Geology 2 Spring 1997

Name:

Lab day & time:

PRE-LAB #9: SANTA YNEZ MTS / SAN MARCOS PASS FIELD TRIPS

This prelab assignment is due at the beginning of your field trip (weekend or lab time). You will not be allowed to participate in the trip if you have not completed this prelab exercise.
During the Santa Ynez Mountains or San Marcos Pass field trip we will be examining our local geological formations. To get ready, you should familiarize yourself with the the nature of the rocks that occur in our region and the basic history that geologists surmise from them. Study the attached stratigraphic section and read through the attached written information about the local geologic history. Answer the following questions.

On the stratigraphic column, different patterns are used to indicate different rock types. Figure out the convention and draw the patterns here:

Conglomerate:

Sandstone:

Shale:

What kinds of rocks make up the Franciscan formation and what process deformed them and added them onto the edge of the continent?

On the stratigraphic section, find the contact between the Franciscan formation rocks and the overlying Jalama formation rocks. What is the nature of this contact?

The rocks that crop out in the Santa Ynez mountains are those of the five Eocene - Oligocene age formations in the section. Which of those five formations are likely to form peaks and cliffs (name them) and which will tend to form the lower valleys and saddles (name them)?
Peak-forming rocks:

Valley-forming rocks:

What is the color of the basal sandstones and conglomerates of the Sespe Formation rocks?
What causes this color?

What does the color of the Sespe rocks imply about the origin of the Sespe formation?

Are there any other formations with similar colors in the section?

What happened during the Pleistocene?

Given that history, what type of contact would you expect to find between the older, deeper, deformed rocks and the Pleistocene and Holocene formations deposited on top? Name it (e.g., "a disconformity").

What are the upper Pleistocene and recent formations made of? What recent conditions led to these kinds of deposits?

GEOLOGY OF THE SANTA BARBARA REGION

I. Introduction

The next pages contain background information about the rocks that crop out in the Santa Barbara - Goleta area and the history that geologists infer from these rocks. This information is given in the form of words and also in a graphical summary called a "stratigraphic column". The information given is for the rocks in the San Marcos Pass area of the Santa Ynez Mountains. Other regions along the coast vary from this history in some of the details, but the broad story is true for all.

II. Stratigraphy and the Geologic Time Scale

Stratigraphy can be most broadly defined as the vertical arrangement of rocks in a given area. Stratigraphy may include rocks of sedimentary, igneous, and metamorphic origin, and can be discussed on a local to world-wide scale.

For convenience in mapping an area, geologists group the local rocks into stratigraphic packages called formations. Recognition of individual formations is somewhat arbitrary, but criteria include appearance (rock types and textures, outcrop characteristics and geometries), similar formative processes, and lateral continuity. Formations are commonly named for the locale where they were first mapped and described.

The stratigraphy of a given area can be graphically represented by a stratigraphic column, such as the one on a following page for the San Marcos Pass region. This stratigraphic column contains several types of information. The various formation names and a graphical representation of their lithologic characteristics are given in the center of the table. The irregular left edge of the central column is a representation of the relative strength or weakness of each layer and this, in turn, is an indication of which layers will tend to form peaks and cliffs and which will form saddles and slopes. Thickness and some general descriptive characteristics are given in the right columns. The far left column gives the age of the formations on the relative geologic time scale. Older formations are lower on the column.

A word about the geologic time scale: this scale is a division of time made by geologists for purposes of correlating non-spatially related formations. It has a historical basis, reflecting where the idea of the geologic time scale developed (Europe in the 18th and 19th centuries), but is now defined using scientific techniques, such as radiometric dating and fossil correlation. The time scale is divided into eras, periods, and epochs, in order of decreasing chronometric range. The epochs can be further subdivided as shown on the column (Lower/Middle/Upper). Approximate ages of the time divisions on the column give an idea of the time range represented by the formations.

GEOLOGIC HISTORY OF THE GOLETA / SANTA BARBARA AREA

This is a brief geologic history of the Goleta / Santa Barbara area. You will find it easier to absorb the information if you keep track of your location on the stratigraphic column as you read through it. Plain English Translation

The basement rocks underlying the area are parts of the Franciscan Formation. The Franciscan consists of a highly mixed assemblage of deep water sedimentary rocks including cherts and greywacke sandstones, plus altered basalt and serpentinite. The Franciscan sedimentary rocks were deposited on the deep ocean floor and the Franciscan basalts and serpentinites are pieces of the oceanic crust. These rocks were deformed, stacked up, and transferred to the edge of North America when their oceanic plate was subducted beneath the rim of the continent. In this area, the formation is Jurassic to Cretaceous in age. Deep ocean floor junk, scraped up and plastered onto the edge of the continent during subduction.

Eocene rocks in the area are all marine and are well-represented, attaining a thickness of over 3000 m (10,000 ft). Most of the Santa Ynez mountain range north of Goleta and Santa Barbara is carved from these rocks. The more resistant sandstones form the higher peaks. The Juncal, Matilija, and Cozy Dell Formations were all deposited in deep ocean water on the continental shelf, in the form of submarine fans. Toward the top of the Eocene section is the Coldwater Formation. The Coldwater is made up of about 80% sandstone and 20% siltstone and shale. Nearshore conditions are indicated in this time by oyster beds and other marine fossils, with occasional streaks of red shale, presumably washed into the shallow marine waters from nearby land. Sediments swept off the land during floods and deposited on the continental shelf.

In Oligocene time, the sea withdrew from the area, leaving a broad coastal plain on which were deposited the conglomerates, sandstones, and silts of the Sespe Formation. The Sespe is coarsest near its base, where conglomerates are interbedded with sandstones. The Sespe beds were deposited in a river-flood plain or deltaic depositional environment. Several lines of evidence suggest that the Sespe is a non-marine unit. Its red color comes from the oxidation of iron within the sediment, which generally takes place in environments where sediments are exposed to the air in alternating wet and dry seasons. Sedimentary structures in the Sespe: cross beds, channel cut and fills, graded beds, irrregular discontinuous strata all suggest river flood plain (non-marine) environments. Finally, marine fossils, such as those seen in the Coldwater, are conspicuously lacking in the Sespe. Vertebrate fossils, such as horse teeth, have been found in the Sespe, but are not proof of a non-marine setting since they could have been washed offshore. ***uplift ***
(Start of San
Andreas
system)
Coastal river flood plain sediments.
Great rocks! (We'll check
these out at
several
places.)

***rifting***

Toward the end of the Oligocene, the sea again spread over the area. The first marine unit is the Vaqueros Formation, which contains abundant broken shell material, and suggests deposition near a gravelly shoreline. The Vaqueros is covered by a series of marine siltstones and shales which indicate deeper water conditions again, starting with the Rincon Shale, a monotonous gray clay shale. Above the Rincon is the Monterey Formation, also a shale but very rich in organic debris, especially the microscopic silica shells of one-celled plants called diatoms. The organic molecules in this debris have subsequently been altered to oil and gas, hence all the oil platforms and the oil and tar seeps in our area. In the Goleta area (and the eastern campus bluffs) the Monterey formation grades upward into the Sisquoc Formation, a gray mudstone, also very rich in diatoms. More continental shelf sediments, this time very rich in diatom shells.

(San Andreas
hits a snag)
***crunch!***

In Quaternary time, the Santa Ynez mountains were faulted, folded, shortened and elevated. As they rose up, the higher and softer parts were eroded. The resulting debris collected in the lowlands, forming the alluvial fans ("fanglomerates") and other deposits ("alluvium") shown at the top of the section. Uplift and folding of mountains, erosion, deposition of debris fans.

There is compelling evidence that the uplift and tectonic deformation continues today. Earthquakes are common, showing tectonic deformations. The rapid uplifts of beach terraces and river bed sediments show the uplift. Near Santa Barbara, Sespe beds can be found that are in fault contact with Pleistocene units, showing that faulting occurred after the deposition of the Pleistocene sediments, i.e., very recently! Continuing uplift, faulting, earthquakes.

This is a brief geologic history of the Goleta / Santa Barbara area. You will find it easier to absorb the information if you keep track of your location on the stratigraphic column as you read through it.	Plain English Translation
The basement rocks underlying the area are parts of the Franciscan Formation. The Franciscan consists of a highly mixed assemblage of deep water sedimentary rocks including cherts and greywacke sandstones, plus altered basalt and serpentinite. The Franciscan sedimentary rocks were deposited on the deep ocean floor and the Franciscan basalts and serpentinites are pieces of the oceanic crust. These rocks were deformed, stacked up, and transferred to the edge of North America when their oceanic plate was subducted beneath the rim of the continent. In this area, the formation is Jurassic to Cretaceous in age.	Deep ocean floor junk, scraped up and plastered onto the edge of the continent during subduction.
Eocene rocks in the area are all marine and are well-represented, attaining a thickness of over 3000 m (10,000 ft). Most of the Santa Ynez mountain range north of Goleta and Santa Barbara is carved from these rocks. The more resistant sandstones form the higher peaks. The Juncal, Matilija, and Cozy Dell Formations were all deposited in deep ocean water on the continental shelf, in the form of submarine fans. Toward the top of the Eocene section is the Coldwater Formation. The Coldwater is made up of about 80% sandstone and 20% siltstone and shale. Nearshore conditions are indicated in this time by oyster beds and other marine fossils, with occasional streaks of red shale, presumably washed into the shallow marine waters from nearby land.	Sediments swept off the land during floods and deposited on the continental shelf.
In Oligocene time, the sea withdrew from the area, leaving a broad coastal plain on which were deposited the conglomerates, sandstones, and silts of the Sespe Formation. The Sespe is coarsest near its base, where conglomerates are interbedded with sandstones. The Sespe beds were deposited in a river-flood plain or deltaic depositional environment. Several lines of evidence suggest that the Sespe is a non-marine unit. Its red color comes from the oxidation of iron within the sediment, which generally takes place in environments where sediments are exposed to the air in alternating wet and dry seasons. Sedimentary structures in the Sespe: cross beds, channel cut and fills, graded beds, irrregular discontinuous strata all suggest river flood plain (non-marine) environments. Finally, marine fossils, such as those seen in the Coldwater, are conspicuously lacking in the Sespe. Vertebrate fossils, such as horse teeth, have been found in the Sespe, but are not proof of a non-marine setting since they could have been washed offshore.	*uplift * (Start of San Andreas system) Coastal river flood plain sediments. Great rocks! (We'll check these out at several places.) *rifting*
Toward the end of the Oligocene, the sea again spread over the area. The first marine unit is the Vaqueros Formation, which contains abundant broken shell material, and suggests deposition near a gravelly shoreline. The Vaqueros is covered by a series of marine siltstones and shales which indicate deeper water conditions again, starting with the Rincon Shale, a monotonous gray clay shale. Above the Rincon is the Monterey Formation, also a shale but very rich in organic debris, especially the microscopic silica shells of one-celled plants called diatoms. The organic molecules in this debris have subsequently been altered to oil and gas, hence all the oil platforms and the oil and tar seeps in our area. In the Goleta area (and the eastern campus bluffs) the Monterey formation grades upward into the Sisquoc Formation, a gray mudstone, also very rich in diatoms.	More continental shelf sediments, this time very rich in diatom shells. (San Andreas hits a snag) *crunch!*
In Quaternary time, the Santa Ynez mountains were faulted, folded, shortened and elevated. As they rose up, the higher and softer parts were eroded. The resulting debris collected in the lowlands, forming the alluvial fans ("fanglomerates") and other deposits ("alluvium") shown at the top of the section.	Uplift and folding of mountains, erosion, deposition of debris fans.
There is compelling evidence that the uplift and tectonic deformation continues today. Earthquakes are common, showing tectonic deformations. The rapid uplifts of beach terraces and river bed sediments show the uplift. Near Santa Barbara, Sespe beds can be found that are in fault contact with Pleistocene units, showing that faulting occurred after the deposition of the Pleistocene sediments, i.e., very recently!	Continuing uplift, faulting, earthquakes.