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Deciphering the Origin of Cyclical Gravel Front and Shoreline Progradation and Retrogradation in the Stratigraphic Record

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Deciphering the Origin of Cyclical Gravel Front and Shoreline Progradation and Retrogradation in the Stratigraphic Record Deciphering the origin of cyclical gravel front and shoreline progradation and retrogradation in the stratigraphic record John J. Armitage1, Peter A. Burgess2, Gary J. Hampson3, and Philip A. Allen3 1Department of Earth Science, Royal Holloway, University of London, Egham, UK; Now at Institut de Physique du Globe de Paris, 1 rue Jussieu, Paris, 75005, France; e-mail: [email protected] 2Department of Earth Science, Royal Holloway, University of London, Egham, UK 3Department of Earth Science and Engineering, Imperial College London, London, UK May 4, 2016 1 1 Abstract 2 Nearly all successions of near-shore strata exhibit cyclical movements of the shoreline, which 3 have commonly been attributed to cyclical oscillations in relative sea level (combining eustasy 4 and subsidence) or, more rarely, to cyclical variations in sediment supply. It has become accepted 5 that cyclical change in sediment delivery from source catchments may lead to cyclical movement 6 of boundaries such as the gravel front, particularly in the proximal segments of sediment routing 7 systems. In order to quantitatively assess how variations in sediment transport as a consequence 8 of change in relative sea-level and surface run-off control stratigraphic architecture, we develop a 9 simple numerical model of sediment transport and explore the sensitivity of moving boundaries 10 within the sediment routing system to change in upstream (sediment flux, precipitation rate) and 11 downstream (sea level) controls. We find that downstream controls impact the shoreline and sand 12 front, while the upstream controls can impact the whole system depending on the amplitude of 13 change in sediment flux and precipitation rate. The model implies that under certain conditions 14 the relative movement of the gravel front and shoreline is a diagnostic marker of whether the 15 sediment routing system experienced oscillations in sea level or climatic conditions. The model 16 is then used to assess the controls on stratigraphic architecture in a well-documented palaeo– 17 sediment-routing system in the Late Cretaceous Western Interior Seaway of North America. 18 Model results suggest that significant movement of the gravel front is forced by pronounced 19 (±50%) oscillations in precipitation rate. The absence of such movement in gravel front position 20 in the studied strata implies that time-equivalent movement of the shoreline was driven by 21 relative sea-level change. We suggest that tracking the relative trajectories of internal boundaries 22 such as the gravel front and shoreline is a powerful tool in constraining the interpretation of 23 stratigraphic sequences. 24 Keywords 25 sea-level, sediment flux, stratigraphy, modelling 26 1 Introduction 27 Change in sediment supply, sea level and subsidence are ubiquitously cited as the main controls 28 on stratigraphic architecture (e.g. Vail et al., 1977; Van Wagoner et al., 1990; Catuneanu et al., 29 2009), but the extent to which any of these controls leave a unique signature within the strati- 30 graphic record is not yet clear (e.g. Burgess et al., 2006; Burgess and Prince, 2015). Periodic 31 changes in relative sea level may be accompanied by climatic change of the same periodicity if 32 both sea level and climate are forced by Milankovitch orbital cycles. Such regional and global 2 33 climatic cycles can have the effect of increasing or reducing surface run-off and sediment supply 34 at the same time as changing sea-level (e.g. Blum and Hattier-Womack, 2009). 35 Sea-level change has clear implications for deposition within the coastal plain as the shoreline 36 transits across this region. The response of the sediment routing system to shoreline migration 37 likely decays upstream of the shoreline (e.g. Fisk, 1944; Blum and T¨ornqvist, 2000; Swenson, 38 2005). Down-stream of the shoreline the associated change in sediment flux into the submarine 39 domain, along with change in water depth will determine stratal geometry, shoreline migration 40 and generation of sequence stratigraphic boundaries (e.g. Heller et al., 1993; Burgess and Prince, 41 2015). 42 Climate change has less clear implications for sediment deposition. Periodic change in pre- 43 cipitation and surface water flow within the fluvial segment could amplify, damp and/or delay 44 the sediment flux signal due the processes of sediment transport (Jerolmack and Paola, 2010; 45 Simpson and Castelltort, 2012; Armitage et al., 2013; Godard et al., 2013; Braun et al., 2015). 46 Furthermore, the timescale of sediment flux perturbation that may be recorded within the fluvial 47 and deltaic segments is a function of the length of the sediment routing system (e.g. Dade and 48 Friend, 1998; M´etivier, 1999; Castelltort and Van Den Dreissche, 2003). It is therefore uncertain 49 from a theoretical stand point whether sediment flux signals from the catchment can be trans- 50 ferred to the shoreline without modification. There is however some observational evidence from 51 sediment routing systems that records of change in coastal to marine sediment accumulation are 52 due to change in sediment flux from the catchment (Covault and Graham, 2010; Covault et al., 53 2011; Carvajal and Steel, 2012). If an upstream signal of increased sediment flux is transferred 54 across the fluvial segment of the sediment routing system, then we could reasonably assume that 55 it will supply more sediment to the shoreline and thus alter the shoreline trajectory. 56 The lack of a unique solution to the interpretation of stratigraphic architecture is a long 57 standing problem within the methodology of sequence stratigraphy, and in the desire to un- 58 derstand how sediment accumulation is a record of past climate, tectonics and internal system 59 dynamics. Previous studies have tried to gain an insight into how strata form using forward mod- 60 els of sediment transport (e.g. Burgess et al., 2006; Paola and Martin, 2012). From measuring 61 the transfer of mass from sediment in transport to deposition from laboratory scale experiments, 62 it has been observed that the application of idealised grain size sorting models may provide a 63 way to analyse the movement of internal grain size boundaries within the sediment-routing sys- 64 tems (Paola and Martin, 2012). Therefore in order to quantitatively assess how variations in 65 sediment transport due to change in relative sea level and surface run-off influence stratigraphic 66 architectures, we will explore the sensitivity of such moving boundaries within a numerical 67 sediment-routing system to change in upstream (sediment flux, precipitation rate) and down- 68 stream (sea level) controls. The internal boundaries that we focus on are (1) the downstream 69 limit of alluvial conglomerates, the gravel front (Paola et al., 1992), (2) the shoreline, and (3) 3 70 the down system limit of shallow marine sandstones, the sand front (Michael et al., 2013). These 71 moving boundaries can be mapped within strata (e.g Michael et al., 2014; Hampson et al., 2014), 72 and therefore have the potential to be used to diagnose past forcing of sediment-routing systems 73 if it is known how they respond to change. 74 In this context we will explore three central questions: 75 (1) How sensitive are the positions of the gravel front, shoreline and sand front to sediment 76 transport mechanisms. In other words, how closely linked via sediment transport are coastal 77 plain, shelf and shoreline morphology and grain size distributions, and how does this linkage 78 relate to control by sediment transport. 79 (2) What is the impact of upstream (sediment flux, precipitation rate) and downstream 80 (sea level) controls on the positions of the gravel front, shoreline and sand front? The aim is 81 to explore the uniqueness of relative sea-level control on shoreline position, implicit in many 82 sequence stratigraphic interpretations. Experimental modelling will help to determine if there 83 are signals upstream of the shoreline that can be used to differentiate driving mechanisms. 84 (3) Do the different parameters that influence shoreline position impart diagnostic charac- 85 teristics to coastal plain and shelf stratigraphic architectures? 86 In the first part of the paper, we investigate these three questions using a generic model 87 of a large sediment-routing system that contains both subaerial and subaqeous depositional 88 domains. Sensitivity tests for the generic model establish a parameter space that is used to 89 investigate a case study from the geological record. In the second part of the paper, we focus on 90 a sediment-routing system within the Western Interior Basin, USA, in which stratal geometries, 91 shoreline migration and sediment budget have been constrained for a period of approximately 92 6Myr during the Late Cretaceous (Hampson, 2010; Hampson et al., 2014). The sediment- 93 routing system represented by the Star Point Sandstone, Blackhawk Formation, lower part of 94 the Castlegate Sandstone and coeval Mancos Shale is exposed in the Book Cliffs of east-central 95 Utah and west-central Colorado, USA. These strata represent the birthplace of outcrop-scale 96 sequence stratigraphy, and are widely visited by academic and industry groups to teach sequence 97 stratigraphic methods and models; the Book Cliffs outcrops therefore provide an ideal case study 98 with which to illustrate the importance of the three questions posed above. 99 2 Methods 100 We couple a 1-D model of sediment transport down depositional dip based on the flow of surface 101 water to a 1-D model of deposition in the submarine domain (Figure 1). Subaerial sediment 102 transport is modeled following Smith and Bretherton (1972) and Armitage et al. (2015), where 103 we assume that sediment flux is a function of both local slope and surface water flux: ∂z q = − (κ + cqn ) , (1) s w ∂x 4 104 where z is elevation, x is the down-system distance, κ is the linear diffusion coefficient, c is the 105 fluvial transport coefficient, n ≥ 1, and the water flux is given by, qw = αx, (2) 106 where α is the precipitation rate (see Table 1).
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