Wednesday, January 20, 2021

Drunk on Petrology - Rockpile Zinfandel


The next Drunk on Geology is for Rockpile Zinfandel from Mauritson Wines

The name of Rockpile is the same as the region in which the vineyards are located. The Rockpile region was a recent AVA denoted in 2002. According to the TTB, an AVA is an American Viticultural Area which is:
"delimited grape-growing region with specific geographic or climatic features that distinguish it from the surrounding regions and affect how grapes are grown."

The specifics of the Rockpile AVA are that it is the northern extent of the Dry Creek Valley AVA that encompasses the areas from 800 to 2,000 feet in elevation with steep, rocky slopes and rocky loam/ clay-loam soils. 

Looking at the geology of the region, most of the Rockpile AVA falls within one specific geological unit. 

Geological map of the Rockpile AVA. Map obtained from the NGMDB

The Rockpile AVA runs along the southwestern shores of Lake Sonoma towards the north. This region is all encompassed by the KJfs formation on the geological map above. The KJfs is the Jurassic to Cretaceous age Franciscan Complex, specifically the greywacke and mélange deposits. The Franciscan Complex is a series of rocks that are termed accretionary (or accreted) terrain. These accreted terrains were added onto North America due to the subduction zone that used to lie on the western shore of California.

Diagram of the subduction zone that was formerly off the western California coast. Image courtesy of the NPS

Starting around 100 million years ago, the Farallon Plate started subducting beneath the North American Plate. As that happened, the islands and other land masses that were on the Farallon plate were scraped off and stuck onto the North American plate in a region known as the accretionary wedge. The Franciscan Complex is such an accretionary wedge that was built up from oceanic crust and pelagic deposits over the course of the Jurassic and Cretaceous. Over time the subduction zone was subducted as well beneath North America, with the remaining parts of the subduction zone only occurring currently along the western coast of Washington, Oregon, and the very northern part of California. 
Extent of the Franciscan Complex in California. Image courtesy of Raymond, 2019.

The rocks within this complex are a large mishmash of materials all thrown together due to the way they accumulated on the continent. As mentioned above, the primary rocks within the Rockpile region are denoted as greywacke and mélange. Greywacke is kind of a bucket term, which is sometimes used for a poorly sorted, grey sandstone. However the Dictionary of Geological Terms 3rd Ed. identifies greywacke (AKA graywacke) as:
"An old term, now generally applied to a dark gray firmly indurated coarse-grained sandstone that consists of poorly sorted angular to subangular grains of quartz and feldspar, with a variety of dark rock and mineral fragments, embedded in a compact clayey matrix having the general composition of slate and containing an abundance of very fine-grained illite, sericite, and chloritic minerals. Graywacke commonly exhibits graded bedding and is believed to have been deposited by submarine turbidity currents.  
The submarine deposition of these rocks makes sense, since they were likely formed along the sea floor before they were scraped off by North America. A turbidity current is an underwater landslide.

On the other hand, the term mélange just typically means a mixture rock blocks of varied sizes and lithologies, not in contact with each other, and surrounded by a finer-grained matrix. Typically these are formed by the process of amalgamation within the accretionary wedge. 
Text from the back of the bottle:
"In 1868 our family established a homestead in the Rockpile area of Northern Sonoma County. The first vines were planted in 1884, starting a tradition of farming that has spanned six generations. Today we craft small production, single vineyard wines that showcase the diversity of Rockpile's rugged terrain. From our vineyards to your glass we thank toy for sharing this tradition" - The Mauritson Family


According to the USGS:

"The sedimentary model suggests that blocks (olistoliths) were transported into the depositional environment of the matrix material by gravity driven debris slides.  The trench associated with the subduction zone provides an area of suitably steep slope for this theory and the converging plates bring the displaced terranes into proximity of the continental margin mélange matrix.  The resulting olistostrome then underwent the deformation described above, disrupting the original depositional character of the matrix/block relationships.  The tectonic model suggests that primary incorporation of blocks into mélange was by tectonic processes.  During and after accretion, lenses of rock derived from incoming exotic terranes were interleaved with continental margin deposits by faulting.  Subsequent deformation during uplift further broke up the lenses of exotic rocks, forming the mélange blocks observed today."

So in summary, the rocks within the Rockpile AVA were formed from submarine landslides that eventually got scraped off the sea floor and jumbled together within the western edge of the North American continent.


Wednesday, January 13, 2021

Drunk on Petrology - Bedrock Old Vine Zinfandel


The next Drunk on Geology is for Bedrock Old Vine Zinfandel by the Bedrock Wine Company

Although I am pretty certain that the owners of the winery chose to use the term "bedrock", with the less geological definition of the word as the foundation of their winery, that is not the way a geologist would see it. And I see it, as a geologist would see it. Bedrock Wine Company also has a large number of vineyards, however only one bear's the name "Bedrock" so we are going to focus on that one.

Bedrock definition (courtesy of the Dictionary of Geological Terms 3rd Ed.):
The solid rock that underlies gravel, soil, or other superficial material.

The problem with determining the bedrock of a region, especially an area within a river valley, such as where the Bedrock Vineyards are located, is that the overlying gravel, soil, and other superficial materials are frequently what is published on geological maps. And that is the case here, so we need to infer what indeed is the bedrock beneath the superficial materials. There are sometimes geologic bedrock maps made which interpret what would be the bedrock below the superficial deposits, however I couldn't find one for this region. This map is also a fairly recent map (2017), so I am fairly confident that we can use it to determine the likely bedrock below the sediment. 

Geological Map of the Bedrock Vineyards locality. Map courtesy of the NGMDB

The red circle on the map above denotes the area of the Bedrock vineyards. As you can see there is a lot of geologic deposits where the only capital letter is a "Q". The "Q" stands for the Quaternary, the time period they were deposited during and these are the superficial deposits as mentioned above. The Quaternary represents the last ~2.5 million years in the Earth's geological history and runs up to today. These are mostly sediment deposited by the Sonoma Creek that runs down the middle of the valley. 

In order to determine the bedrock from a map like this we need to first look at the bedrock along the outside edges of the stream valley deposits. These are going to be the sediments that begin with capitol letters besides just "Q". You can determine what all of these abbreviations by looking at the original map and legend HERE.

Text from the back of the bottle:
"From vines averaging over 80 years in age planted throughout the diverse state of California, this wine aims to reflect the perfume, freshness, and spice that makes Zinfandel so utterly delicious. Fermented with native yeasts and minimally racked, this wine should provide great short-term drinking but will age gracefully for 5-10 years."
The most notable rock formations are QPge, PMpu, Msvrah, and Msvas. Since QPge spans both sides of the river sediments, I am inclined to believe that this is the most likely bedrock directly below the winery. Even though I said that the deposits starting with a "Q" represented overlying sediment, there is also a capitol "P" on these. The "P" indicates that the unit spans more than just the Quaternary. 

The QPge represents the Glen Ellen Formation. The Glen Ellen Formation is a unit comprised of a mixture of tuffaceous clay, mud, bouldery to pebbly gravel, and sand and silt deposits with interbedded conglomerates. These sediments were deposited in a variety of nonmarine environments, including coalescing alluvial fans, fan deltas, streams, and lakes (per Sweetkind et al., 2010). The Glen Ellen formation is Pliocene to Pleistocene in age and is identified as younger than 3.2 million years old. These rocks were likely deposited in a prehistoric version of the Sonoma Creek in which is currently found in the valley now. 

The PMpu is the Pliocene-Miocene age Upper Petaluma Formation. The Petaluma Formation is dominated by deposits of moderately to weakly consolidated silty to clayey mudstone, with local beds and lenses of poorly sorted sandstone. The environment in which these rocks were deposited consists of transitional marine and nonmarine sediments that were deposited in estuarine, lacustrine, and fluvial depositional settings (per Sweetkind et al., 2010). These rocks represent a much higher sea level than the Glen Ellen Formation, with a mixture of marine and nonmarine rocks deposited within the valley. 

Msvrah (?Rhyolite of Adobe Canyon) and Msvas (Andesite of Shocken Hill) are both a part of the Sonoma Volcanics. The Sonoma Volcanics were deposited across a long range of time from the Miocene through the Pliocene and are described by the USGS as:
"... a thick and highly variable series of continental volcanic rocks, including andesite, basalt, and minor rhyolite flows and interbedded coarse- to fine-grained pyroclastic tuff and breccia, redeposited tuff and pumice, and diatomaceous mud, silt, and sand."

So overall, it is likely the bedrock for the Bedrock Vineyard is the Glen Ellen Formation, however the Upper Petaluma Formation is also likely. These are likely sprinkled with local volcanic deposits from the Sonoma Volcanics as well. 

Tuesday, January 12, 2021

Drunk on Petrology - Chalk Hill Syrah


The next Drunk on Geology is for Chalk Hill Syrah from the Chalk Hill Estate Winery

The name of the Chalk Hill Estate Winery comes from Chalk Hill upon which it sits, but the winery encompasses a rather large area with several different types of soil and bedrock. From their own website:
"In amazing proximity, the soils range from shallow to deep, from thin to thick, from gravel and rock to heavy clay. An old river bed caps one hill; an unusual serpentine vein ranges through another and, under the topsoil, is a distinctive layer of chalk-colored volcanic ash which inspired the name of Chalk Hill, the appellation, and the estate."

Their description of the winery is geologically fantastic, so lets take a closer look at the actual geologic map of the region. 
The winery is located within the red circle on the geologic map above. This region, and the surrounding area, is a mishmash of geology. Starting with the purple unit that crosses the middle of the red circle, we have the serpentine vein that is mentioned in the description.  

Geologic Map of the Chalk Hill Winery region. Map located using the NGMDB

Although identified by the winery as "unusual", the serpentine band is actually part of a Middle Jurassic formation known as an ophiolite, of which ophiolites are fairly common across California. This is because of how California formed. The diagram below illustrates the subduction zone that was formerly off the western coast of California. 

Diagram of the subduction zone that was formerly off the western California coast. Image courtesy of the NPS

The crust of the Earth is made up of plates that move around and a subduction zone is when one plate goes beneath the another plate. When that occurs, a lot of things happen. As the plate descends, it starts to melt. That melted rock then rises back up producing volcanoes, in this instance it is how the Sierra Nevada mountains formed. But as that plate is descending, the overriding plate (in this instance North America) is scraping the top layer of the down-going plate (the Farallon Plate). The scraped material includes any islands in the ocean, other land masses, and even pieces of oceanic crust. This material is known collectively as "accreted terrane", and is how a vast portion of California was built up over time.  

Looking at the pieces of the oceanic crust in particular, as the oceanic crust is formed, it is created at the Mid Ocean Ridge (as seen on the diagram above), by a series of continuous volcanic eruptions. These volcanic eruptions produce layers of different rocks and minerals depending on how the eruption interacts with the other rocks and water in the area. This eventually creates a layer-cake of rocks that are known as an Ophiolite Sequence.

Idealized ophiolite sequence. Image courtesy of Arenas et al., 2007.

One of the lowermost levels of an ophiolite sequence is the Serpentinite layer. The serpentinite is formed from the hydrothermal (hot water) metamorphism of mantle rocks, producing these green mineral rich rocks (most frequently comprised of antigorite, lizardite, and chrysotile) with a surface that looks similar to snake skin (hence the name). 

One of the other main rock formations in the region of the winery is the KJgvs, which is part of the Jurassic-Cretaceous Great Valley complex. This particular series of rocks consists of interbedded sandstone, siltstone, and shale formed within the Great Valley forearc basin as seen in the subduction zone diagram above. These are non-marine rocks including river complexes across the valley. 

Most of the rocks within this region, however, are part of the Sonoma Volcanics,  including the Tsb rock, which is a basalt. According to the USGS:
The Sonoma volcanics constitute a thick and highly variable series of continental volcanic rocks, including andesite, basalt, and minor rhyolite flows and interbedded coarse- to fine-grained pyroclastic tuff and breccia, redeposited tuff and pumice, and diatomaceous mud, silt, and sand. A prominent body of rhyolite flows and tuff with some obsidian and perlitic glass called the St. Helena rhyolite member, occurs in the upper part.
The final location we will talk about would the the eponymous Chalk Hill. Chalk Hill is to the east of the winery with the winery sitting near its base along the Chalk Hill road (as near as I can tell). And as the Chalk Hill Winery website describes, Chalk Hill is indeed not made up of chalk. Chalk is a well known sedimentary rock, most famously found as the White Cliffs of Dover. Chalk itself is made up of tiny fossils of animals called coccolithophores. The microscopic animals form a ball covered in little calcium carbonate plates called coccoliths. When the animal dies, the plates come apart and if enough of the animals are present in the environment, they can accumulated over time to form the rock known as chalk, which is a white to grey colored chalky rock.

However, that is not what we have here. We have a rock that resembles the color and texture of chalk, but is not actual chalk. This rock is made up of ash and other pyroclastic deposits from the nearby volcanic zones produced from the subduction zone that continued off to the west. Ash, also a white to grey chalky rock, has a strong resemblance to chalk, hence the name of the region and the hill. When ash falls, it quickly hardens and produces a rock layer known as tuff. Although the term tuff can refer to any pyroclastic (volcano produced) rocks. The tuff of Chalk Hill was deposited mostly within water bodies in the region as part of the Sonoma Volcanics. These ash deposits have been dated to ~4 to 5 million years ago. 

Wednesday, January 6, 2021

Drunk on Petrology - Soda Rock's Lord Snort Blanc


My next Drunk on Geology post is for Soda Rock's Lord Snort Blanc from the Soda Rock Winery

The winery was named "Soda Rock" back in in the 1930's following the repeal of prohibition, a name that has been passed down through several different owners. The original name of the winery descended from the Soda Rock Post Office and General Store, a building which the winery took over after it had fallen into disrepair. The name of Soda Rock appears to be named after an outcrop of rock along the nearby Russian River, identified on topographic maps simply as "Soda Rock". 

Soda Rock feature on the topo maps. Image from the NGNDB.

Since the actual Soda Rock lies within the Russian River it is a bit difficult to identify which specific geologic formation it belongs to. However we can look at the nearby geological formations and see what fits best. The closest, and most likely, is the Pleistocene and Pliocene age (~5-2 million years old) Glen Ellen Formation (QTge on the map below). The Glen Ellen Formation is identified as "compact lenticular poorly sorted silly clay, sand, poorly cemented conglomerate, and local fairly loose sand and gravel. There is also a tuffaceous member of the Glen Ellen Formation within the region, although not mapped right in this area.

Soda Rock feature on the geological maps. Image from the NGNDB.

The other potential rock near that area are the slightly more pinkish rocks on the map above. These are Pliocene and Miocene age (~20-5 million years old) Sonoma Volcanics (Tsb on the geologic map). Which are mainly andesite and basalt lava flows, pyroclastic rocks, and mud flows with interbedded volcanic sediments. Thick sections of poorly permeable tuff breccia and rubbly and fractured zones in lava yield water to wells. 

Soda Rock from Google Street View

Based on the view from Google Earth, which luckily had a "street view" along the Russian River, we can get a fairly good view of the Soda Rock itself. Unfortunately, this is no substitute for going out and viewing the rock in person. So any guess as to what the rock actually is, is entirely a guess on my part, but this looks to me more like a volcanic deposit. This means that the Soda Rock itself is more likely to be part of the Sonoma Volcanics, than a sedimentary deposit, which would be part of the Glen Ellen Formation.

The term "soda" is also a geological term, although one that is not often used in scientific circles (from what I am familiar with). It seems to be a more commercial/industrial term and it refers to sodium, specifically sodium carbonate (Na2CO3). Sodium within rocks also has a tendency to produce a grey, ashy looking rock, of which the term "soda ash" is also another commercial term for the word "soda". So, regardless of geology, the Soda Rock in the river, does appear to have a grey, ashy look to it, hence rendering it understandable that one would call it "soda". 

Text from the bottle:
"Located in the heart of the renowned Alexander Valley in Northern Sonoma County, Soda Rock Winery's rich heritage dates back to 1869 when the winery site housed the Soda Rock Post Office and General Store.

Today Soda Rock Winery's Village Heritage continues as owners Ken and Diane Wilson bring new life to this historic site with the best that Alexander Valley has to offer."

As a side note, I picked the Lord Snort Blanc as the wine for this post because the whole reason we stopped at this winery was the fantastic pig sculpture just outside the main wine tasting building, dubbed Lord Snort. A sculpture by artist Bryan Tedrick that is 20,000 pounds steel statue measuring 32 ft wide, and 20 ft tall. Lord Snort made his debut at the Burning Man Festival and eventually found his way to the winery.

And of course we had to take our picture in front of Lord Snort with our Lod Snort Blanc. 

Friday, January 1, 2021

Drunk on Seismology - Ram's Gate Vineyard

My next Drunk on Geology post is for the Ram's Gate Vineyard itself. When touring the vineyard I noticed a little sign denoting the "Faultline Vineyard", and while the winery doesn't have a geological themed name or label, I thought this would be a fun addition.

Upon further inspection it appears that the entire winery is located along the southernmost extent of the Rogers Creek Fault. The Rogers Creek Fault is the northern extension of the much better known Hayward Fault, one of the most active faults in California. The two faults connect beneath San Pablo Bay and it was unknown until recently if they were physically connected or just ran parallel to each other. The size of a potential earthquake is directly tied to the size of the fault. The larger the fault, the larger the potential earthquake. And with these two major faults being identified as two parts of the same fault, that increases the size of the potential earthquake along the Hayward-Rogers Fault Zone significantly (from a ~6.7 to a ~7.4). 

Fault map of the Ram's Gate Vineyard showing the Rogers Creek Fault Zone.

A 2018 study of the fault provided a recent, in-depth review of the fault with updated fault maps from San Pablo Bay up through Healdsburg, part of the Sonoma Valley winery region. In the map above you can see the fault trace running right through the center of the Faultline Vineyard, which is located to the east of the main tasting room building. 

Here is a shot towards the west, facing the main tasting room building along the main path of the Faultline Vineyard. In this photo the fault trace above is located to the right of the photo. The Rogers Creek Fault is an active fault with the most recent earthquake likely occurring sometime during the mid 1700's and has a recurrence interval between 131 and 370 years.

Here is a photo looking across the fault towards the north, which would be located approximately midway down the middle of the photo running from left to right across the vineyards.