Monday, 25 May, 2026/Edition 112

Blach shale deposition is important to petroleum source rocks and marks significant climate-ocean changes in Earth’s history. This week, we will look at two Cretaceous black shales. Also, this edition of Core Elements introduces a new segment, “The Lighthearted Layer,” for fun reading. Enjoy it!

Rasoul Sorkhabi

Editor, Core Elements

Caribbean Link to 88-Ma Global Oceanic Anoxic Event

Landscape of the Nicoya Peninsula/Wikimedia Commons

A paper in Global Planetary Change traces the cause of a global oceanic anoxic event (OAE) of Coniacian-Santonian age (90–84 Ma) to the peak Caribbean Large Igneous Province (ILP) volcanism at 88 Ma.

Cretaceous OAEs:

• OAEs are important markers of global climate warming, planktonic proliferation, ocean water acidification, and organic-rich shale deposition.
• Three OAEs have been recorded during the Cretaceous:
  • OAE1 (120–100 Ma)
  • OAE2 (94 Ma)
  • OAE3 (88–84 Ma)

What they did:

• Goran Andjić and colleagues studied the Chumico Formation at three localities on the Nicoya Peninsula of Costa Rica’s Pacific coast.
• They applied zircon U-Pb dating and radiolarian biostratigraphy to tuffs and volcaniclastic sediments interbedded with the black shales of the Loma Chumico Formation.
• They also determined the concentrations of mercury and total organic carbon, as well as the ratios of stable carbon isotopes on coeval carbonate samples from Hole 700B and Hole U1513A.
  • These holes were drilled on the northeastern slope of the Northeast Georgia Rise in the South Atlantic.

About the Loma Chumico Formation:

• This formation overlies a mafic basement of 94–89 Ma, composed of basalt and intruded by dolerite and gabbro rocks. The mafic basement exhibits a typical plume-derived LIP.
• It is a deepwater, volcaniclastic, and black shale formation deposited in the Tempisque Forearc Basin in northern Costa Rica.

What they found: Geochronologic coincidence and geochemical signature (such as high mercury abundance) of the samples indicate that Caribbean LIP volcanism between 88 and 86 Ma probably triggered the organic-rich OAE3 event.

Why it matters:

• Cretaceous OAE3 is the latest of its kind in Earth’s history.
• While OAE3 has been recorded in marine and terrestrial sediments, its relationship with a coeval black shale deposition and LIP was not empirically established.

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The Geochemistry of Pre-Salt Petroleum Source Rocks Offshore Brazil

 Pre-salt oil and gas fields of Brazil/Agência Nacional Do Petróleo

A group of scientists from Brazil has published a paper in Marine Geoscience and Energy Resources that details organic matter preservation in Lower Cretaceous pre-salt shale formation in the Santos and Campos basins offshore Brazil.

What they did:

• The researchers collected samples of Jiquia Shale and analyzed 50 samples from four wells.
• They employed XRC, XRD, and RockEval methods to obtain geochemical and mineralogical data.

Jiquia Shale: This lacustrine sedimentary rock was deposited during the Jiquia Stage (biozones NRT-008/009 to NRT-010) in the Middle Aptian under predominantly anoxic bottom-water conditions.

What they found:

• The study indicated contrasting depositional architectures across the Santos and Campos basins.
• In the Santos Basin, the Jiquia stage sediments were developed in a predominantly carbonate-dominated system (74.6 ± 10.7 percent total carbonates) with consistently higher TOC values (5.1 ± 1.1 percent).
• In the Campos Basin, the sediments were developed in a mixed carbonate-siliciclastic system (38.3 ± 13.0 percent carbonates) with more variable and lower TOC contents (3.1 ± 1.3 percent).
• The Jiquia stage shale concentrated in the Coqueiros Formation (Campos Basin) and Itapema Formation (Santos Basin) is a major pre-salt source rock.

Why it matters:

• Pre-salt reservoirs in the Santos and Campos basins are among the most prolific oil and gas accumulations in Brazil.
• This paper contributes to our knowledge of pre-salt source rocks charging these reservoirs.

Introducing: The Lighthearted Layer

Book cover courtesy of Amazon

The Lighthearted Layer is a new segment in Core Elements meant to take a step back from technical geoscience to share a piece of wit and/or wisdom. I hope you enjoy it and find it interesting! If you want to contribute a piece of geoscience-related wit or wisdom for this section, email editorial@aapg.org

Now, for today’s segment…

When Richard Selley published his classic text Ancient Sedimentary Environments in 1970, he included this confession in the acknowledgments:

“I am extremely grateful to Professor J. Sutton and the Imperial College authorities who granted me leave of absence to retire from the academic hurly-burly to the peace and quiet of the oil industry to write this book.”

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