Abstract:
Drilling at Limalok (Site 871) and Wodejebato (Site 874) Guyots, Republic of the Marshall Islands, during Ocean Drilling Program (ODP) Leg 144, revealed that approximately 289 m and approximately 163 m of shallow-water limestones overlie volcanic basement at these sites, respectively. Site 871 was drilled in the central lagoon at Limalok at a water depth of 1255 m, whereas Site 874 was drilled on an inner perimeter ridge at Wodejebato at a water depth of 1374 m. Shipboard biostratigraphy indicates that neritic limestones from Site 871 are late Paleocene to middle Eocene, whereas limestones from Site 874 are Maastrichtian. We have combined petrography and isotope geochemistry (delta (super 18) O, delta (super 13) C, (super 87) Sr/ (super 86) Sr) to constrain the diagenetic histories of these two guyots. Carbonates from Sites 871 and 874 primarily consist of grainstones, packstones and wackestones which exhibit partial to complete moldic and vuggy porosity. Site 871 samples are moderately well-cemented with equant, microspar calcite. Cements in Site 874 carbonates are generally coarser and more abundant, primarily having prismatic and radiaxial equant to bladed forms. The vast majority of Site 871 samples are non-cathodoluminescent, whereas Site 874 samples from 116 to 163 m display dull to bright cathodoluminescence. Oxygen and carbon isotope values of matrix and cement samples from Site 871 range from -5.28 per mil to 0.50 per mil and -0.68 per mil to 2.33 per mil, respectively. At Site 874, most samples have delta (super 18) O values which cluster between -1.97 per mil to -0.20 per mil; however, six bivalve samples have lower delta (super 18) O values ranging from -4.34 per mil to -3.07 per mil. Ridge carbonate delta (super 13) C values range from 0.92 per mil to 3.13 per mil. Depleted oxygen isotope values (<-3 per mil) are consistent with either diagenetic alteration with an isotopically depleted fluid (i.e., meteoric water) or alteration of an isotopically depleted marine carbonate at low water/rock ratio. Sr isotope ratios from Site 871 samples are consistent with an early Paleogene age; however, the temporal resolution of seawater Sr isotopes is minimal during this time. For Site 874, strontium isotopic analyses of 36 samples indicate that the (super 87) Sr/ (super 86) Sr ratio varies from 0.707869+ or -0.000009 (Maastrichtian, approximately 67 Ma) near the top of the sequence to 0.707730+ or -0.000012 (latest Campanian, approximately 72 Ma) near the base. The Campanian/Maastrichtian boundary is most likely between 136.66 and 143.66 mbsf, based upon Sr isotopes.