Castle Rock State Park – Part II

This is the second post in a series about the geology of Castle Rock State Park.  It was submitted by a reader named Jonathan.  In the first post, Jonathan explained why he chose to write about Castle Rock State Park and he also showed where it’s located.  In this post, Jonathan will discuss the geologic history of the park.

According to geologist Richard Stanley, the rock formations of Castle Rock State Park are a component of the tertiary strata of La Honda basin, a “marine embayment that persisted through much of Tertiary time in the area of the modern Santa Cruz Mountains.” In his paper, ‘Evolution of the Tertiary La Honda Basin, Central California’ (1990), Stanley summarizes the history of this area:

The complicated geology and geologic history of the La Honda basin reflect the fact that, throughout its history, the basin has been located at or near the tectonically active plate boundary between the North American continent and various oceanic plates of the Pacific basin. The La Honda basin originated during the Paleocene, perhaps during an episode of wrench tectonism associated with oblique subduction and arrival of the Salinia terrane. Major restructuring of the basin during the Oligocene—including uplift and erosion of the basin margins, movement along the Zayante-Vergeles fault, and deposition of two sand-rich deep-sea fans—apparently resulted from the approach of the Farallon-Pacific spreading ridge and its collision with the California continental margin. During the late Oligocene and early Miocene, widespread volcanism and marine transgression accompanied an episode of regional transtension along the San Andreas fault system. Deposition of shallow marine sandstones and deeper-water siliceous mudstones occurred during much of the Miocene and Pliocene but was interrupted at least three times by brief episodes of uplift and erosion associated with transpressional wrench tectonism along the San Andreas fault. Marine deposition ended and uplift of the modern Santa Cruz Mountains began during the late Pliocene in response to the most-recent episode of regional transpression.

Stanley refers to ‘sand-rich deep-sea fans,’ which, in ‘A Dictionary of Earth Sciences’ (A. Allaby & M. Allaby, 1999) is defined as a “fan-shaped body of sediment that accumulates at the lower end of a submarine canyon, either at the foot of the continental slope or on the continental rise” (figure 4). In other words, a deep-sea fan can be thought of as an underwater version of alluvial fan.

Figure 4

According to Stanley, the sandstone, mudstone and conglomerate that were deposited by these deep-sea fans formed sedimentary rock, which was uplifted in the Santa Cruz Mountains as Vaqueros Sandstone that we see today in Castle Rock State Park.

Geologic maps of Castle Rock State Park and the surrounding region indicate that most of the Park’s outcrops are Vaqueros Sandstone (figure 5).

Figure 5

Additionally, because fossils contained within sedimentary rocks vary from the bottom to the top of layers, fossils found within the Park’s outcrops allow geologists to date the rock to be approximately 23 million years old (figure 6).

Figure 6