Geologic Fact Sheet: The Geologic Soul of Pescadero Creek Vineyard
Geologist Bill Cotton
December 27, 2013
What is the type of bedrock in Pescadero Creek Vineyard?
Pescadero Creek Vineyard is rooted in mudstone bedrock (i.e. shale) that has been identified as the Santa Cruz Mudstone geologic formation. Not a very sexy rock name but "mudstone" is essentially the same as shale, the difference being mostly how the rock breaks apart (shale breaks up in layers and mudstone crumbles into small shards) and the amount of fine clay in the rock (mudstone has a bit more). The unique aspect of the bedrock at your site compared to other nearby rock deposits is the fact that it represents a very restricted geologic origin. Of all of the sedimentary rocks found in the Santa Cruz Mountains the shale and mudstone bedrock represent an origin in a deep ocean environment far from the ancient Miocene shoreline. The other sedimentary formations in the mountains, such as siltstone, sandstone, and conglomerate rock types, all were deposited in much more chaotic environments similar to beaches and near-shore areas of today or in a few cases in very remote abyssal areas of deep oceans.
[Short Sedimentary Rock Lesson : Sedimentary rocks, from coarse-grained to fine-grained clastic rocks, include Conglomerate (boulders, cobbles & pebbles mixed with sand & gravel), Sandstone (sand- size sediments (e.g. beach sand), Siltstone (silt- size grains which can’t be seen without a magnifying glass) and Mudstone/ Shale (mostly very fine silt- and clay-sized particles and some form of chemical ppt. and the last sedimentary rock type include the non-clastic chemical rocks such as Limestone/Chert/Dolostone mostly composed of CaCO3/SiO2/CaMgCO3 mixed with clay. ]
Geologists use rock types and their regional distributions as geologic clues that define a region’s past geologic history that in turn is used to reconstruct the paleogeography of an area. For example, coarse-grained sediments found along modern beaches and surf zones are equivalent to the lithified sedimentary bedrock that make up the conglomerate and sandstone bedrock exposed in some parts of the Santa Cruz Mountains. Therefore when we find those rock types, we conclude that that region of the mountain was once an ancient shoreline.
Where was it deposited?
Because of the very fine-grained nature of Pescadero Creek Vineyard’s bedrock we can conclude that it reflects a unique depositional environment that was situated many 100s of miles off shore from some ancient shoreline. The ancient shoreline was close to the present day California-Nevada state line. The California coastal area was like a vast borderland (e.g. Catalina-like) with offshore islands separated by deep ocean basins. We know that the shale was deposited in an ocean (i.e marine origin) because it contains tiny marine fossils (Diatoms (silicious algae/phytoplankton), fish scales, foraminifera, and silicious sponge spicules). Because of the abundance of these tiny siliceous fossil remains, the bedrock is described a "siliceous mudstone". The very fine silt and clay make- up of the rock and the extremely small and delicate fossil content all point to a site of deposition as one in a quiet marine basin far from the influence of the continental shoreline. As the Coastal California shoreline was hugging the California/Nevada border the sediment debris that make up the Santa Cruz Mudstone was accumulating far off shore and well beyond the curvature of the ancient ocean surface in a deep ocean basin. The entire thickness of the Santa Cruz Mudstone deposit is about 3000 feet which is about one-half of the entire sedimentary rock sequence in the mountains. It likely took about 5 million years to accumulate all of the mudstone bedrock (~7 inches/1000 years). This is exceedingly slow and it reflects the uniquely protected depositional setting of the deposit.
When was it deposited?
We know the geologic age of the Santa Cruz Mudstone from its stratigraphic relation to other bedrock formations, its fossil content, and a radiometric age date (6.7+/- 0.6 million yrs). The general age extends from the Upper Miocene to the Lower Pliocene and spans a geologic time interval from about 8 M years ago to about 4 +/- . So during this time interval the off-shore marine basin of deposition was very slowly receiving the finest clastic grains of silt and clay and the tiny skeletal remains of diatoms, forams, fish scales, and sponges.
When did it become uplifted into Mountains?
We know that tectonic forces have played a significant role in the long geologic history of the Santa Cruz Mountains. The long-term geologic history of the mountain system is complex. We know that the region had four periods of mountain building during the last 65 million years (Tertiary Period). We are still in the last orogenic period characterized by slowly raising mountains and lots of earthquakes. Your vineyard bedrock represents part of the last phase of sediment accumulation and mountain building episodes, one that is continuing today. The mountainous geomorphic region that we call the Santa Cruz Mountains is likely about 3+/- million years old having formed after the last marine deposit was laid down in the Late to Middle Pliocene.
The real driving force that uplifted the Santa Cruz Mountains is the San Andreas Fault, the active geologic boundary between the Pacific Plate and the North American Plate. The SAF started (i.e. came on land) in southern California (near San Diego) approx. 30 million years ago (during the Oligocene) and slowly progressed north finally arriving in the Bay area about 15M years ago. Strong tectonic compressive forces along the San Andreas Fault folded and faulted all of the Santa Cruz Mountain rocks uplifting them to produce the present day Santa Cruz Mountains.
Along with the development of the Santa Cruz Mountain topography most of the bedrock geology west of the San Andreas Fault has its origins hundreds of miles south of their present locations. Lateral movement along the San Andreas Fault has rafted the Santa Cruz mudstone (& other Santa Cruz Mountain bedrock) approximately 225 miles from its original place of deposition in central California.