Site 1000 is located in 927.2 m of water in Pedro Channel along the northern Nicaraguan Rise. The northern Nicaraguan Rise is bordered by the Cayman Trough to the north and the southern Nicaraguan Rise and Colombian Basin to the south. Pedro Channel is the widest and deepest channel along the east-northeast trending Nicaraguan Rise. Shallow-water (30 m) carbonate banks bound Pedro Channel on three sides. These banks include Pedro Bank to the east, Rosalind Bank to the west, and Serranilla Bank, Alice Shoal, and Banco Nuevo to the south. A largely complete 695.6-m section of Miocene to Holocene sediment was recovered at Site 1000. This sediment consists predominantly of periplatform ooze, chalk, and limestone. Site 1000 was logged from 277 to 675 mbsf. SCS reflection Line CH9204-30 crosses Site 1000 at Shotpoint 1495.
The synthetic seismogram shows very good agreement with the seismic data, and thus provides a link between the seismic sections and the core data. This link provides critical information about how the core lithologies relate to the seismic facies, as well as age control for the correlated seismic horizons (~0-700 milliseconds below seafloor [ms bsf]). The main seismic event used for correlation is a set of high-amplitude reflections starting at 320 ms bsf or a two-way traveltime (TWT) of 1570 ms. In addition to these events, reflectors at 1435, 1520, 1640, 1720, 1760-1780, and 1870-1900 ms correlate reasonably well between the synthetic trace and the SCS data (Fig. 20).
Figure 20 illustrates correlation between SCS Line CH9204-30 and the synthetic-generated traces. Based on this correlation, several characteristics of the ODP Site 1000 core can be correlated with the seismic data. Several lithologic units were defined for Site 1000 (Fig. 21; Sigurdsson, Leckie, Acton, et al., 1997). Subunit IA consists of nannofossil and micritic oozes with foraminifers and pteropods. Subunit IB contains micritic nannofossil ooze with foraminifers to foraminiferal micritic ooze with nannofossils, volcanic ash layers, and normally graded turbidites. Subunit IC varies from micritic nannofossil chalk with clay and foraminifers to clayey nannofossil chalk with micrite. Subunit ID consists of micritic nannofossil chalk with foraminifers. Subunit IIA contains calcareous limestone with foraminifers to nannofossil micritic limestone with clay and foraminifers, with interbedded minor volcanic ash layers. Subunit IIB has calcareous limestone with foraminifers and nannofossil micritic limestone with clay and foraminifers, interbedded volcanic ash layers, and normally graded sandy turbidites. Of these, two units appear to elicit a distinctive response in the seismic data. Lithostratigraphic Subunit IA from 0 to 51 mbsf correlates with a seismic interval that has high-continuity reflectors (Fig. 21). Subunit IA was identified as containing more clay minerals and larger magnitude variations in carbonate content than the underlying stratigraphic subunit. It is possible that these variations could result in the seismic reflectors present over that interval. Lithostratigraphic Subunit ID from 486 to 513 mbsf also correlates with a seismically distinctive interval containing high-amplitude, high-frequency, continuous reflectors (Fig. 21). This subunit is differentiated from subunits above and below by its massive bedding and higher carbonate content.
Another interesting correlation between the core and the seismic data is at ~513 mbsf. Lithologically, this depth marks the transition from chalk to limestone. It coincides with physical properties increases in velocity and density and decreases in water content and porosity (Sigurdsson, Leckie, Acton, et al., 1997). These variations are also evident in the sonic, density, and resistivity logs from downhole measurements (Sigurdsson, Leckie, Acton, et al., 1997). Seismically, this transition is marked by the disappearance of high-frequency reflections (Fig. 21). It is likely that high-frequency energy is unable to pass far beyond this distinct lithologic boundary because of attenuation.
Despite the fact that it comprises only a small portion of the recovered sediment, the presence of turbidites in the cores also seems to correlate with distinctive seismic facies. Four distinctive cored intervals contain turbidites: 60-80, 245-270, 320-340, and 591-696 mbsf. The intervals 60-80, 245-270, and 320-340 mbsf consist of planktonic foraminifer turbidites (Fig. 21). These thin- to medium-bedded turbidites (<30 cm) comprise a maximum of 3% of the core recovered in the core containing the highest number of turbidites. The planktonic foraminifer turbidite intervals correlate with a low-amplitude, lower continuity seismic facies. The interval from 591 to 696 contains coarse-grained turbidites with bank-derived foraminifers (Fig. 21). This interval correlates with a high-amplitude, locally continuous seismic facies.
The origin of the low-amplitude, lower discontinuity seismic facies that typifies the three planktonic foraminifer turbidite intervals may result from the localized disruption of the physical properties that would otherwise result in a moderate-amplitude, continuous seismic facies. The sonic and density logs across the three planktonic foraminifer turbidite intervals show minimal changes relative to the overlying and underlying turbidite-free sedimentary units. All three turbidite intervals contain pelagic material and current-winnowed, fine-grained carbonates, as do the overlying and underlying turbidite-free sedimentary units. In the turbidite-free sections, the impedance contrasts resulting in seismic reflections are often caused by variations in degree of cementation, porosity, or water content, all of which affect the velocity and density (Slowey et al., 1989). Over the turbidite intervals, however, these properties could be disrupted locally by deposition of the turbidite. This might result in the discontinuous, low-amplitude seismic response.
For intervals containing the coarse-grained, bank-derived turbidites, the case appears to be different. The turbidites tend to range in thickness from a few to several tens of centimeters, though none is >60 cm (Sigurdsson, Leckie, Acton, et al., 1997). Both the sonic and density logs show large-amplitude variations over this turbiditic interval. The seismic response is a result of the higher velocity and density values in the turbidites relative to intervals of sediment above and below without the coarse-grained turbidites. The high-amplitude seismic reflections over this interval result from the interference pattern of numerous impedance contrasts and do not represent distinct turbidite layers.
Core recovery of a significant paleoceanographic event, the "Carbonate Crash," is present between 380 and 460 mbsf (Sigurdsson, Leckie, Acton, et al., 1997). The carbonate crash is marked by a ~15% decrease in carbonate content in the sediment over this interval. The impedance log and reflection coefficient over this interval have lower amplitude variations compared with the overlying and underlying intervals (Fig. 16). This interval correlates to a zone of relatively low-amplitude reflectors on the synthetic seismogram (Fig. 20). This is evident in the CH9204 seismic data as a zone of low-amplitude to transparent seismic facies compared to the facies above and below. The high-amplitude reflector at ~440 ms TWT on Trace 1495 (Fig. 16) does not correlate well with the synthetic seismogram. The source of this reflector remains unidentified.
Several lithologic facies in the core do not appear to have any visible effect on either the synthetic seismogram or the seismic facies. Numerous ash layers were recovered from Site 1000, with a maximum thickness of 53 cm, but typically much thinner with a 5.1-cm mean thickness (Sigurdsson, Leckie, Acton, et al., 1997). Most notably, these ash layers show peak accumulation rates over intervals 280-300, 510-540, and 600-696 mbsf. None of the intervals containing ash layers have an effect on the velocity or density that would affect a seismic response.
There is a significant event in the seismic (1570-1600 ms TWT), synthetic (330-350 ms TWT), and log data (295-320 mbsf) that does not appear to correlate with any major lithologic properties. The logs over this interval show widely varying values for the sonic, density, photoelectric effect, and gamma-ray logs compared to units above or below this interval. The variation is also evident in the impedance log and reflection coefficient (Fig. 16). The seismic response is a series of high-amplitude, high-frequency, high-continuity, parallel reflectors (Fig. 21).