Data report: interpretation of cored intervals with greater than one hundred percent core recovery: IODP Expedition 313 used as an example methodology

Abstract

Conventionally, depths of core recovered during a drilling and coring expedition are determined by placing the top of the recovered core at the top of the core barrel run that recovered it. During many ocean drilling expeditions, certain intervals of core recovered from a drill run are recorded as 100% or greater, resulting in an overlap in core depths between adjacent runs. There are various reasons for these overlaps; significantly, the presence of clay that expands on recovery, as well as other possibilities including gas expansion, slipped cores recovered by subsequent runs, and disturbances during drilling leading to retrieval of surplus loose material. These intervals of overlap require cautious treatment to avoid potential confusion in analysis if different cores are sampled at apparently equivalent depths. Scientists need to be precise in the explanation of their method of dealing with any such overlaps.During Integrated Ocean Drilling Program Expedition 313, a mixture of siliciclastic sediments were recovered from three sites on the New Jersey shallow shelf. This report identifies intervals of core overlap from the conventional depth determination and analyzes their origin by combining core lithologic observations with notes taken during drilling operations and recorded drilling parameters. Drilling parameters represent an underutilized resource in the interpretation of core recovered during drilling, especially for measurements in which assumptions are made on the quality of the core. Downhole logs provide independent depth control and, through analysis in tandem with equivalent core petrophysical measurements, an evaluation can be made of how to optimally deal with overlapping core intervals. The advantages of the methodology presented go beyond the Expedition 313 examples illustrated here and could be applied to other expeditions.