Previous   |    Next

doi:10.2204/iodp.proc.320321.207.2013

Introduction

Sediments deposited in the equatorial Pacific Ocean store some of the best long-term records of Earth’s climate. These sediments record ocean-wide variations, as shown by the observation that in this region sediment physical properties correlate over large distances (Moore and Pälike, 2006). These lithostratigraphic correlations complement bio-, chemo-, and magnetostratigraphic correlations and assist in the construction of a common timescale. Integrated Ocean Drilling Program (IODP) Expeditions 320 and 321 (Pacific Equatorial Age Transect) sampled the sediment mound deposited near the paleoequator to validate and extend the astronomical calibration of the Cenozoic geologic timescale and to improve, date, and intercalibrate stratigraphic datums (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]).

This data report applies a Monte Carlo method to obtain a detailed lithostratigraphic correlation by matching two sediment records. The distinguishing feature of the method is that it does not produce a single optimal correlation, but rather a sample of possible correlations that result in a good match. The average of these samples gives the best correlation, and their variability measures the uncertainty that is inherent to the correlation, highlighting intervals where the match is relatively poor and the correlation less reliable.

The method is applied to high-resolution bulk density measurements from core samples and downhole logs from Expedition 321 Sites U1337 and U1338, which targeted the early Miocene to present time interval and were drilled 600 km apart (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). A combination of core and log measurements best constrains site-to-site correlations, but complications occur because these two data types are placed on different depth scales. The cumulative length of drill pipe is used for core depth, whereas the measured length of the wireline cable determines depth in downhole logs. Moreover, to obtain as complete a record as possible, a composite depth scale is constructed by splicing core sections taken from different holes at the same site. This process typically expands the actual thickness of the cored interval by 10%–15% (Lisiecki and Herbert, 2007; Westerhold et al., 2012). Reconciling these different depth scales requires a detailed correlation to precisely align the high-resolution core and log records at each site prior to site-to-site correlation. The Monte Carlo method is applied to first correlate core and downhole log data at each site, so that all data are placed on the same depth scale. Core and downhole log data are then used to obtain a reliable lithostratigraphic correlation between the two sites.

Top of page   |    Previous   |    Next