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Detailed correlation and accurate depths are a cornerstone of many Integrated Ocean Drilling Program (IODP) expeditions, and Expedition 313 New Jersey Shallow Shelf was no exception. Siliciclastic sediments were drilled and recovered at three sites on the New Jersey shallow shelf, with overall expedition aims that included the evaluation of sequence stratigraphic facies models. Detailed correlation is important for sequence stratigraphic studies both at the scale of the margin (e.g., Mountain et al., 2010; Miller et al., 2013) but also for smaller scale studies, especially where key intervals fall within intervals of overlap (e.g., Nilsson et al., 2013; Inwood et al., submitted).

Overlaps in core depth can originate from different processes (Ruddiman et al., 1987), including gas expansion (e.g., Flood, Piper, Klaus, et al., 1995), sediment rebound (e.g., Moran, 1997), clay swelling (e.g., Mountain et al., 2010; Saffer et al., 2011; Daigle and Piña, 2016), or drilling disturbances such as a slipped core being recovered by a subsequent run or reaming (e.g., see the “Methods” chapter [Expedition 313 Scientists, 2010a]). During paleoceanographic expeditions, it is common to produce a composite depth splice from more than one borehole, the final result of which often indicates 10%–20% expansion (Hagelberg et al., 1992, 1995; Lisiecki and Herbert, 2007), thus indicating the importance of appropriately dealing with core expansion for each scientific study. The purpose of this data report is to produce an overview of the three Expedition 313 sites, identify all intervals of core overlap, and evaluate their origin, along with discussion of the optimum way to deal with depth in these intervals.

In intervals of clay lithology, it is common to recover more than 100% core, which results in an apparent overlap in depth and is generally due to the hydration of clay minerals causing expansion (Anderson et al., 2010). Swelling clay is often noted during drilling or borehole logging operations, including during Expedition 313 (see the “Methods” chapter [Expedition 313 Scientists, 2010a]). Clay can refer to the size of the particles in a rock or to a clay mineralogy. Clay minerals belong to the group of hydrous aluminosilicates and have maximum particle dimensions of <0.005 mm (e.g., Meunier, 2006); kaolinite, smectite, and illite are the three main clay groups (Grim, 1953). For Expedition 313, clay was defined by particle size, but most of the recovered New Jersey sediments contain some component of clay minerals, including kaolinite, micas, illite, mixed-layer clays, and smectite (Mountain et al., 2010). For example, clay minerals make up as much as 59.2% of the clay recovered from Hole M0027A, which was measured by X-ray diffraction (XRD) (Mountain et al., 2010). The degree of clay expansion (swelling clay) varies depending on the clay type. For example, some clay (e.g., bentonite) can swell to >20 times its original size, although the quantification of the degree of swelling based on clay composition is not straightforward (Anderson et al., 2010).

Overlaps in nonclay lithologies (e.g., in sandy intervals) can be due to drilling-related disturbances. Many of these disturbances are apparent from observation of the core. For example, typical features are described and illustrated for International Ocean Discovery Program Expedition 354 (France-Lanord, Spiess, Klaus, Schwenk, and the Expedition 354 Scientists, 2016), including fall-in (material falls into the borehole), core biscuits (broken-up pieces of core), flow-in (material from outside the borehole flows into the drilled sediments), voids, fractures, and soupy (liquid) cores. Such drilling disturbances in the core were described and recorded graphically on the visual core description (VCD) images by the Expedition 313 sedimentologists (see “Core descriptions”). However, visual observation of these features does not necessarily identify all disturbances, and reports of disturbance do not always correlate with the measured degree of core extension (Lisiecki and Hebert, 2007). Drilling data, including notes taken during drilling operations as well as semiquantitative drilling parameters (physical forces and pressures during drilling), can be essential in identification of drilling disturbance. However, these data are rarely incorporated into later scientific studies, even where incorporation could be valuable. This is likely due to a combination of a lack of availability of requisite data/information (e.g., drilling parameters) and to a lack of knowledge. Additionally, drilling parameters are collected in time rather than in depth, which adds complexity to the analysis and integration with the core and downhole logging data that are collected in depth. Past research has shown the value of incorporating drilling parameters for the interpretation of characteristics such as cavities or formation strength (e.g., Sugawara et al., 2003; Inwood et al., 2008; Kinoshita, Tobin, Moe, and the Expedition 314 Scientists, 2008), especially where technology exists to convert these measurements directly into depth by systems that record the time and depth of the drill string below the rig floor (e.g., Flemings, Behrmann, John, and the Expedition 308 Scientists, 2006; Inwood et al., 2008). During Expedition 313, drilling parameters were recorded in time, but they can still be of use. A number of interrelated measurements were taken, including number of revolutions of the drill bit per minute (head rpm), torque pressure, water pressure, and weight on bit (see the “Methods” chapter [Expedition 313 Scientists, 2010a]). Water pressure (pullback pressure or hydraulic pressure) is the inverse of bit weight and is necessary to prevent the drill bit from resting on the formation without any control. Torque relates to oil pressure on the drill motor and depends on the gear used. Higher head rpm can indicate a faster drilling rate. These parameters provide semiquantitative information in that, providing external parameters are constant, a change in an individual parameter indicates a change in formation properties (e.g., lithology).

Once overlaps and their origins have been identified, it is common to decide on a method to avoid the confusion of two data points at seemingly equivalent depths (refer to IODP Depth Scales Terminology at One simple method is to ignore the data from one of the cores within the overlap interval (either the upper or lower core). An alternative for numerical data is to average measured data in these intervals. However, neither of these scenarios are ideal for more-detailed studies or for intervals around key surfaces where a wrong assumption may wrongly correlate a misleading feature. Scaling in these intervals is one way of treating these intervals, for which there are several variations, such as scaling per core or by an overall factor. Downhole logging data can be valuable because they represent independent depth control, and where analogous measurements exist on both the recovered core and downhole logs, an accurate solution can be proposed with confidence (e.g., Gilbert and Burke, 2008; Inwood et al., 2008; Fontana et al., 2010; Clary et al., 2017). For Expedition 313, both natural gamma radiation (NGR) and magnetic susceptibility measurements could be used in this way to help interpret core overlaps, as both measurements were acquired in situ during downhole logging and on the recovered core material.

An overview (Fig. F1) of all three Expedition 313 sites indicates the location of and interpretations for core overlaps (provided in larger scale in in OVERLAP in “Supplementary material”). Examples of the usefulness of the drilling data are illustrated for two intervals in Hole M0027A (Figs. F2, F3). The latter part of this report focuses on two key intervals of clay expansion in Holes M0027A and M0028A (Figs. F4, F5) and illustrates the value of downhole magnetic susceptibility logging data to correct depths in overlaps in Hole M0027A (Fig. F6).