Monday, 3 March 2025/ Edition 48

Regional geology is a fulfilling part of any geologist’s work, because it involves field work, outdoor life, mapping, sample collection, and solving geologic problems. It also integrates various branches of geology to unravel something hidden about Earth. Let's look at two geologic stories from California and Colorado.

Rasoul Sorkhabi

Editor, Core Elements

Angel Island in California Reveals Deep Ocean Rocks

TierneyMJ/ Shutterstock.com

Have you ever been to Angel Island in California? It has amazing geology. A new study published in the GSA Bulletin describes the geological age, sedimentology, and structural geology of the Franciscan accretionary complex on Angel Island.

What’s in a name: The geologic term “Franciscan Assemblage” came about after the city of San Francisco was coined in 1895 by U.C. Berkeley geologist Andrew Lawson, who also named the San Andreas Fault as the western limit of the Franciscan Complex.

The Franciscan Complex is a forearc accretionary wedge that formed from the Pacific-side subduction in California during Jurassic and Cretaceous times. The accretionary complex displays deep ocean floor rocks which rarely outcrop and are valuable to geologists.

New findings: Previous researchers mapped Angel Island as an undivided blueschist metasedimentary unit thrust over low-grade metasediments along the Quarry Point Thrust.

• But new field shows that Angel Island rock formations include the entire diversity of the Franciscan Complex.

• The new lithostratigraphy consists of seven structurally juxtaposed units from sub-metamorphic to blueschist rocks.

• Detrital zircon U-Pb dating indicates that structurally higher blueschists have maximum depositional ages of 108–110 Ma.

• The structurally lower rock units below the Quary Point Thrust have similar provenance (sediments source area) but younger maximum depositional ages of about 97 Ma.

Why it matters:

• These are the first high-resolution detrital zircon U-Pb ages from the Franciscan Complex.

• The study highlights the importance of geologic mapping combined with geochronology.

Go deeper: Read the full study by Das and colleagues here.

Sponsored

Unlock Carbon Storage Potential

Join GVERSE GeoGraphix at CCUS 2025 to explore cutting-edge geoscience technology that accelerates carbon capture, maximizes efficiency, and drives innovation.

The Colorado Rockies Awaken

Ronnie Chua/ Shutterstock.com

Denver is called Mile High City, thanks to the Colorado Plateau and the Colorado Rocky Mountains.

Laramide Orogeny: These mountains formed during the basement-uplift Laramide Orogeny from 80–40 Ma. However, geomorphic and sedimentological data indicate a prominent shift from net deposition to net erosion in the Pliocene to early Pleistocene. Was this a result of tectonic activity or climate forcing?

In a recent article in GSA Today, Marder and colleagues take on a long-standing enigma in American West geology: If the orogeny ended 40 million years ago, why do we observe younger geologic features, such as high topographic relief and river incision, in the ancient foreland basin?

What they did: The researchers collected samples from fluvial terraces in the Colorado Rocky Mountains and used Berillyim-10 dating for erosion rates and thermoluminescence dating for bedrock incision rates.

What they found: Researchers identified two distinct geomorphic styles along the entire 350-km-long mountain front:

1. A slowly-eroding, lower-gradient landscape at higher elevations

2. A steep-rapidly eroding landscape at lower elevations

Tectonic model: The researchers support the tectonic model, which suggests domal uplifts in the Colorado Rockies as far-field effects of regional geodynamic processes, which especially include:

• Residual effects of the Farallon-Kula plate subduction

• Rio Grande rifting

Why it matters: These geomorphic styles are more consistent with a tectonic rejuvenation that climate forcing of landscape erosion because:

• The lower and steeper channel landscape indicates a mountain-scale increase in the rate of base-level lowering relative to the Colorado Rocky Mountains.

• There is also a gradual increase in channel steepness from north to south consistent with tectonic models of the Colorado Rockies that suggest a southward increase in rock uplift over the past 5 million years.

Broader significance: This study has far-reaching implications for post-orogenic display of topographic rejuvenation in the Rockies and other mountain belts and supports the primacy of tectonic dynamism over climate forcing in shaping high mountain topography.

Sponsored

Redefine Subsurface Exploration

Discover GVERSE GeoGraphix at BEOS 2025 and experience smarter, faster subsurface exploration with advanced geoscience software built for precision and performance.

Quiz of the Week

Last week’s quiz question was: How do we define the outermost boundary of Earth’s atmosphere? How high is it above Earth’s surface?

There’s no single answer. The edge of the atmosphere is defined based on various criteria:

• According to the NOAA, the outermost layer of the atmosphere is the exosphere which (in calm conditions) extends from 600 to 10,000 kilometers above Earth’s surface. (Solar storms can lower it to 1,000 kilometers.)

• Some scientists consider the top of the thermosphere (thermopause at about 600 kilometers, below the exosphere) as the edge of Earth’s atmosphere, because atoms in the exosphere can escape into space.

• Other scientists put the upper limit of Earth’s atmosphere at 190,000 kilometers above Earth’s surface, about halfway to the Moon. Beyond this, the Sun’s radiation pressure exerts more control on hydrogen atoms than Earth’s gravity. 

• NASA and “international law” put the upper height of the atmosphere at 100 kilometers above the mean sea level. This boundary, called the Kármán Line (after Theodor von Kármán), separates spacecraft flights from the lower flights. The “effective” Kármán Line below which rocket-plane winged flights occur is 80 kilometers.

Now, for this week’s quiz: How is the edge of the Solar System defined? How far is it from the Sun?

Please send your response by March 6 to editorial@aapg.org.

👍 If you enjoyed this edition of Core Elements, consider supporting AAPG's brand of newsletters by forwarding to a friend or colleague and signing up for our other newsletters here.

➡️ Was this newsletter forwarded to you? Subscribe to Core Elements here.

✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.