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Dolomite formation in pelagic limestone and diatomite, Romanche fracture zone, Equatorial Atlantic

Daniel Bernoulli, Luca Gasperini, Enrico Bonatti and Peter Stille
Dolomite formation in pelagic limestone and diatomite, Romanche fracture zone, Equatorial Atlantic
Journal of Sedimentary Research (November 2004) 74 (6): 924-932


Two dredge samples from the Romanche Fracture Zone in the equatorial Atlantic document dolomite formation in pelagic sediments in a deep-sea environment. The samples come from a highly deformed sedimentary succession, the Romanche Sedimentary Sequence, constituting the transverse ridge accompanying the transverse valley of the fracture zone to the north. These sediments were folded and uplifted to or near the seafloor prior to the growth of an early Miocene carbonate platform which unconformably overlies them. Dolomitized pelagic limestones of Early Cretaceous age preserve solution molds and unaltered tests of radiolaria (opal-A) and ghosts of calcareous nannoplankton in a dolomicritic groundmass. Carbon isotope data indicate a normal marine source of carbon, and oxygen isotope compositions are consistent with precipitation from cool, marine pore waters at or near the sediment-water interface. The (super 87) Sr/ (super 86) Sr ratio suggests dolomitization at around 25 Ma, presumably when the sediments were exhumed to or near to the seafloor. In contrast, a dolomite-cemented diatomite of presumably late Eocene age shows no relics of a carbonate precursor. Dolomite crystals grew freely in the originally highly porous rock, cementing it into a tight fabric. The absence of compaction suggests that cementation by dolomite took place soon after deposition and before significant burial of the sediment. Carbon and oxygen isotope compositions of this dolomite suggest that its formation also occurred from cool, marine waters. The (super 87) Sr/ (super 86) Sr ratio shows the value of seawater around 35 Ma. Under the assumption that dolomite cementation was by seawater, it occurred shortly after deposition at or near the seafloor.

ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 74
Serial Issue: 6
Title: Dolomite formation in pelagic limestone and diatomite, Romanche fracture zone, Equatorial Atlantic
Affiliation: Basel University, Geology Institute, Basel, Switzerland
Pages: 924-932
Published: 200411
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 56
Accession Number: 2005-004661
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
S01°00'00" - N01°00'00", W17°00'00" - W14°00'00"
Secondary Affiliation: Istituto di Geologia Marina, ITA, ItalyUniversite Louis Pasteur, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200502
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