Abstract
Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east-central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well-preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non-seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non-seep sites.
| Original language | English |
|---|---|
| Pages (from-to) | 185-203 |
| Number of pages | 19 |
| Journal | Lethaia |
| Volume | 54 |
| Issue number | 2 |
| Early online date | 2 Sept 2020 |
| DOIs | |
| Publication status | Published - 3 Mar 2021 |
Bibliographical note
Funding Information:We thank Tom Linn (Glendive, Montana), Joyce and Jim Grier (North Dakota State University, Fargo) and Neal Larson (Larson Paleontology Unlimited, Keystone, SD) for introducing us to AMNH loc. 3911 and for their invaluable assistance in the field. We thank Mariah Slovacek, Anastasia Rashkova, Steve Thurston and Bushra Hussaini (AMNH) for help with preparation of specimens and SEM, photography and figures, and cataloguing assistance. Colin Carney and Dyke Andreasan (USC‐SIL) are gratefully acknowledged for assistance with stable isotope analysis. The authors also thank Matt Garb, Jamie Brezina, Kirk Cochran, Corinne Myers, Shannon Brophy, Alison Rowe, Kayla Irizarry, Remy Rovelli, Dustin Perriguey, Yang Gao and Nicholas Hugo for assistance in the field and useful discussions. This project was supported by an NSF Biological Sciences REU awarded to Delaney Ryan at the AMNH, and a Lerner‐Gray Postdoctoral Fellowship at the Richard Gilder Graduate School and AMNH awarded to James Witts. Additional support from the Newell Fund (AMNH) is gratefully acknowledged.
Funding Information:
We thank Tom Linn (Glendive, Montana), Joyce and Jim Grier (North Dakota State University, Fargo) and Neal Larson (Larson Paleontology Unlimited, Keystone, SD) for introducing us to AMNH loc. 3911 and for their invaluable assistance in the field. We thank Mariah Slovacek, Anastasia Rashkova, Steve Thurston and Bushra Hussaini (AMNH) for help with preparation of specimens and SEM, photography and figures, and cataloguing assistance. Colin Carney and Dyke Andreasan (USC-SIL) are gratefully acknowledged for assistance with stable isotope analysis. The authors also thank Matt Garb, Jamie Brezina, Kirk Cochran, Corinne Myers, Shannon Brophy, Alison Rowe, Kayla Irizarry, Remy Rovelli, Dustin Perriguey, Yang Gao and Nicholas Hugo for assistance in the field and useful discussions. This project was supported by an NSF Biological Sciences REU awarded to Delaney Ryan at the AMNH, and a Lerner-Gray Postdoctoral Fellowship at the Richard Gilder Graduate School and AMNH awarded to James Witts. Additional support from the Newell Fund (AMNH) is gratefully acknowledged.
Publisher Copyright:
© 2020 Lethaia Foundation. Published by John Wiley & Sons Ltd