Lecture 8: The Great Valley Forearc Basin

Phase 3: The California Subduction Factory (Continued)

Lecture 8: The Great Valley Forearc Basin

Topics:

  • I. Isostasy and exhumation of the Sierran Batholith

  • II. Seismic reflection profiles

  • III. The Great Valley sequence

  • IV. Turbidites & submarine fans

Overview

  • The lecture focuses on sedimentary rocks in two key areas:

    • Subsurface of the Central Valley

    • Exposed rocks along western margin of the Central Valley in the Coast Range

The Great Valley Basin

  • Description:

    • The Great Valley is a sedimentary basin resembling a huge bathtub filled with sedimentary rocks.

    • The thickness of these sedimentary rocks is approximately between 10-15 km (6-9 miles or ~40,000').

    • The sedimentary structure is a broadly U-shaped set of down-warped layers.

    • The basin structure is characterized as an asymmetric wedge, thickest along the western margin adjacent to the Coast Ranges.

    • The wedge tapers to a feather edge on the eastern border towards the Sierra Nevada range.

    • Sedimentary layers are exposed along the western edge of the Great Valley and tilt eastwards, with some layers becoming almost vertical.

    • Initially deposited horizontally, these layers were tilted upwards due to the rising Coast Ranges ramming against the Great Valley rocks.

    • Future discussions will cover the formation of the Coast Ranges and deformation processes.

Source of Sediments

  • Origin of Sediments: Investigating how sediments filled the basin and their geological history.

I. Isostasy and the Exhumation of the Sierran Batholith

  • Description of the Sierran Batholith:

    • Intrusive igneous rocks of the Sierran Batholith formed 3-15 km beneath the surface in magma reservoirs at temperatures around 750-900°C (1800-2000°F).

    • Average depth is about 10 km (6 mi).

  • Exhumation Explained:

    • Exhumation refers to the process through which deeply buried rocks are brought to the surface, involving:

    • Erosional exhumation

    • Tectonic exhumation such as through fault slip (exposing rocks in the footwall of a normal fault).

    • Typical erosional exhumation rates are between 0.01 to 1 mm/year (up to 1 km per Myr).

  • Key Process - Isostatic Uplift:

    • Less-dense crust floats on denser mantle, causing uplift when mass is removed from the crust.

    • Example Explanation:

    • If a mountain range 2 km high loses 500 m due to erosion, you’d expect an elevation of 1.5 km, but due to isostatic uplift, new height can rebound to approximately 1.9 km.

    • Erosion creates buoyant rises in the underlying rock, bringing deeper rocks closer to the surface over millions of years.

    • Sediments from weathering of the ancestral Sierras were transported down west-flowing rivers into the Great Valley basin.

II. Seismic Reflection Profiles

  • Geological Mapping Technique: Seismic reflection profiling helps identify subsurface geology where direct drilling is limited (4 - 5 km max).

  • Process Overview:

    • Artificial seismic waves produced at or near the Earth’s surface using methods such as:

    • “Thumper truck” generating sound waves

    • Controlled explosions in drilled holes

    • Air guns if underwater.

    • Waves travel deeper into the Earth and reflect off subsurface discontinuities (bedding planes, faults, igneous intrusions).

  • Travel Time Measurement:

    • The reflected waves return to the surface and are recorded by sensitive microphones called geophones linked to a computer.

    • The time delays recorded allow geologists to deduce depth and structure by creating seismic cross-sections.

  • Distinction of Data Types:

    • 2D seismic data yields single cross-sections, while 3D seismic data creates volumetric images from multiple intersecting profiles.

  • Cost and Funding:

    • Collection of seismic data is highly expensive: around $150,000 per square mile in 2020 dollars for 3D data.

    • This process is often funded by scientific agencies or the oil and gas industry seeking new resources.

III. The Great Valley Sequence and the Ancestral Great Valley

  • Formation and Age:

    • The Great Valley has existed as a sedimentary basin since at least 150 Ma (mid-Mesozoic).

    • The oldest known rocks in the Great Valley are younger than Sierran magmatism, which began around 210 Ma or the start of Franciscan subduction zone metamorphism at ~180 Ma.

  • Accumulation of Sediment:

    • Approximately 10-15 km of sediment has accumulated since 150 Ma, originating mainly from erosion and volcanic activity from the adjacent Sierra Nevada.

    • The Mesozoic and early Cenozoic sedimentary deposits are collectively termed the Great Valley Sequence.

  • Composition of the Great Valley Sequence:

    • It consists of:

    • Sandstone (originally sand)

    • Shale (originally clay-rich mud)

    • Conglomerate (originally pebbles and cobbles)

    • Deposited primarily in deep marine environments confirmed by marine fossils and radiometric dating of zircon grains.

  • Basin Characteristics:

    • The basin is classified as a forearc basin located between the ancestral Sierra Nevada and the western accretionary wedge (Francisan complex).

    • The forearc basin lies in front of the volcanic arc with sediment transport restrictions due to surrounding geological formations.

IV. Turbidites & Submarine Fans

  • Basin Evolution:

    • Great Valley is asymmetrically shaped, deeper on the western side and shallower on the eastern flank.

  • Sediment Transport Mechanisms:

    • Sediments originated mainly from ancestral Sierra Nevada via rivers.

    • Erosion material was transported through submarine canyons into the forearc basin by turbidity currents.

  • Turbidity Currents:

    • Turbidity currents consist of dense slurries of sediment and water that convey materials rapidly downhill.

    • Triggering events can include storms, volcanic eruptions, earthquakes, or underwater landslides.

  • Sediment Deposition:

    • Deposits form alternating layers of sand and clay called turbidites; each couplet forms under short intervals (minutes to hours) followed by long periods before the next deposition.

    • Modern analogs for turbidity currents can be observed in areas like Monterey Canyon, enhancing understanding of ancient depositional processes in the Great Valley.