruce; and Romans, Brian
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Deepwater Paleocene-Eocene deposits of the German Rancho Formation form part of the coastal outcrops of the Gualala Block between Fort Ross and Point Arena. Deepwater sands and conglomerates were deposited axially in the elongate Gualala Basin in a forearc setting. Two sections of contrasting facies are exposed in relatively continuous sections at Salt Point State Park (SPSP) and Gualala Point Regional Park (GPRP).

At SPSP, the deposits are characterized by packages of thick-bedded amalgamated and dewatered sands, clast-supported conglomerates, and packages comprising thin- and medium-bedded planar- and ripple-laminated sandstones. Chaotic debrite facies are also present. Numerous erosive surfaces separate different facies, and this together with the occurrence of high-energy facies (pebble-size clast-supported conglomerate, concentrations of large mud rip-up clasts, and debrites), leads us to interpret the succession as the fills of nested deepwater channels. These channel elements have distinct facies associations that allow us to recognize channel axis, channel off-axis, channel margin, channel abandonment facies. There are no overbank or levee facies recognized within the channel complex, which we interpret to indicate that these high energy facies were confined within a larger-scale erosional conduit.

The facies associations at GPRP are significantly different, with packages of thick-bedded, largely nonamalgamated sandstones separated by intervals of medium-to thick-bedded argillaceous sandstone beds. These argillaceous sands show a variety of sedimentary and textural fabrics characteristic of hybrid event beds. Additionally, in contrast to the deposits at SPSP clast-supported conglomerates are rare. The presence of hybrid event beds and the lack of identifiable erosional surfaces, amalgamation surfaces, clast-supported conglomeratic facies, or channel margin facies, suggest that these deposits were deposited in less confined setting.

Although it is difficult to correlate the two outcrop sections stratigraphically or spatially, the differences in reservoir architecture and facies are likely due to variable bathymetric relief in this structurally complex basin. Furthermore, these outcrops provide facies models for channelized and unchannelized deposits that can be applied to analogous subsurface reservoirs deposited in complex bathymetric settings.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013