Accommodation- versus supply-dominated systems for sediment partitioning to deep water

Abstract

Several decades of studies on shelf-margin evolution have led to recognition that both accommodation-dominated and supply-dominated sediment-delivery systems are capable of transporting sediments from the shelf down into deep-water basins. The former case relies on falling sea level and lowstands to move deltas to the shelf edge, whereas the latter depends on well-supplied deltas reaching the shelf edge regardless of sea-level rise. However, it remains unclear how to distinguish between the two sediment-dispersal alternatives, and which of these is more efficient in delivering sediments to deep water. We explore sediment-volume partitioning into deep-water areas by analyzing >1600 runs of a geometric delta model with varying eustatic, shelf-morphologic, and sediment-supply conditions. Previous studies suggest that greenhouse eustatic (low amplitude and frequency) conditions generate lower shelf accommodation, and permit the shoreline to arrive at the shelf edge quickly. Further investigation reveals that (1) this argument works only for the supply-dominated system, and (2) the proportion of total sediment that reaches deep water is not correlated to the frequency of sea-level change, but depends strongly on the shelf width and the amplitude of sea-level change. We suggest a ratio between (1) the product of shelf width and the amplitude of sea-level change and (2) total sediment supply to quantitatively characterize the sediment dispersal system. A ratio of 0.4 forms a good boundary between accommodationand supply-dominated systems in the modeling results, and in three well-studied ancient systems (the Maastrichtian Washakie Basin, Wyoming, USA; the Pliocene paleo–Orinoco margin, Trinidad and Tobago; and the Miocene New Jersey margin, northeastern USA). This work also suggests that the sediment mass balance becomes more important for continental margin building regardless of sea-level scenarios over the longer term. INTRODUCTION Understanding the sediment supply mechanism and sediment distribution of deep-water systems is important for predicting and estimating hydrocarbon resources, deciphering past climatic and tectonic signals, and preventing geohazards. Recent studies have proposed possibilities for driving cross-shelf shoreline transit, thus creating a delta–deep water linkage (Covault and Graham, 2010; Sweet and Blum, 2016; Fig. 1A). In conventional sequence stratigraphy, sea-level fall below the shelf break was considered the only mechanism to drive the shoreline across the shelf, delivering sediments to the preexisting shelf edge and further forming deep-water fans (Vail et al., 1977; we term this the “accommodation-dominated system”). The alternative possibility is that high sediment supply also can push the shoreline to the shelf margin, and that deep-water deposition can ensue regardless of sea-level changes (Burgess and Hovius, 1998; Carvajal and Steel, 2006; we term this the “supply-dominated system”). A shelf-penetrating canyon is another possibility for providing long-lived connections transporting sediments to deep water. In this study, we focus on the first two possibilities. Even though these two end members are widely accepted, it is still not well understood how efficient they each are at delivering sediments to deep water over multiple sea-level cycles. This lack of understanding stems from the limited documentation on sediment-volume partitioning across complete shelf-margin clinoforms (e.g., Carvajal and Steel, 2012). Furthermore, even though there is a tendency to link the frequency and amplitude of eustatic sea-level change with the two types of system (icehouse conditions with accommodation-dominated, and greenhouse conditions with supply-dominated), some cases also demonstrate the importance of additional parameters such as sediment supply and shelf width (e.g., Covault and Graham, 2010). We explore the sediment distribution across numerically modeled shelf-margin clinoforms to (1) compare the efficiency of accommodationand supply-dominated sediment dispersal systems; (2) understand how upstream and downstream boundary conditions can influence the formation of deep-water fans and continental margin building; and (3) learn how to quantitatively distinguish between these two systems. METHODOLOGY We modified the geometric model designed by Kim et al. (2009) (Fig. 1) that was able to accurately estimate the shoreline position in their Experimental EarthScape basin (https:// www .safl .umn .edu /facilities /experimental -earthscape -xes -basin) experiments. The conservation of sediment mass is expressed as q t b dx L s = − ( ) ∫ η 0 , (1) where qs is the sediment flux (km2/m.y.), t is the simulated model time (m.y.), η is the topographic elevation of the sediment surface (km), b is the elevation of the basement (km), and L denotes total length of the basin (km). A preexisting shelfmargin clinoform is used as the initial basement (Fig. 1). The shoreline position, s, is determined under the prescribed slope conditions for the topset and foreset of deltaic clinoforms that are solved numerically for each time step as η η = + − ( ) ( ) < s t tan if s x S x s, (2a) η η = − − ( ) ( ) > s f tan if x s S x s, (2b) where St and Sf are gradients (degrees) of the topset and foreset, respectively. The elevation of the shoreline (ηs) equals sea level (see detailed CITATION: Zhang, J., Kim, W., Olariu, C., and Steel, R., 2019, Accommodationversus supply-dominated systems for sediment partitioning to deep water: Geology, v. 47, p. 419–422, https:// doi .org /10 .1130 /G45730.1 Manuscript received 21 October 2018 Revised manuscript received 31 January 2019 Manuscript accepted 20 February 2019 https://doi.org/10.1130/G45730.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 11 March 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/5/419/4680629/419.pdf by Univ of Texas-Austin user on 21 October 2019 420 www.gsapubs.org | Volume 47 | Number 5 | GEOLOGY | Geological Society of America descriptions and limitations in the GSA Data Repository1). We input a wide range of eustatic conditions, shelf width, and magnitude of sediment supply to evaluate the influence of each variable on the partitioning of sediment volumes to the deep-water area. We calculated the ratio of the volume of sediment partitioned beyond the shelf edge to the total sediment budget for each run (Figs. 2 and 3A). Most model runs used qs = 20 km2/m.y. and a total run time of 1 m.y. (Fig. 2), but qs = 5–50 km2/m.y. was also tested in some model runs (Fig. 3A). The amplitude of sea-level change varied from 0 to 150 m in 10 m increments. The duration for cycles of sealevel change was from 50 to 500 k.y. in 50 k.y. increments. The shelf width ranged from 20 to 200 km in 20 km increments. We distinguished the accommodationand supply-dominated systems in the experimental results by their shoreline behavior (Fig. 1; see animations in the Data Repository). In the accommodation-dominated system, shoreline advancement across the entire shelf occurs only when sea level falls below the shelf edge (Fig. 1C). The shoreline in the supplydominated system, after it arrives at the shelf margin, docks at the shelf edge for the remainder of the simulated model time (Fig. 1D). SEDIMENT VOLUME PARTITIONING UNDER DIFFERENT EUSTATIC AND SHELF CONDITIONS FOR ACCOMMODATIONAND SUPPLY-DOMINATED SYSTEMS The proportion of the sediment budget partitioned into deep water, estimated for each run with qs = 20 km2/m.y., is more sensitive to the shelf width and amplitude of sea-level change than to the frequency of sea-level change (Fig. 2). The deep-water sediment proportion decreases with increasing shelf width (Fig. 2B). For a narrow shelf, the deep-water sediment proportion is high (up to ~90%) and the greenhouse (low-frequency and low-amplitude) and icehouse (high-frequency and high-amplitude) eustatic conditions produce similar results (e.g., <10% differences for 20 km shelf width). For moderately wide shelves, the deep-water sediment proportion in icehouse conditions is maintained, whereas the proportion continuously decreases in greenhouse conditions. For wide shelves, very limited sediments are delivered into deep water in greenhouse conditions. The proportion of sediments transported into deep water is also closely related to the amplitude of sea-level change (Fig. 2C). An increasing amplitude of sea-level change causes the deep-water sediment proportion to decrease and then increase. The deep-water sediment proportion is not closely related to the frequency of sea-level change under certain shelf widths and amplitudes of sea-level change (Figs. 2A and 2D). For an 80 km shelf width under 40 m amplitude of eustatic sea-level change, the deep-water sediment proportion ranges only from 70% to 71% (Fig. 2D). The modeling results suggest that narrow shelves and a low amplitude of sea-level change favor the formation of supply-dominated systems (Fig. 2). Wider shelves and a higher amplitude of sea-level change produce greater shelf accommodation (Sømme et al., 2009). Therefore, in the supply-dominated system, the deep-water sediment proportion decreases with increasing shelf width and amplitude of sea-level change (Figs. 2B and 2C). When shelf accommodation increases beyond a threshold value, beyond which the sediments cannot fill the space created, the supply-dominated system changes to an accommodation-dominated system. The volume of deep-water sediment in the accommodationdominated system is decided by how long the shoreline remains at or below the shelf edge (i.e., the amplitude of sea-level change versus the water depth at the shelf edge), rather than by the shelf accommodation. Therefore, the relationship between deep-water sediment proportion and the amplitude of sea-level change is not monotonic. Previous studies argued that longer cycle duration would allow more time