Abstract

Trajectories of successive clinoform rollovers are widely applied to predict patterns of spatio-temporal sand distribution. However, the detailed internal architecture of individual clinothems is rarely documented. Understanding the textural complexities of complete topset-foreset-bottomset clinothem sequences is a key factor in understanding how and when sediment is transferred basinward. This study used high-resolution, core-based analyses of 267 samples from three research boreholes from quasi-coeval topset, foreset, and bottomset deposits of a single Miocene intrashelf clinothem recovered during Integrated Ocean Drilling Program (IODP) Expedition 313, offshore New Jersey, USA. Topset deposits were subdivided into three sedimentary packages based on grain character and facies analysis, consisting of upper and lower river-dominated topset process-regime packages separated by a middle wave- and storm-dominated process-regime package. Temporal variability in topset process regime exerts a quantifiable effect on grain character across the complete depositional profile, which was used here to correlate topset deposits with time-equivalent sedimentary packages in foreset and bottomset positions. River-dominated sedimentary packages have higher sand-to-mud ratios; however, the grain character of river-dominated sedimentary packages is texturally less mature than that of wave- and storm-dominated deposits. Differences in grain character between packages dominated by different process regimes increase basinward. The novel use of quantitative grain-character data allows intraclinothem time lines to be established at a higher resolution than is possible using chronostratigraphic techniques. Additionally, stratigraphic changes in grain character were used to refine the placement of the basal sequence boundary. These results challenge the idea that clinoform trajectories and stacking patterns are sufficient to describe spatio-temporal sand-body evolution across successive clinothems.

INTRODUCTION

Clinothems form the principal architectural building blocks of many shelf-to-basin successions (e.g., Gilbert, 1885; Rich, 1951; Bates, 1953; Asquith, 1970; Mitchum et al., 1977; Pirmez et al., 1998; Adams and Schlager, 2000; Bhattacharya, 2006; Patruno et al., 2015), and they are routinely subdivided geometrically into topset, foreset, and bottomset deposits. Clinothems form valuable archives of basin-margin evolution: The trajectories and geometries of consecutive clinoform rollovers and their resultant stacking patterns are widely applied to predict spatio-temporal sand distribution, in both the subsurface and in outcrop (e.g., Steel and Olsen, 2002; Johannessen and Steel, 2005; Helland-Hansen and Hampson, 2009; Jones et al., 2015; Koo et al., 2016; Chen et al., 2017; Pellegrini et al., 2017). Clinoform trajectory models have been developed to account for observed form in terms of the balance between the rates of sediment supply and the generation of accommodation space (e.g., Burgess and Hovius, 1998; Mellere et al., 2002; Steel and Olsen, 2002; Bullimore et al., 2005; Carvajal and Steel, 2006; Uroza and Steel, 2008; Helland-Hansen and Hampson, 2009; Ryan et al., 2009). The role of topset and shelf process regime in determining clinoform architecture and timing of sediment transfer has recently been emphasized as an important parameter to consider (e.g., Dixon et al., 2012a; Jones et al., 2015; Hodgson et al., 2018; Cosgrove et al., 2018).

Previous investigations of clinothem sequences have focused on understanding basin-scale relationships using multiple successive clinothems (e.g., Steel and Olsen, 2002; Johannessen and Steel, 2005; Helland-Hansen and Hampson, 2009; Dixon et al., 2012b; Koo et al., 2016; Chen et al., 2017; Pellegrini et al., 2017; Cosgrove et al., 2018). Understanding the internal architectural complexity of complete topset-foreset-bottomset clinothem sequences (including grain-size, grain shape and sand and mud content) is a key factor in understanding how and when sediment is transferred basinward, and in providing better constraint on the spatio-temporal sedimentary correlations of stratal units and their bounding surfaces. However, developing high-resolution intrasequence chronostratigraphic correlations is problematic, particularly in sand-rich successions. Stratigraphic changes in sedimentary facies can provide a means by which to correlate strata between wells, but this is fraught with uncertainty because of the transitional nature of facies change and the possibility for sediment bypass and nondeposition in one part of a clinothem that is time equivalent to deposits in other parts (Fig. 1). Biostratigraphic or chronostratigraphic constraints typically lack the necessary resolution to permit correlations of intrasequence surfaces. The limited understanding of intrasequence architecture is exacerbated by a paucity of sedimentological and stratigraphic documentation of individual clinothems with preserved coeval topset, foreset, and bottomset deposits (e.g., Carvajal and Steel, 2009; Carvajal et al., 2009; Wild et al., 2009; Grundvåg et al., 2014; Prélat et al., 2015; Koo et al., 2016).

To understand intraclinothem architecture at high resolution, both stratigraphically (up core) and longitudinally (dip parallel), and to determine linkages to topset process regime, this study utilized samples from three research boreholes recovered during Integrated Ocean Drilling Program (IODP) Expedition 313, offshore New Jersey, USA (Fig. 2). The cored intervals targeted topset, foreset, and bottomset deposits of a single Miocene clinothem sequence (m5.4; Fig. 3) using integrated analysis of grain character (size and shape; cf. Fildani et al., 2018) and core-based interpretation of sedimentary textures and structures. The aim of this study was to highlight how quantitative grain-character data can be used to better understand the cause(s) of intrasequence textural complexities. Specific research objectives were as follows: (1) to understand how topset process-regime signals (including depositional architecture and grain character) are propagated downdip into foreset and bottomset deposits; (2) to illustrate how topset process-regime variability impacts sediment texture down the complete two-dimensional (2-D), dip-parallel depositional profile; (3) to demonstrate the use of grain character to correlate intraclinothem, time-equivalent surfaces; and (4) to discuss how high-resolution grain-character data can be used as an additional tool to refine the placement of sequence boundaries.

Geological Setting

The Miocene United States (U.S.) middle Atlantic margin, spanning the shelf region offshore New Jersey, Delaware, and Maryland, is a siliciclastic-dominated prograding passive margin. This region has been tectonically quiescent since the opening of the Atlantic Ocean in the mid-Jurassic (Watts and Steckler, 1979). Therefore, the Mid-Atlantic margin offers a valuable natural laboratory in which to study mixed-energy coastal system successions in a tectonically stable setting (Katz et al., 2013). Furthermore, the succession preserves detailed microfossil and strontium isotope records, which provide good chronostratigraphic age control (Browning et al., 2013).

Rifting commenced during the Late Triassic (ca. 230 Ma; Sheridan and Grow, 1988; Withjack et al., 1998), with seafloor spreading active from the Middle Jurassic (ca. 165 Ma). The Jurassic section, in the region of the Baltimore Canyon Trough (Fig. 2), is mainly composed of limestones of shallow-water origin (8–12 km thick). The margin was fringed by a barrier reef complex until the mid-Cretaceous (Poag, 1985). During the Cenozoic, the tectonic history was dominated by simple thermal subsidence, sediment loading, and crustal flexure (Watts and Steckler, 1979; Reynolds et al., 1991). The Late Cretaceous to Paleogene interval was marked by generally low rates (∼5000 km3 m.y.–1; shelf width >150 km) of siliciclastic and carbonate accumulation (Poag, 1985). Global and regional cooling resulted in a significant switch from carbonate ramp deposition to starved siliciclastic deposition during the late middle Eocene in onshore regions to earliest Oligocene further offshore on the slope (Miller and Snyder, 1997). The late Oligocene to Miocene interval was characterized by a dramatic increase in sedimentation rates (Poag, 1985; Miller and Snyder, 1997), the causes of which are poorly constrained, although some authors have suggested it was the result of tectonic activity in the hinterland (Poag and Sevon, 1989; Sugarman et al., 1993). The late Oligocene to Miocene increase in sedimentation rates resulted in the growth of a siliciclastic sedimentary prism, consisting of multiple clinothem sequences, which prograded over the low-gradient shelf. The clinothems accumulated in an intrashelf setting, forming a seaward-thickening shelf prism (Hodgson et al., 2018). Intrashelf clinothems, or subaqueous deltas, are situated seaward of the shoreline break and landward of the continental break and typically have reliefs of tens of meters (Helland-Hansen and Hampson, 2009; Henriksen et al., 2009; Patruno et al., 2015; Hodgson et al., 2018).

IODP Expedition 313 drilled three research boreholes (Sites M27, M28, and M29), positioned to target the topset, foreset, and bottomset deposits of the Miocene intrashelf clinothems. The clinothems are well imaged on a grid of seismic profiles (Monteverde et al., 2008), which display the distinct sigmoidal geometries of the clinothem sequences (Fig. 3). Core sites M27, M28, and M29 were drilled in a transect along the trace of seismic line Oc270 529 (Fig. 2). Expedition 313 mapped 25 regional seismic surfaces of Oligocene to Miocene age, which correspond with changes in sedimentary facies in the associated core holes (Mountain et al., 2010). Integrated Sr-isotope stratigraphy and biostratigraphy (see Browning et al., 2013) was used to date sequences with a resolution of ±0.25–0.6 m.y. This study focused on sequence m5.4, which is of Miocene age (Mountain et al., 2010; Browning et al., 2013), and it is discussed in detail below.

Sequence m5.4

Sequence m5.4 was deposited over ∼1.1 m.y. (17.7–16.6 Ma), with brief depositional hiatuses at its base and top (Browning et al., 2013). Integrated seismic data and stratigraphy suggest m5.4 is a composite sequence, composed of three higher-order depositional sequences (m5.4–1, m5.34, and m5.33; Miller et al., 2013a) of ∼100 k.y. duration; the higher-order sequences have been dated by regression of Sr-isotope data. Interpretations from previous studies of the stratigraphic depths of the composite sequence boundaries (m5.4 and m5.3) are illustrated in Figure 4, alongside interpretations of the higher-order sequence boundaries. For the purposes of this investigation, the placement of the m5.4 and m5.3 sequence boundaries will follow those presented in Miller et al. (2013a), who recognized sequence boundaries based on integrated core, seismic, and log data. The alternative published placements are described below.

Site M27

At site M27, the basal sequence boundary of m5.4 is placed at 295.01 meters composite depth (mcd) at an erosional surface (Miller et al., 2013a), which has been tied to synthetic seismogram data (Miller et al., 2013b). Originally, the m5.4 sequence boundary was placed at 271.23 mcd by Mountain et al. (2010). This surface was subsequently suggested by Miller et al. (2013a) to define the base of a higher-order sequence (m5.33). Sequence m5.4-1 is interpreted to have been cut out at Site M27 (Miller et al., 2013a); as such, m5.4 at Site M27 is a composite sequence consisting of the m5.34 (23.88 m thick; 295.01–271.13 mcd) and m5.33 (15.04 m thick; 271.23–256.19 mcd) sequences. Sr-isotope age estimates are 17.0–16.9 Ma and 16.6–16.5 Ma for sequences m5.34 and m5.33, respectively (Browning et al., 2013). The placement of the overlying m5.3 sequence boundary is equivocal and has been placed at 236.15 mcd (Mountain et al., 2010), 249.76 mcd (Miller et al., 2013b), and 256.19 mcd (Miller et al., 2013a). The 256.19 mcd sequence boundary placement was favored by Miller et al. (2013a) due to core expression, where a strongly bioturbated contact separates silt from an overlying coarse glauconite sand; this placement was also followed by Hodgson et al. (2018) and Proust et al. (2018).

Site M28

At Site M28, sequence m5.4 (151.30 m thick; 512.30–361.00 mcd) is bounded by two high-amplitude reflectors (m5.4 and m5.3) and has been recognized in previous studies (Monteverde et al., 2008; Mountain et al., 2010; Miller et al., 2013a, 2013b) based on termination styles of seismic reflectors at its base and top. Two alternative bases of sequence m5.4 were proposed by Mountain et al. (2010) at 495.20 mcd, where a thin sand bed overlies a clayey silt, and by Hodgson et al. (2018) at 519.70 mcd, where a sharp-based sand forms a fining-upward package of stratified sands with a deeply burrowed basal contact.

M5.4-1 is differentiated from the m5.4 sequence boundary but shares the same basal reflector. Sequence m5.4-1 (512.30–479.00 mcd) is ca. 17.7–17.6 Ma in age (Browning et al., 2013). M5.34 (479.00–405.00 mcd; 17.6–17.4 Ma; Browning et al., 2013) is interpreted to be a sequence boundary, as determined from seismic reflector termination patterns, including onlap, downlap, and the erosional truncation of the m5.4 sequence boundary (Miller et al., 2013b; Miller et al., 2018). M5.33 (405.00–361.00 mcd) is 16.7–16.6 Ma in age and is associated with a basal unconformity representing an ∼0.7 m.y. hiatus (Browning et al., 2013). Terminations onto adjacent seismic profiles (onlap and erosional truncation) are associated with the M5.33 sequence boundary, as illustrated by a strike line taken at Site M28 (Miller et al., 2018).

Site M29

At Site M29 (19.18 m thick; 662.37–643.19 mcd; Miller et al., 2013b), Sr-isotope dating suggests sequence m5.4 has an age of 17.7–17.6 Ma; this age range corresponds to composite sequence m5.4-1 at Site M28, although this is poorly constrained (Browning et al., 2013). The basal m5.4 sequence boundary is placed at 662.37 mcd (Miller et al., 2013a), where a silty glauconite sand is overlain by a silt; the 662.37 mcd basal boundary is also supported by synthetic seismogram data (Miller et al., 2013b). Sequence m5.4 was originally interpreted by Miller et al. (2013a) to pinch out after Site M28 and reappear at Site M29, as per the preceding description. However, an alternative interpretation was provided by Hodgson et al. (2018), in which sequence m5.4 is not present at Site M29; that interval in the core (spanning at least 662.37–649.16 mcd, with a coring gap from 649.16 to 644.28 mcd) is interpreted to represent the upper part of underlying sequence m5.45. At Site M29, the upper sequence boundary (m5.3) is placed at 643.19 mcd, where a sharp-based glauconite sand is deeply burrowed into an underlying silt; this sequence boundary is also associated with a large impedance contrast (Miller et al., 2013b). However, synthetic seismograms place the m5.3 seismic sequence boundary in a coring gap at 648.00 mcd (Miller et al., 2013b).

METHODS

This investigation employed two principal methodological approaches: (1) quantitative grain-character analysis and (2) paleoenvironmental interpretations of lithofacies, based on the visual core descriptions by the Expedition 313 sedimentologists and original core observations of lithology and sedimentary structures. According to the Miller et al. (2013a) scheme, the seismic sequence targeted in this investigation, sequence m5.4, spans the depths 295.00–256.19 mcd (38.81 m thick), 512.33–363.00 mcd (149.33 m thick), and 662.37–643.19 mcd (19.18 m thick) in cores M27, M28, and M29, respectively. An additional ∼5 m of stratigraphy was also described from below the basal m5.4 sequence boundary (300.00–295.00 mcd and 667.00–662.37 mcd in core M27 and core M29, respectively). In core M28, an additional ∼12 m of stratigraphy has been described (525.00–512.33 mcd), in order to include the alternative m5.4 sequence boundary proposed by Hodgson et al. (2018) at 519.70 mcd. Similarly, above the overlying sequence boundary for m5.3, an additional ∼5 m of stratigraphy is also described (256.19–251.00 mcd, 361.00–356.00 mcd, and 643.19–638.00 mcd in cores M27, M28, and M29, respectively).

Facies Associations and Depositional Environments

Here, we present interpretations of lithofacies and depositional environments based on assemblages of sedimentary structures, sedimentary texture and composition, fossil content, and ichnofabric. These lithofacies show variability up core within sequence m5.4. Paleoenvironmental interpretations were based on the following: (1) a classic wave-dominated shoreline model (e.g., Reineck and Singh, 1972; McCubbin, 1982; Browning et al., 2006), which recognizes upper shoreface (0–5 m), lower shoreface (5–10 m), offshore transition (10–30 m), and offshore environments (>30 m); and (2) mixed river/wave delta facies models (e.g., Galloway, 1975; Bhattacharya and Walker, 1992). These have been summarized in Mountain et al. (2010) and Proust et al. (2018).

Grain-Character Analysis

The semilithified samples were subjected to a mechanical and chemical disaggregation process to remove organic matter and prepare them for grain-character analysis (see Cosgrove et al., 2018). Grain-character analysis was undertaken using a CamsizerXT (Retsch Technology), which is an optically based dynamic image analysis instrument capable of measuring grain sizes from 0.001 to 8 mm with an accuracy of ±1% (Moore et al., 2011). The grain-character analysis of the CamsizerXT yielded: (1) a grain-size distribution for each individual sample, with 105 logarithmically divided grain size classes spanning 0.001–8 mm, and (2) a fully quantified grain-shape value (sphericity and roundness) for each grain-size class within that grain-size distribution. The raw output data of the CamsizerXT were subsequently analyzed using GRADISTAT computer software (Blott and Pye, 2001). GRADISTAT allows rapid analysis of grain-size statistics from multiple sediment samples and provides values of the mean, mode, and sorting of the grain population, in addition to a grain-size cumulative frequency distribution for each sample. Grain-shape values were analyzed with Microsoft Excel software.

Within sequence m5.4, 63, 219, and 49 sediment samples were recovered from cores M27, M28, and M29, respectively. Due to the downdip change in clinothem thickness, the number of recovered samples varied between cores M27 and M29. Each core was subdivided into three sedimentary packages; this subdivision was based on the average grain-size distribution and corresponds to changes in sedimentary facies. The number of samples from each sedimentary package is displayed on the accompanying figures. Additionally, the exact core depth of each sample used in this investigation is provided in the Supplemental Information1.

RESULTS

Core facies observations and descriptions are presented for sequence m5.4 at Sites M27, M28, and M29 in Tables 13, respectively. The tabulated lithofacies descriptions were supplemented by the sedimentary logs, which are presented in Figures 5A, 5B, and 5C for Sites M27, M28, and M29, respectively, and representative core photos (Fig. 6).

Topset Deposits (Core M27): Description

The core observations and descriptions are presented in Table 1.

Topset Deposits (Core M27): Interpretation

The topset deposits of sequence m5.4 form two broad facies associations: a coarser-grained facies (295.00–∼294 mcd and 273.00–256.19 mcd; Figs. 6A and 6C) and an intervening finer-grained facies (∼294–273.01 mcd; Fig. 6B). Within the coarse facies, the cross-lamination separated by undulating surfaces is interpreted as asymmetrical ripples formed by a unidirectional flow of fluvial origin. A fluvial source for the coarse facies is also supported by the presence of significant quantities of terrestrial material, including wood chunks and plant debris, concentrated within these stratigraphic intervals (e.g., Plink-Björklund and Steel, 2004; Rossi and Steel, 2016). The sand-rich nature of the coarse facies and the presence of gravel-sized detrital mineral grains (quartz and glauconite; Figs. 6A and 6C) suggest periods when river-flood events dominated, during which coarse sediment was rapidly deposited in a shoreface setting (e.g., Cosgrove et al., 2018).

Within the fine facies (Fig. 6B), the sand and silt interbeds are interpreted to be storm beds in a lower shoreface setting; convex-up laminations are interpreted to be hummocky cross-stratification. The presence of storm-beds, hummocky cross-stratification, and frequent shell-debris supports a wave- and storm-dominated process regime (e.g., Dott and Bourgeois, 1982; Harms et al., 1982).

Foreset Deposits (Core M28): Description

The core observations and descriptions are presented in Table 2.

Foreset Deposits (Core M28): Interpretation

The deposits of core M28 present either a coarse- or a fine-grained facies association with transitional changes observed between the facies. The foreset deposits of core M28 display a coarse-grained, glauconite-bearing facies (512.33–∼495 mcd and ∼420.8–361.00; Figs. 6D and 6F) and an intervening fine-grained facies (∼495–∼420.8 mcd; Fig. 6E).

The coarse-grained facies is predominantly expressed as medium-grained muddy sand that contains gravel-sized quartz and glauconite grains (Fig. 6D), and it represents deposition by mixed sediment gravity flows. The poorly sorted and unstratified nature of the coarse-grained facies suggests deposition by debris flows (Mulder and Alexander, 2001; Fig. 6F). Additionally, the presence of mud-chips and large volumes of detrital mineral grains (quartz and glauconite) suggests updip erosion and entrainment (Hodgson et al., 2018). Rare coarse-grained beds that display normal grading, and cross- and parallel-laminations, are interpreted to be the result of high-concentration turbidity currents (Mulder and Alexander, 2001). The abundant terrestrial debris and amount of mica suggest that fluvial processes at the shelf edge were responsible for the deposition of the coarse-grained facies. The dominant fluvial processes responsible for the deposition of the coarse-grained facies are suggested to be river-flood events that induced remobilization of shelf-edge deposits (cf. Normark and Piper, 1991; Zavala et al., 2006).

Within the finer-grained facies (Fig. 6E), the presence of both low-angle cross-laminations and convex-up laminations (hummocky cross-stratification) indicates wave and storm reworking (e.g., Dott and Bourgeois, 1982; Harms et al., 1982). Additionally, the discrete sharp-based, normally graded sand beds interbedded with coarse-grained silt indicate episodic sediment flux associated with storm events (Reineck and Singh, 1972). The finer-grained facies are interpreted to represent deposition on a wave- and storm-dominated shelf. This sedimentary package can be tentatively associated with the finer-grained package found in the topset deposits of core M27.

Bottomset Deposits (Core M29): Description

The core observations and descriptions are presented in Table 3.

Bottomset Deposits (Core M29): Interpretation

The deposits of core M29 present either coarse- or fine-grained facies associations; however, interpretations of the exact stratigraphic segregation of these facies are somewhat subjective because no abrupt facies changes are present. The bottomset deposits of core M29 display a coarse-grained, glauconite-bearing facies (662.37–∼658.50 mcd and ∼651.6–643.19 mcd; Figs. 6G and 6I) and an intervening fine-grained facies (∼658.5–∼651.6 mcd; Fig. 6H). The coarse-grained facies is typified by structureless glauconite-bearing sand interbedded with planar-laminated glauconite sand. The coarse-grained intervals are interpreted to represent rapid deposition of glauconitic sands from high-density turbidity currents and debris flows (Hodgson et al., 2018). The fine-grained facies is dominated by a structureless silt, predominantly representing deposition from suspension fallout, either from surface plumes or low-density turbidity currents.

Process Summary

Across the depositional profile, the fine-grained facies show a predominant wave and storm influence, recognized by: (1) abundant hummocky cross-stratification in the deposits of cores M27 and M28; (2) sandy-silt interbeds, representing episodic sediment flux associated with storm events; and (3) silt-dominated foreset and bottomset deposits, indicating an absence of direct fluvial sediment delivery. In contrast, the coarse-grained facies shows a predominant river influence, recognized by: (1) the strong terrestrial influence displayed by this facies (abundant plant and wood debris); (2) the grain-size variation (granule- and pebble-sized quartz and glauconite grains); (3) unidirectional current indicators (asymmetrical ripples); and (4) debritic and turbiditic deposits in foreset and bottomset deposits, interpreted to result from river-flooding events and hyperpycnal flows, respectively.

Grain Character

Grain-character data are presented for the topset, foreset, and bottomset deposits of sequence m5.4. The observed facies changes correspond with changes in grain-size distribution. The changes in grain-size distribution noted below were used to subdivide sequence m5.4 deposits into three subunits (a, b, and c) at each site.

Topset Deposits (M27)

The grain-size distribution of samples from 295.00–294.26 mcd displays two principal peaks at 0.057 mm (very coarse-grained silt) and 0.35 mm (medium-grained sand; Fig. 7A). In contrast, from 294.25 to 273.00 mcd, the grain-size distribution displays one broad peak spanning 0.098–0.21 mm (very fine- to medium-grained sand; Fig. 7B). From 272.99 to 256.19 mcd, the grain-size distribution displays two principal peaks at 0.063 mm (very fine-grained sand) and 0.27 mm (medium grained-sand; Fig. 7C). These changes in grain-size distribution define three sedimentary packages in the topset deposits (core M27) of sequence m5.4, informally referred to as M27a (295.00–294.26 mcd), M27b (294.25–273.00 mcd), and M27c (272.99.00–256.19 mcd; Fig. 8). Additional differences in grain character in sedimentary package M27b relative to sedimentary packages M27a and M27c include a finer mean grain size (Fig. 9A), a lower sand-to-mud ratio (M27b = 70:30 [Fig. 10B], M27a = 71:29 [Fig. 10A], M27c = 74:26 [Fig. 10C]), and a higher mean sphericity (Fig. 9A; Table 4).

Foreset Deposits (M28)

The grain-size distribution of samples from 512.33–495.00 mcd displays two principal peaks at 0.0625 mm (very fine sand) and 0.25 mm (medium sand; Fig. 7D). In contrast, from 494.99 to 415.00 mcd, the grain-size distribution of the samples displays one broad peak spanning 0.0682–0.193 mm (very fine- to fine-grained sand; Fig. 7E). From 414.99 to 63.00 mcd, the grain-size distribution comprises two principal peaks at 0.0625 mm (very fine-grained sand) and 0.297 mm (medium-grained sand; Fig. 7F). These changes in grain-size distribution were used to define three sedimentary packages within the foreset deposits (core M28) of sequence m5.4, informally referred to as M28a (512.33–495.00 mcd), M28b (494.99–415.00 mcd), and M28c (414.99–363.00 mcd; Fig. 8). Additional differences in grain character of M28b compared to M28a and M28c include a finer mean grain size (Fig. 11A), a lower sand-to-mud ratio (M28b = 73:27 [Fig. 10E], M28a = 88:12 [Fig. 10D], M28c = 84:16 [Fig. 10F]), better sorting (Fig. 11B), and more spherical (Fig. 11C) and well-rounded (Fig. 11D) grains (Table 5).

Bottomset Deposits (M29)

The grain-size distribution of samples from 662.37–658.50 mcd displays two principal peaks at 0.0625 mm (very fine sand) and 0.273 mm (medium sand; Fig. 7G). In contrast, from 658.49 to 651.64 mcd, the grain-size distribution displays one sharp, asymmetric peak at 0.0682 mm (very fine sand; Fig. 7H). A bimodal distribution returns from 651.63 to 643.19 mcd, where the grain-size distribution displays two principal peaks at 0.0682 mm (very fine sand) and 0.297 mm (medium grained-sand; Fig. 7I). The changes in grain-size distributions were used to define three sedimentary packages in the bottomset deposits of core M29, informally referred to as M29a (662.37–658.50 mcd), M29b (658.49–651.64 mcd), and M29c (651.63–643.19 mcd; Fig. 8). Additional differences in grain character of sedimentary package M29b, relative to sedimentary packages M29a and M29c, include a finer mean grain size (Fig. 12A), a lower sand-to-mud ratio (M29b = 37:63 [Fig. 10H], M29a = 84:16 [Fig. 10G], M29c = 81:19 [Fig. 10I]) and more spherical and rounded grains (Figs. 12C and 12D; Table 6).

Correlations across Topset-Foreset-Bottomset Profiles

In lieu of higher-resolution biostratigraphic and chronostratigraphic age control, quantitative grain-character data were used here as a lithostratigraphic tool to objectively subdivide the stratigraphy, and to correlate genetically related sedimentary packages, from topset through foreset to bottomset deposits of seismic sequence m5.4 (Fig. 8). The sedimentary packages are separated by two intraclinothem surfaces, based on abrupt changes in grain-size distribution, informally referred to in this study as surfaces 2 and 3; an additional surface (surface 1) corresponds to the basal m5.4 composite sequence boundary. Surface 2 separates sedimentary packages M27–M29a and M27–M29b and occurs at 294.26, 495.00, and 658.50 mcd in cores M27, M28, and M29, respectively. Surface 3 separates sedimentary packages M27–M29b and M27–M29c and occurs at 272.99, 415.00, and 651.64 mcd in cores M27, M28, and M29, respectively (Fig. 8). The correlated sedimentary packages between the surfaces are M27–M29a and M27–M29c, which correspond to the coarse-grained facies and topset deposits that have bimodal grain-size distributions, and which are coincident with river-dominated facies, and sedimentary package M27–M29b, which has a unimodal grain-size distribution and corresponds to the fine-grained package in the topset deposits that are wave-dominated.

Sedimentary Package M27–M29a (River-Dominated Topset Deposits; Coarse-Grained Facies)

A downdip transect through sedimentary package M27–M29a, which is bounded by surface 1 (m5.4 sequence boundary of Miller et al., 2013a) and surface 2 (Fig. 8), reveals the following: (1) an increase in mean grain size from topset (0.14 mm) to foreset (0.16 mm) to bottomset (0.16 mm) deposits (Fig. 13A); (2) an increase in sorting from foreset (2.4σ) to bottomset (2.0σ) deposits (Fig. 13B); (3) the lowest sphericity and most angular grains retained in foreset deposits (Figs. 13C and 13D); (4) consistently bimodal grain-size distribution throughout the depositional profile that varies minimally downdip (Fig. 7J); and (5) an increasing sand-to-mud ratio from topset (71:29; Fig. 10A) through to foreset (88:12; Fig. 10D) and bottomset (84:16; Fig. 10G) deposits.

Sedimentary Package M27–M29b (Mixed Wave- and Storm-Dominated Topset Deposits; Fine-Grained Facies)

The downdip profile of sedimentary package M27–M29b, which is bounded by surfaces 2 and 3 (Fig. 8), reveals the following: (1) a decrease in mean grain size from topset (0.11 mm) and foreset (0.11 mm) to bottomset deposits (0.061 mm; Fig. 14A); (2) an increase in sorting from topset to foreset and bottomset deposits (Fig. 14B); (3) an increase in sphericity and roundness downdip (Figs. 14C and 14D); (4) a grain-size distribution that is consistently unimodal and narrows and fines downdip (Fig. 7K); and (5) a variable sand-to-mud ratio from topset (30:70; Fig. 10B) through foreset (27:73; Fig. 10E) to bottomset deposits (37:63; Fig. 10H).

Sedimentary Package M27–M29c (River-Dominated Topset Deposits; Coarse-Grained Facies)

The downdip profile of sedimentary package M27–M29c, which is bounded by surface 3 and the overlying m5.3 sequence boundary (Miller et al., 2013a; see also Fig. 8 herein), reveals the following: (1) The coarsest grain sizes (0.24 mm) are found within foreset deposits relative to topset (0.13 mm) and bottomset (0.21 mm) deposits (Fig. 15A); (2) the most poorly sorted deposits are retained in the foreset deposits (Fig. 15B); (3) the least spherical and most angular grains are found within bottomset deposits (Fig. 15C and 15D); (4) the average grain-size distribution is consistently bimodal and varies minimally downdip (Fig. 7L); and (5) the sand-to-mud ratio is lowest in topset deposits (26:74; Fig. 10C) and varies by <5% between foreset (84:16; Fig. 10F) and bottomset (81:19; Fig. 10I) deposits.

In summary, sedimentary package M27–M29b displays: (1) a finer mean grain size; (2) better sorting; and (3) higher mean values of sphericity and roundness, and it consistently displays a unimodal average grain-size distribution (Figs. 7 and 8) relative to sedimentary packages M27–M29a and M27–M29c. The foreset and bottomset deposits of sedimentary package M27–M29b are significantly more mud-prone relative to sedimentary packages M27–M29a and M27–M29c, which contain >80% sand (Fig. 10).

Up-Core Grain-Size Patterns

The new high-resolution, quantitative grain-size data presented in this investigation are shown in Figure 8.

Site M27

From 300.00 mcd to 295.00 mcd, the dominant grain size is fine-grained sand, which typically makes up ∼60% of the total grain-size composition. At 298.19 mcd, the grain size coarsens abruptly, associated with an increase in the medium sand content from ∼10% to ∼30%, and the introduction of coarse sand, which forms ∼7% of the overall grain-size composition. At 295.00 mcd, there is the surface 1 to m5.4 sequence boundary (Miller et al., 2013a), which is overlain by a sand-rich package, and an abrupt increase in the medium sand content (Fig. 8). The coarse-grained sandy package (composed of ∼45% and ∼10% medium- and coarse-grained sand, respectively) terminates at 294.26 mcd (surface 2). Directly overlying surface 2, there is an increase in the overall silt content (from ∼15% to ∼30%) and a decrease in the medium sand content (from ∼45% to ∼10%) within a fining-upward trend (294.26–285.06 mcd); this is overlain by a coarsening-upward package (285.05–279.78 mcd). A final fining-upward package (279.78–272.99 mcd) terminates at surface 3 (272.99 mcd; this study). Surface 3 is marked by a decrease in silt content (∼55% to ∼18%) and increase in coarse and very coarse sand, which form ∼30% of the total sediment composition. Grain-size trends overlying surface 3 show a general fining-upward motif (272.98–256.19 mcd), which terminates at the overlying sequence boundary m5.3 (256.19 mcd). Overlying the Miller et al. (2013b) m5.3 sequence boundary, there is general coarsening-upward trend to 251 mcd, and there is an increase in the medium sand content from ∼5% below the m5.3 sequence boundary to a maximum of ∼25% at 251.52 mcd (Fig. 8).

Site M28

From 525.00 to 519.70 mcd, there is a fining-upward sandy package, which typically consists of ∼35% medium sand. At 519.70 mcd (alternative m5.4 sequence boundary as proposed by Hodgson et al., 2018), there is a marked increase in medium sand content (∼55%). From 519.70 to 515.89 mcd, there is a fining-upward trend that results in an increase in the overall silt content from ∼12% at 516.80 mcd to ∼52% at 514.74 mcd (Fig. 8). From 514.71 to 512.33, there is a general coarsening-upward trend. The 512.33 mcd sequence boundary (Miller et al., 2013a) is shared by surface 1 (this study) and directly overlies a very coarse sand at 512.97 mcd. Grain-size trends overlying surface 1 show a general coarsening-upward trend to 495.00 mcd (surface M28b; this study). Surface 2 is associated with an overlying fining in mean grain size and the disappearance of the coarse sand fractions. Surface 1 corresponds with the placement of the original m5.4 surface identified in Mountain et al. (2010) at 495.20 mcd, where a thin sand bed overlies a clayey silt. Grain-size trends overlying surface 2 show two fining-upward packages (495.00–459.95 and 459.94–432.00 mcd), which are overlain by a package of fine sand (432.00–415.00 mcd) that terminates at surface 3 (415.00 mcd; this study). Surface 3 is associated with a marked increase in medium sand content from ∼7% to ∼45% of the total sediment composition. Overlying surface 3, there are two coarsening-upward packages (415.00–392.00 and 392.00–363.00 mcd), which terminate at sequence boundary m5.3 (363.00 mcd; Miller et al., 2013a). Directly overlying the m5.3 sequence boundary, the gravel and very coarse sand are no longer present. From 363.00 to 358.00 mcd, the grain-size composition is dominated by fine and medium sand (Fig. 8).

Site M29

From 667.00 to 662.37 mcd, there is a general fining-upward trend; at this stratigraphic interval, the grain-size composition is dominated by fine and medium sand. At 662.37 mcd, there is the m5.4 sequence boundary (Miller et al., 2013a), shared by surface 1 (this study). Directly overlying surface 1, there is a decrease in silt content from ∼50% to ∼12% and an increase in the medium sand content from ∼10% to ∼60%. From 662.37 to 658.50 mcd, there is a sedimentary package dominated by medium sand, which forms ∼50% of the total sediment composition (Fig. 8). The sand-rich package terminates at surface 2 (658.50 mcd; this study). Directly overlying surface 2, there is a marked increase in the silt content (from ∼5% to ∼75%). Overlying surface 2, the grain size is dominated by silts (658.50–651.64 mcd), which form ∼60% to ∼80% of the total sediment composition; the silts are occasionally punctuated by thin lenses of medium and coarse sand (Fig. 8). The silt-rich package terminates at surface 3 (651.64 mcd). Directly overlying surface 3, the silt content drops from ∼75% to ∼5%, and coarse sand is present, forming ∼25% of the total sediment composition. Overlying surface 3, there is a fining-upward trend (651.64–649.51 mcd), followed by a coarsening-upward package (649.50–643.19 mcd), which terminates at the overlying sequence boundary m5.3 (643.19 mcd). From 643.19 to 638.00 mcd, there is an overall fining-upward trend (Fig. 8).

DISCUSSION

How Does Intraclinothem Topset Process Regime Influence Downdip Grain Character?

Vertical Process Variability and Grain Character

The core expression of sequence m5.4 topset deposits indicates that either fluvial-dominated (M27a and M27c) or wave- and storm-dominated (M27b) processes were active within the same seismic sequence. The presence of both fluvial and wave-and-storm process regimes within one seismic sequence indicates that sequence m5.4 is an example of a mixed-energy system (e.g., Ainsworth et al., 2008, 2011; Olariu, 2014; Gomis-Cartesio et al., 2017), rather than a clinothem described by a single process regime (see examples cited in Dixon et al., 2012a). Concomitant changes in quantitative grain character occur in association with changes in the dominant process regime.

River-Dominated Sedimentary Packages

The sand-to-mud ratios show river-dominated sedimentary packages are dominated by bypass of sand-grade sediment across topsets and preferential deposition within foreset and bottomset deposits (Fig. 10). The shapes of the grain-size distribution profiles vary minimally between foreset and bottomset deposits (Figs. 7J and 7L); this suggests the bulk transfer of sand fractions across topsets. However, the foreset deposits are relatively coarser, indicating preferential deposition of the coarser grain-size fractions (0.25–0.75 mm; medium- and coarse-grained sand) in the slope setting. This may reflect the rapid dissipation of gravity-flow energy, resulting in slope deposition of the coarsest grain-size classes.

The high sand-to-mud ratios are associated with debritic and turbiditic foreset and bottomset deposits, which are typically glauconite-bearing sands. The association between reworked glauconite and river-dominated topsets was documented by Mountain et al. (2010), who interpreted clinoform sequences that had poorly sorted glauconitic sand in the clinoform rollover position as river-dominated features. The presence of recycled glauconite within foreset and bottomset deposits supports topset glauconitic sands as a sediment source for downdip deposits (Hodgson et al., 2018; Proust et al., 2018).

Although the sand content is relatively high within the river-dominated sedimentary packages, river-dominated deposits are less well sorted, and grain shapes tend to be less spherical and more angular than that of the wave- and storm-dominated sedimentary package (Figs. 13, 14, and 15). The lower textural maturity of the river-dominated deposits reflects a shorter transport time from hinterland erosion to deposition within foreset and bottomset deposits (Hodgson et al., 2018).

The river-dominated deposits exhibit a consistent bimodality in average grain-size distribution (Figs. 7J and 7L), which suggests a dual sediment source. This is interpreted to reflect a fine sand component associated with hinterland erosion and a coarser glauconite sand component associated with reworking from topset and clinoform rollover deposits. The bimodality in average grain-size distribution is present throughout the topset to bottomset profile in both river-dominated packages (Figs. 7J and 7L). In both cases, there is a slight coarsening from topset to foreset deposits, and then the average grain-size distribution remains constant between foreset and bottomset deposits (Figs. 7J and 7L). The coarsest grains are not sequestered in topset deposits, suggesting bypass of the coarsest-grained sediment fractions, possibly through channels. The average grain-size distribution profile of the river-dominated deposits reflects sourcing of the coarsest grain-size fractions (typically reworked glauconite and quartz) from the clinoform rollover seaward of the core M27 intersection. Additionally, the average grain-size distribution profiles also indicate relatively efficient sediment transport beyond the shelf break, associated with sediment bypass (cf. Stevenson et al., 2015).

Wave- and Storm-Dominated Sedimentary Packages

The sand-to-mud ratios of the wave- and storm-dominated sedimentary package (M27–M29b) are consistently lower than those of the river-dominated packages; the difference in sand content becomes greater downdip, where the bottomsets of the river-dominated packages contain up to 47% more sand than the wave- and storm-dominated sedimentary package (Fig. 10). The sand-to-mud ratios of the wave- and storm-dominated sedimentary package show the retention of sand-grade sediment within topset and foreset deposits, with limited basinward sand bypass (Fig. 10). The bottomset deposits are associated with mud-grade sediment, attributed to deposition from suspension fallout. Compared to the river-dominated deposits, the foreset and bottomset deposits of the wave- and storm-dominated sedimentary package are less sand-rich (foreset and bottomset deposits contain an average of 13% and 45.5% less sand, respectively; Fig. 10), but the textural maturity is higher (Figs. 13, 14, and 15). The relatively higher textural maturity displayed by the wave- and storm-dominated deposits reflects reworking processes landward of the shelf edge, associated with the redistribution of sediment and a longer residence time within the sediment transport system (Bhattacharya and Walker, 1992; Diebert et al., 2003; Li et al., 2011). The downdip average grain-size distribution profiles of wave and storm deposits display prominent fining from topset to bottomset (Fig. 7K). This reflects the inefficiency of wave- and storm-dominated systems at transferring sand beyond the shelf edge, and the dominance of shore-parallel sediment redistribution (Coleman and Wright, 1975; Bhattacharya and Giosan, 2003), resulting in relatively thick, sand-prone topset deposits. The documentation of significant variability in sediment composition and texture within the bottomset deposits of a single clinothem sequence suggests that nuanced changes in topset process regime may represent a hitherto overlooked contributing factor in the depositional evolution of clinothem sequences.

Lateral Variability in Process Regime

The core data set presented here is from a single 2-D dip-parallel transect and captures only one portion of the along-strike variability; however, a network of 2-D seismic reflection lines permits the three-dimensional (3-D) architecture of sequence m5.4 to be constrained (Monteverde et al., 2008). Recent studies have highlighted that shallow-marine systems, both modern and ancient, can display prominent lateral variability associated with changes in the interactions among fluvial, wave, and tidal processes (Ta et al., 2002; Ainsworth et al., 2008, 2011; Olariu, 2014; Jones et al., 2015).

The propensity for systems to exhibit lateral variability, associated with changes in the dominant process regime, has the potential to increase grain-character heterogeneity both along strike and downdip. In the context of improved prediction of downdip facies from updip sedimentary facies, the interactions of temporal and lateral process-regime change could introduce significant variability not only in sand content, but also in grain character. For example, the influence of shore-parallel variability may be expressed as a lateral transition from a river-dominated topset system to a wave-dominated system further along strike; however, downdip of the wave-dominated system, a fan fed by the river-dominated system could be intersected.

Despite this, sequence m5.4 is a rare example of a chronostratigraphically constrained clinothem, in which the sedimentological and stratigraphic characteristics of coeval topset, foreset, and bottomset deposits have been documented. Future studies of the relationship between along-strike variability in process regime and grain-character variability will require exceptional outcrop control, or integrated 3-D seismic reflection data sets and core-hole data with strike and dip control.

Classification of Mixed Process-Regime Clinothems

Cosgrove et al. (2018) determined that sequence m5.4 formed a rising clinoform trajectory and was a wave-dominated feature. Across seismic sequence m5.4, the majority of the cored topset, foreset, and bottomset deposits preserve indicators consistent with wave-dominated topset deposits, including minimal transport of coarse-grained sediment into deep-water settings (e.g., Helland-Hansen and Hampson, 2009). However, designating the entire sequence as belonging to this end-member category fails to accurately describe the stratigraphic or geographic variability. The stratigraphic intervals in sequence m5.4 that have a river-dominated topset process regime (sedimentary correlations M27–M29a and M27–M29c) are associated with the effective transport of coarse sand into the deep-water setting. Therefore, characterizing a clinothem by a single process or a clinoform trajectory fails to account for the inherent stratigraphic and lateral variability in mixed process-regime systems.

Autogenic and Allogenic Topset Process-Regime Change

The cause of changes in topset process regime may be controlled by allogenic or autogenic forcing mechanisms. Allogenic controls, i.e., those which are external to the sedimentary unit, primarily document the effects of eustatic variability and changes in hinterland climatic and tectonic regime, which modulate the production and discharge of sediment from source regions (e.g., Castelltort and Van Den Driessche, 2003; Armitage et al., 2011). The effects of allogenic forcing mechanisms could feasibly result in changes in the topset process regime and consequently account for the stratigraphic expression of the surfaces and sedimentary packages displayed in sequence m5.4. The scenario in which allogenic processes result in the observed intraclinothem surfaces, in addition to their regional basinward extent, would support their interpretation as sequence boundaries within a m5.4 composite sequence (Miller et al., 2013a, 2013b; Miller et al., 2018).

The intraclinothem surfaces could also be the result of autogenic controls, such as river avulsion and/or switching of wave-dominated delta lobes (e.g., Olariu, 2014; Hampson, 2016); this would mean that the intraclinothem surfaces are not sequence boundaries. Autogenic mechanisms have been shown to generate surfaces and stratigraphic architectures that are challenging to distinguish from those generated through allogenic processes (e.g., Muto and Steel, 2002). The identification of autogenic and/or allogenic generation of the intraclinothem surfaces, in this instance, remains tentative given the lack of strike control to test the regional extent of the surfaces and resolution of the chronostratigraphic data.

How Can High-Resolution Grain Character Data Be Used as an Additional Correlation Tool?

Miller et al. (2013a) determined sequence m5.4 to be a composite sequence, composed of three higher-order sequences (m5.4-1, m5.34, m5.33) of ∼100 k.y. duration. However, the placement of the intraclinothem stratigraphic surfaces was associated with varying degrees of uncertainty (Fig. 4). The placement of the intraclinothem sequence boundaries at Site M27 is primarily based on stacking pattern analysis. However, trends above the m5.33 sequence boundary are acknowledged to be unclear (Miller et al., 2013b). Furthermore, the Sr-isotope error margins and the single sample used to date sequence m5.33 (Browning et al., 2013) render the chronostratigraphic data weak.

At Site M28, sequence boundary m5.4-1 (17.7–17.6 Ma; Browning et al., 2013) is suggested to share its basal reflector with sequence boundary m5.4 (Miller et al., 2013a). Sequence boundary m5.34 (479.00 mcd; Miller et al., 2013a) is interpreted from seismic reflector terminations; however, only a minor impedance contrast, a weak core expression (Miller et al., 2013b), and no significant chronostratigraphic hiatus (Browning et al., 2013) are present. The placement of the m5.33 sequence boundary is based on the criteria of onlap and downlap (Miller et al., 2013a), which coincides with a coring gap (∼405 mcd; Miller et al., 2013a). In light of the coring gap and the error associated with Sr-isotope data (±0.61–0.4 m.y.; see Browning et al., 2013), the placement is ambiguous.

At Site M29, the correlation of sequence boundaries m5.4-1, m5.34, and m5.33 is more tentative. Sr-isotope data permit correlation with both the m5.4-1 and m5.34 sequence boundaries (17.7–17.6 Ma; Browning et al., 2013; Miller et al., 2013a). Additionally, there are weak/no core sequence boundaries proposed for m5.34 and m5.33, respectively, which, combined with chronostratigraphic data, provides unconvincing evidence for a composite sequence at M29.

Grain-character data provide an alternative approach to the subdivision of clinothems (Fig. 10). The alternative intraclinothem surfaces presented here were correlated across the complete depositional profile and correspond to changes in facies, grain size (Figs. 1315), grain shape (Figs. 1315), grain-size distribution (Fig. 7), sand-to-mud ratios (Fig. 10), and stratigraphic stacking pattern (Fig. 8). The placement of the intraclinothem surfaces in this investigation differs from those proposed previously (m5.4-1, m5.34, and m5.33; Miller et al., 2013a). The differences in the stratigraphic placement of the intraclinothem surfaces between previous investigations and this study are attributed to (1) the different methodologies used to identify the intraclinothem surfaces and (2) the stratigraphic resolution available to each investigation. The intraclinothem surfaces that separate the sedimentary packages presented in this investigation are attributed to changes in the dominant topset process regime. The results presented here highlight the potential application of quantitative grain-character data sets as supplementary correlation tools. Abrupt changes in grain size (many qualitatively observed) are widely used as a means of subdividing the stratigraphic record; however, additional grain-character attributes (including sorting, sphericity, and roundness) are overlooked. The lack of data sets that utilize sorting, sphericity, and roundness as additional correlation tools reflects the general lack of quantitative data sets in the wider literature.

How Can High-Resolution Grain Character Data Be Used as an Additional Tool to Refine Sequence Boundaries?

Different approaches exist for the placement of sequence boundaries across depositional profiles (e.g., Catuneanu et al., 2009; Hodgson et al., 2016; Barrett et al., 2018). However, in this instance, previous authors have followed a similar approach (Monteverde et al., 2008; Mountain et al., 2010; Miller et al., 2013a, 2013b; Hodgson et al., 2018; Miller et al., 2018) to place sequence boundaries within the Expedition 313 data set. Despite the integrated data set, which has identified sequence boundaries in multichannel seismic profiles and in core and sedimentary logs, uncertainty remains regarding the placement of the m5.4 sequence boundary (Fig. 4). At Site M28, sequence boundary m5.4 has been placed at 495.20 mcd by Mountain et al. (2010), at 512.33 mcd by Miller et al. (2013a), and an alternative at 519.70 mcd by Hodgson et al. (2018). The ambiguity surrounding the exact placement of the sequence boundary m5.4 is exacerbated by the absence of strong supporting seismic impedance contrast and chronostratigraphic data, and the nonuniqueness of core-based approaches to identify sequence boundaries (e.g., Browning et al., 2006). The surface of Mountain et al. (2010) corresponds to a thin sand bed overlying a clayey silt. The surface of Miller et al. (2013a) corresponds to a contact of fine sand overlying clayey silt; this is associated with a minor impedance contrast and a minimal chronostratigraphic time gap (ca. 17.9 Ma below and 17.7 Ma above). The alternative surface of Hodgson et al. (2018) corresponds to a bioturbated contact, where an upward-fining, sharp-based sand is overlain by silt.

Grain-Size Trends

Mountain et al. (2010), Miller et al. (2013a), and Hodgson et al. (2018) used abrupt changes in stacking patterns and grain size to aid placement of the m5.4 sequence boundary. The higher-resolution, fully quantitative grain-size data presented in this investigation reveal more detailed up-core grain-size trends and stacking patterns (Fig. 16). In the original semiquantitative cumulative lithology, the Hodgson et al. (2018) alternative sequence boundary (519.70 mcd) appears to immediately overlie a clay- and silt-rich horizon, which forms part of a fining-upward package. However, the candidate sequence boundary does not directly correspond with a prominent change in stacking pattern when the detailed cumulative grain-size data presented in this investigation are considered (Fig. 16). The new cumulative grain-size data do not indicate the presence of a large clay/silt peak directly underlying the proposed sequence boundary (Fig. 16). Instead, the total silt content remains relatively low (12%), and a more subtle increase in the very fine and fine sand content is observed. This subtle but significant difference in grain size is likely a product of the different (quantitative and semiquantitative) methodologies used in this investigation versus those presented in Miller et al. (2013b). The candidate sequence boundary proposed by Hodgson et al. (2018) does correspond with an increase in the medium sand content (30% and 47% directly below and above the proposed surface, respectively; Fig. 16). However, this does not correspond with an overall change in stacking pattern, as is suggested by the original semiquantitative cumulative lithology. The new data presented here place the candidate sequence boundary proposed by Hodgson et al. (2018) within a fining-upward package, which peaks at 515.89 mcd and corresponds with a large peak in the silt content. The ∼1.5 m sampling interval used to produce the original semiquantitative cumulative lithology was not of sufficient resolution to capture this.

The candidate sequence boundary proposed by Mountain et al. (2010; 495.00 mcd) was placed using general lithology alone, and it was placed within a very fine to fine sand package. The new cumulative grain-size data presented here indicate that the candidate sequence boundary of Mountain et al. (2010) corresponds to a broad change in stacking pattern and falls where a coarsening-upward package (containing coarse and very coarse sand) abruptly transitions into a relatively finer-grained package (Fig. 16). The abrupt change in grain size at 495.00 mcd is interpreted to represent intraclinothem surface 2, associated with a change in the topset process regime and the depositional style associated with this change.

In the original semiquantitative cumulative lithology, the candidate sequence boundary proposed by Miller et al. (2013a; 512.33 mcd) appears to overlie a coarsening-upward package and was placed at the junction between a clay/silt peak and very fine/fine sand package (Fig. 16). The new cumulative grain-size data presented here show the same overall stacking pattern: a coarsening-upward package that terminates at the 512.33 mcd and is overlain by a blocky, sand-rich package. There is also a peak in the fine sand content (40% of the total sediment composition) directly overlying the candidate sequence boundary of Miller et al. (2013a). The clay/silt content presented in the original semiquantitative cumulative lithology is significantly higher than that of the new high-resolution grain-size data; this is attributed to differences in technique used for data acquisition. The candidate sequence boundary at 512.33 mcd broadly corresponds with a change in stacking pattern across the original qualitative general lithology, the original semiquantitative cumulative lithology, and the new high-resolution grain-size data presented here, which supports this position as the m5.4 sequence boundary.

From the lower-resolution, semiquantitative cumulative lithology, it is clear that previous authors have identified potential sequence boundaries based upon perceived abrupt changes in grain size and lithology, alongside core-based criteria (Fig. 16). The higher-resolution quantitative data set presented here displays the same broad-scale (approximately tens of meters) trends in stacking as those presented by Mountain et al. (2010) and Miller et al. (2013b), but it refines and improves: (1) the accuracy of the grain-size composition and (2) the stratigraphic locations of changes in stacking pattern.

Grain Character

The fully quantitative nature of this data set enables changes grain size (including sand-to-mud content and mean grain size) to be calculated across the three candidate sequence boundaries. Of the three proposed sequence boundaries, the greatest change in sand-to-mud content occurs at the Miller et al. (2013a) boundary at 512.33 mcd, where a net change of 10% in sand-to-mud content is recorded, compared to 6.8% and 0.8% change at the boundaries of Mountain et al. (2010) and Hodgson et al. (2018), respectively (Fig. 17). In addition to the changes in sand-to-mud ratios, changes in mean grain size across the proposed sequence boundaries have been calculated (Fig. 18). The largest change in mean grain size across the three candidate sequence boundaries again occurs across the 512.33 mcd sequence boundary (Miller et al., 2013a), where a 29% change in mean grain size is observed, compared to a 20% and 18% change at the 495.20 mcd (Mountain et al., 2010) and 519.70 mcd (Hodgson et al., 2018) boundaries, respectively (Fig. 18A). One of the fundamental tenets for determining sequence boundaries is based upon the identification of abrupt stratigraphic changes in grain size; hitherto, a quantitative assessment of these parameters has been unavailable due to the relatively low sampling densities and qualitative/semiquantitative methodologies employed by previous authors. Quantitative analysis of sand-to-mud ratios and mean grain-size changes across the three proposed sequence boundaries supports a preferred sequence boundary at 512.33 mcd (Miller et al., 2013b), which displays the greatest overall change in grain size (Figs. 17 and 18A).

Up-core grain-shape characteristics (sphericity and roundness; Figs. 18B and 18C) are also shown alongside the potential sequence boundaries proposed by Mountain et al. (2010), Miller et al. (2013a), and Hodgson et al. (2018). Similarly, the grain-shape data support placement of the Miller et al. (2013a) sequence boundary at 512.33 mcd. The grain shapes become increasingly angular and less spherical up core to the 512.33 mcd sequence boundary; at the 512.33 mcd sequence boundary, there is a significant drop in both grain sphericity (from 0.91 to 0.78; 15.5%) and grain roundness (from 0.61 to 0.33; 59.6%) to their lowest levels within the stratigraphic section (Fig. 18). The dramatic difference in grain shape across this stratigraphic horizon supports a fundamental change in sediment source and/or transport regime, consistent with a depositional hiatus and sequence boundary; this is supported by the core expression of the 512.33 mcd sequence boundary (see Miller et al., 2013a). The other candidate sequence boundaries exhibit significantly less change in grain shape across the proposed sequence boundaries: 1.8% and 5.5% change in roundness and 1.1% and 0.1% change in sphericity for the Mountain et al. (2010) and Hodgson et al. (2018) candidate sequence boundaries, respectively (Fig. 18).

The high-resolution grain-character data presented here may provide an additional, complementary approach, to be used in conjunction with core criteria, to refine the placement of sequence boundaries, and/or to determine the most statistically likely sequence boundary from a number of candidate sequence boundaries. Additionally, the higher-resolution up-core grain-size data presented here highlight the fact that lower-resolution, semiquantitative lithological data may dramatically oversimplify grain-size trends and promote the somewhat arbitrary placement of sequence boundaries in core sections.

CONCLUSIONS

Integrated grain-character data and core facies have been used to describe a mixed process-regime Miocene clinothem sequence offshore New Jersey, USA. The quantitative, high-resolution grain-character data have enabled the topset, foreset, and bottomset deposits to be subdivided into three sedimentological packages, based on shared grain-character attributes. The topset core expression indicates that sedimentary packages M27–M29a and M27–M29c were deposited under a river-dominated process regime, as indicated by (1) widespread topset asymmetric ripple lamination; (2) terrestrial, woody organic matter; and (3) abundance of detrital quartz and glauconite sand grains. The topset core expression indicates that sedimentary package M27–M29b was deposited under a wave- and storm-dominated process regime, as indicated by (1) widespread hummocky cross-stratification; (2) rhythmically laminated topset deposits; and (3) symmetrical ripple lamination.

The correlations of sedimentary packages across the topset-foreset-bottomset profile using high-resolution grain-character data provides a unique perspective into intraclinothem architecture and basin fill within a single seismic-scale clinothem. The dominant topset process regime exerts a fundamental control on the distribution of grain character. In this system, the sedimentary packages associated with river-dominated topset conditions have higher sand-to-mud ratios across the downdip profile; however, the grain character is texturally less-mature relative to the wave- and storm-dominated sedimentary package. The differences in grain character between the river-dominated and wave- and- storm-dominated sedimentary packages are exaggerated downdip.

The sedimentary packages are separated by intraclinothem surfaces, which were determined objectively using changes in the average grain-size distribution, and which are concomitant with stratigraphic changes in the facies, grain-size composition, and grain shape and sorting parameters. The identification of coeval sedimentary packages at subseismic resolution from grain character alone is a novel methodology for subdividing the stratigraphic record and provides a high-resolution correlation of strata within a sand-rich sequence. However, the data set does not resolve whether the intraclinothem surfaces are formed through autogenic or allogenic controls.

The high-resolution, quantitative grain-character data are also shown here to be an additional tool to help refine the placement of sequence boundaries. In this instance, the grain-character data were used to support the preferred placement of a sequence boundary position from three previously postulated candidate sequence boundaries. This was achieved by quantitatively assessing grain-size and grain shape change across the proposed sequence boundaries. Additionally, the new data presented here helped to refine and improve interpretations of stacking patterns and grain-size trends.

ACKNOWLEDGMENTS

We thank the Integrated Ocean Drilling Program (IODP), ECORD Science Operator (EOS), and the Bremen Core Repository (BCR) for assisting us with sample collection. We thank the Sorby Environmental Fluid Dynamics Laboratory (University of Leeds) for assistance with sample analysis. Finally, we thank an anonymous reviewer and reviewer C. Messina, Associate Editor A. Fildani, and Editor R. Russo for their constructive comments, which significantly improved the paper.

1Supplemental Information. Data table showing the core depth of all samples used in this investigation for grain character analysis. Please visit https://doi.org/10.1130/GES02046.S1 or access the full-text article on www.gsapubs.org to view the Supplemental Information.
Science Editors: Raymond M. Russo, David Fastovsky
Associate Editor: Andrea Fildani
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.