LARGE, CALCITE-CEMENTED SANDSTONE 'CHIMNEYS' IN THE SANTA MARGARITA SANDSTONE, SCOTTS VALLEY, CALIFORNIA: CASE STUDY OF A PLUNDERED LATE MIOCENE COLD SEEP

A unique outcrop of the upper Miocene Santa Margarita Sandstone in Scotts Valley, California, contains abundant calcite-cemented concretions that resemble authigenic structures found at cold seeps, including chimney-like columns up to 2 m in diameter. This site has been known locally as ‘The Ruins’ since the mid-1800’s, when optimistic entrepreneurs proclaimed the concretions to be remnants of a ‘great and magnificent’ ancient structure consisting of 50 columns capped by a dome. Misguided exploration of the site by Gold Rush era speculators and treasure hunters predictably produced no artifacts and regrettably resulted in removal or destruction of most of the original columns. We recently mapped, sampled and analyzed the remaining 12 chimneys and numerous slab-like concretions, which are exposed over ~160 square meters in the uppermost portion of the otherwise poorly indurated Santa Margarita Sandstone. The surviving columns crop out along a SW-NE trend on a sandy hillside and are distinctly chimney-like, with circular cross sections and central cavities; the tallest rises 1.5 m above the surface. A broad horizon of discontinuous, slab-like concretions, 0.2-1.7 m in length, stratigraphically overlies the chimneys. All concretions consist of coarse-grained sandstone cemented by low-Mg calcite (1.5-2% MgCO₃), and they typically contain abundant remains of the echinoid Astrodapsis spatiosus. The stable isotope values of the cements (δ¹⁸O from -5.15‰ (chimney) to -2.32‰ (slab); δ¹³C from -19.89‰ (chimney) to -1.95‰ (slab)) are consistent with a cold seep origin. We conclude that the once-famous ‘Ruins’ concretions represent an exhumed, localized seep field that included focused rising columns (chimneys) and diffusive flow (slabs) of methane-rich fluids through shallow marine sediments. The chimneys overlie and follow the trend of the middle Miocene-aged Bean Creek Fault, suggesting that fluid flow was fault/fracture controlled. This sole example of hydrocarbon seepage in the Santa Margarita Sandstone occurred during a period of complex transpressional deformation in the southern Santa Cruz Mountains and foreshadowed development of an extensive regional fluid system (marked by seepage, oil migration and injection of Santa Margarita sands/fluids into overlying sediments) in the latest Miocene.