Site 1001 is located in 3271.0 m of water on the edge of the Southern Nicaraguan Rise, just north of the Hess Escarpment. ODP drilling at Site 1001 recovered a 522.8-m section composed of clay, ooze, chalk, limestone, chert, and basalt (Sigurdsson, Leckie, Acton, et al., 1997). The recovered sediment consists of a middle Miocene to Pleistocene section overlying a Cretaceous to upper Eocene section, separated by an unconformity. The section was logged from 83 to 470 mbsf. SCS reflection Line EW9417-10 crossed Site 1001 at Shotpoint 1500. Two major Caribbean seismic reflectors, A" and B", have been mapped throughout the Site 1001 site survey (Cunningham, 1998). These horizons were penetrated at Deep Sea Drilling Project Site 152, located roughly 40 km east-northeast of Site 1001 (Edgar, Saunders, et al., 1973). Although recovery at Site 152 was poor, the A" horizon was correlated with lower Eocene chalks with minor chert or silicified cherty Paleocene limestone. The B" horizon with associated with Upper Cretaceous basalt.
The synthetic seismogram at Site 1001 facilitates correlation of the core data with the SCS data. Reflectors within the SCS data that appear to correlate with the synthetic seismogram include those at 2350, 2410, 2440-2465, 2520, 2540, 2775, and 2790 ms. Figure 22 illustrates the correlation between the synthetic seismogram and core-derived depths and ages.
The primary objective of creating synthetic seismograms over Site 1001 was to correlate the Eocene-Miocene unconformity identified in the core with the A" reflector and the basaltic crust with the acoustic basement or B" reflector interpreted in the seismic data (Fig. 23). The A" reflector is clearly evident on both the seismic data from SCS Line EW947-10 at 2540 ms and the synthetic seismogram as the largest peak in the synthetic. The other significant correlation is the end of the synthetic seismogram at 2805 ms with basement recovered at 485 mbsf. The end of the synthetic seismogram correlates with the top of basalt recovered in the core, and the B" basement horizon from the SCS interpretation (Cunningham, 1998). One interesting note is the lack of any seismic response at the chalk to limestone transition (lithostratigraphic Units II to III) as was evident in the Site 1000 SCS data and synthetic seismogram. The absence of a major change in the impedance across this interface at Site 1001 results in the lack of a seismic response.
Chert layers were recovered in core from Site 1001 at ~166, 200, 240, 295, and 450-470 mbsf; however, none of these appears to have any impact on the velocity profile at this site (Fig. 13). The chert layers at ~166 mbsf are also associated with the lower Eocene to middle Miocene unconformity. Both the velocity and density measurements increase from the Miocene section into the Eocene. The changing physical properties of the Miocene and Eocene sediment rather than the presence of chert causes the impedance contrast created across this boundary. In most cases, the chert layers are so thin that they have minimal effect on the sonic and density logs across those intervals. Without a velocity or density change in the logs, there is no impedance contrast and the cherts have no effect on the synthetic seismogram.