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Abstract: Early to Middle Miocene Carbonate Chemostratigraphy and Compositional Changes of Shelf-Derived Turbidites from ODP Leg 165, Site 1000, Northern Nicaraguan Rise and Site 999, Colmbian Basin

MUTTI, MARIA, University of Southern California, Los Angeles

One of the objectives of OPD Leg 165 was to reconstruct the Neogene evolution of the Nicaraguan Rise, recovering the top of a drowned platform (of inferred mid Miocene age) and a continuous peri-plaform section overlying the top of the drowned platform, to help establish a Caribbean paleoceanographic record. OPD Site 1000 recovered a 965-m-thick sedimentary sequence, ranging in age from early Miocene to recent. Site 1000 is located in Pedro Channel, one of a series of channels that dissects the carbonate shelves and isolated carbonate banks that define the east-northeast-trending Northern Nicaraguan Rise (NNR). Based upon the interpretation of high-resolution seismic grids and analyses of dredged shallow-water limestones cropping out on the seafloor, Pedro Channel is interpreted to have formed by the partial foundering during the middle Miocene of a large carbonate platform, that included the shelves of Honduras and Nicaragua, and the island of Jamaica. However, drilling results indicated that this platform is older and deeper than predicted.

The recovered succession at Site 1000 consists dominantly of peri-platform sediments and sedimentary rocks, interbedded with intervals of redeposited pelagic and neritic carbonate sediments from the slopes and tops of the adjacent shallow water banks. Site 999 is situated offshore from the Nicaraguan Rise, in the pelagic environment of the Colombia Basin, and the recovered succession there is 1066,4 m thick and consists primarily of pelagic rocks.

Bulk isotope data were measured on samples systematically collected at 0.5m resolution from both Site 1000 and Site 999. The investigated interval spans from the Oligo/Miocene boundary through the Middle/Upper Miocene boundary, and covers more than 700 m between the two sites. The data show marked and well constrained d13C excursions, than can be correlated between the two sites. A shift from d13C=1^pmil to d13C=2.2^pmil occurs in Site 1000 from depth 660 to 600 m, and is comprised within nannofossil zone CN 3 (between 18.2 and 15.6 Ma). This excursion lasts until 510 m, immediately below the datum for nannofossil zone CN 5a (13.5 Ma), when the data show a progressive a gradual shift towards lower d13C values. Shifts of similar amplitude and timing were also measured in samples from Site 999.

Much of the peri-platform sequence recovered at Site 1000 is turbidite free, but the total thickness and number of turbidite layers varies significantly downcore. In fact, shelf-derived turbidites occur below 600 m, then again above approximately 400 m depth but are absent in the interval in between. The point-counting of grains within turbidite beds has helped to reconstruct the evolution of the platform and to constrain the time of its drowning during nannofossil zone CN3.

This data can be linked with the d13C record and shows how the onset of the positive d13C excursion predates the platform drowning event by an estimated time of 1-2 m.y. Turbidites are absent during the time of the positive carbonate excursion, possibly suggesting that changes in the shallow water environments were coupled with the processes causing the d13C changes. These data suggest that chemostratigraphy used in combination with traditional sedimentological analyses can be a useful tool in predicting the timing of neritic facies changes.