| Title: | Mid-Cretaceous igneous events and geological responses: The devil is in the details |
| Author: | Roger L. Larson |
| Date Submitted: | 04/30/2002 |
| Address: | Graduate School of Oceanography,
University of Rhode Island
Narragansett
RI
USA
02882
|
| Phone: | (401) 874-6165 |
| Email: | rlar@gsosun1.gso.uri.edu |
| Co-Authors: | Duncan, Robert A., Oregon State University, rduncan@coas.oregonstate.edu; Erba, Elisabetta, Universita di Milano, elisabetta.erba@unimi.it |
| Affiliation: | University of Rhode Island |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725552 |
| Keywords: | oceanic plateaus, radiometric dating, black shales, stable isotope ratios |
| Abstract: | Increased rates of volcanism associated with oceanic plateau formation and sea floor spreading in mid-Cretaceous time probably caused a variety of geological responses, including black shale episodes and changes in isotopic ratios of carbon, oxygen and strontium. However, small but possibly significant mismatches between radiometrically dated oceanic basalts and stratigraphically dated geological responses make it difficult to establish cause and effect relationships with certainty. In particular, radiometrically dated basalts from the Ontong Java and Manihiki Plateaus average 123 Ma, while the Selli black shale (OAE1a) and associated isotopic ratio anomalies occurred 2-3 m.y. later. Precise paleotemperature estimates are still needed for the Barremian-Early Albian interval to assess the potential effect of this huge volcanic episode on paleoclimate. Basalts from the Kerguelen Plateau suggest major volcanic activity, much of which was subaerial, beginning at about 118-119 Ma, which possibly continued at decreased rates and contributed to OAE1b and elevated paleotemperatures in the Albian. Basalts from the Caribbean Plateau, which were previously determined to have radiometric ages of about 88-91 Ma, have been redated and with one exception now have radiometric ages of about 92-95 Ma. Thus, the case is considerably stronger, but not yet certain, that Caribbean Plateau volcanism contributed to the Bonarelli black shale episode (OAE2) and associated isotopic anomalies near the Cenomanian/Turonian boundary at 93.5 Ma. These new age 40Ar-39Ar whole rock age determinations are based on incremental heating experiments that, with one exception, produce Ar-recoil patterns from which only total fusion ages can be extracted. These ages are more precise than previous estimates, but still produce the same estimated duration of volcanic activity of about 3 m.y. Additional improvements in radiometric dating will rely on finding samples from which feldspar can be separated. A substantial amount of mid-Cretaceous and older oceanic crust, especially in the Pacific Basin has been subducted in the past 120 m.y. This has erased much of the potential evidence for the increased igneous activity from the geological record. Thus, there will always be unverifiable assumptions in any hypotheses that hope to explain the causes and effects of mid-Cretaceous events. More samples from the existing ocean floor record, which can be dated with increasingly precise radiometric and stratigraphic techniques, are clearly necessary to assess in more detail these mid-Cretaceous relationships. A significant part of the time allotted for this talk will be devoted to a group discussion of future IODP drilling and potential drill sites to address these issues. |
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