DETECTING AND INFERRING AGES OF MARINE WAVE-CUT PLATFORMS ON SANTA ROSA ISLAND, CA

Santa Rosa Island, part of the California Channel Islands, is composed mostly of Tertiary marine sediments and volcanic rocks mantled by Quaternary and possibly Neogene marine deposits. Marine terraces are a common feature, and previous research has concentrated on those fringing the island generated during MIS 5a (80 ka) and 5e (120 ka). New geologic mapping has identified several topographically higher, low-gradient, low relief surfaces throughout the island. These surfaces display features diagnostic of marine incursion and terracing. Determining their ages will constrain intra-island and inter-island/mainland variations in surface uplift rates and fault-related strain. We utilize simple toolsets to expand on our observations and infer ages of as-yet undated terraces while quantitative dating us underway.

We use terrain analyses of a 1-m lidar dataset to predict the spatial extent of terrace surfaces and correlate between nearby, known terrace localities. Analyses consist of a combination of traditional methods to identify low-relief, low-gradient surfaces, and newer differential geometry tools that identify planar, yet tilted, surfaces. Both identify wave-cut platforms as identified in the field. Histogram analysis shows distinct elevation zones for these surfaces, particularly at lower elevations. The elevation histogram “peaks” can be inverted using known elevation-age relationships from nearby islands and the mainland to infer the ages of suspected terraces. We also use a simple one-dimensional numerical model, driven by the oxygen-isotope curve and user-specified uplift rates, to infer terrace age-elevation relationships and geometric relations based on model predictions of terrace generation and preservation through varying uplift rates and relative sea level.

Although these tools provide insights into probable ages and uplift rates of marine terraces, they are limited by the presence of younger supra-terrace alluvial and eolian deposits that skew elevations of the ground surface, and the likelihood of false-positive terrace identification of older Tertiary strata with similar geomorphic expression.