Abstract

Sequence stratigraphy remains the foremost methodology used to describe the stratigraphic record and to interpret the controls on deposition; yet, it relies on long-standing assumptions that few studies have sought to validate. Here, we present results from testing hypotheses related to the deep-water depositional sequence model by revisiting the seismic-based type locality: the Mississippi Fan, in the central Gulf of Mexico. By independently testing the relationship between cycles of relative sea-level change and those of sedimentation, we demonstrate that >98% of Neogene–Quaternary deep-water sequences do not accumulate in a manner prescribed by long-held sequence stratigraphic conventions. Instead, over the past 5.5 Myr, sequences show a temporal mismatch in frequency, phase, and amplitude with cycles of relative sea-level change, a concept we refer to as stratigraphic aliasing. Divergences are attributed to variable rates of sedimentation, which were responsible for creating cycle frequencies that were both higher and lower than those of relative sea-level change, and that resulted in two modes of Mississippi Fan accumulation: lower average sedimentation rates in older sediments, and an opposite trend in younger successions. The latter mode occurred ∼2.2 Myr after the onset of North American glaciation, a period marked by significant continental drainage reorganization and salt-tectonic deformation. Based on our conclusions, we recommend that the future of sequence stratigraphy be rooted firmly in assessing the reproducibility of preexisting spatio-temporal predictions and in the rigorous use of multiple working hypotheses.

INTRODUCTION

For over 40 years, depositional systems have been thought to be controlled by the interaction of eustasy and subsidence (i.e., “relative sea level” of Jervey, 1988), with variability in sediment supply providing a secondary effect (see Vail et al., 1977; Posamentier et al., 1988; Van Wagoner et al., 1990). According to this view, relative sea-level change drives reciprocal sedimentation, whereby condensed sections and nearshore deposits accumulate during rises and high stands, and erosional surfaces and deep-water accumulations deposit during falls and low stands (Figs. 1A and 1B). While active debate regarding both controlling mechanisms and their stratigraphic responses has been ongoing since the late 1980s (see Haq et al., 1987; Christie-Blick et al., 1988), the community remains divided on the level of applicability of sequence stratigraphy (see Madof et al., 2016; Burgess, 2016), on its status as a workflow as opposed to a paradigm (see Burgess and Prince, 2016; Catuneanu and Zecchin, 2016), and on its ability to be treated as a set of testable hypotheses (this study).

The Mississippi Fan, in the central Gulf of Mexico, serves as the type locality for the seismic-based model of the deep-water depositional sequence. In addition to Weimer (1990), other workers identified Mississippi Fan lobes from downlapping reflections at the base of gullwing-shaped deposits (Garrison et al., 1982) and related accumulations to falls in relative sea level (Bouma et al., 1985). While these concepts have been applied globally to deep-water systems, few studies have sought to validate the initial findings. Here, we revisited the Mississippi Fan in an attempt to independently reproduce the relationship between relative sea-level change and sedimentation, and to test whether the latter is controlled by changes in the former. By testing long-standing conventions, we offer new insights into the time-varying interactions between purported drivers and accumulation, and into the effect of climatic and tectonic signals on modulating inherently nonstationary deposition.

MISSISSIPPI FAN, GULF OF MEXICO

The Neogene–Quaternary Mississippi Fan in the central Gulf of Mexico is one of the most well-studied submarine fans on Earth. Because of the abundance of subsurface data, it has been used to understand deep-water stratigraphic architecture and cyclicity, as well as relative sea-level change and sedimentation.

Architecture and Cyclicity

The Mississippi Fan is a submarine accumulation >4 km in thickness situated outboard of the Sigsbee Escarpment. Based on downlapping reflections (i.e., geometric evidence for sequence boundaries), Weimer (1990) interpreted 17 laterally compensatory deep-water sequences (Fig. 2A) and inferred each to be composed of channelized (coarse-grained) deposits flanked by levee-overbank (fine-grained) deposits. By incorporating age control, Weimer and Dixon (1994) showed that sequence duration generally decreased from 0.66 Myr (5.5–4.84 Ma; sequence number 1) to 0.023 Myr (0.23–0.0 Ma; sequence number 17) and increased in lateral extent and overall thickness through time (Figs. 2B and 2C). In addition to the duration of sequences, Weimer and Dixon (1994) interpreted two major hiatuses, from 3.0 to 1.9 Ma and from 0.45 to 0.086 Ma. While Weimer and Dixon (1994) may have incorrectly assumed that seismic reflections were laterally continuous synchronous surfaces, alternative interpretations have not yet been proposed.

Sea Level and Sedimentation

We calculated relative sea-level change and sedimentation rates independently for the last 5.5 Myr of growth of the Mississippi Fan (Figs. 3A and 3B). The former was constructed by adding the eustatic record of Miller et al. (2005, their fig. 3, created from the benthic foraminiferal δ18O record) to the back-stripped subsidence rate of the northern Gulf of Mexico of Diegel et al. (1995, their fig. A-3). While specific details regarding the construction of the subsidence curve remain unclear, it serves as the most robust (if not the only) example for the area. Sedimentation rates for each Mississippi Fan sequence were calculated by dividing maximum preserved thickness (from Fig. 2A) by duration (from Fig. 2B); rates span two orders of magnitude, with older sequences displaying lower average accumulation rates (i.e., 104 cm/Myr), and younger ones showing an opposite trend (i.e., 106 cm/Myr). Based on our analysis, the bulk of long-term relative sea-level change in the Mississippi Fan was caused by subsidence related to salt withdrawal, rather than by eustatic change.

ASSESSING DEVIATIONS FROM CONVENTIONS

To test the effect of accommodation on Mississippi Fan deposition, conditions both conforming to, and deviating from, conventional sequence stratigraphic models must be evaluated. Although no framework currently exists to address the latter, we considered mismatches between relative sea-level change and sedimentation, and the incompleteness of the stratigraphic record.

Stratigraphic Aliasing

Sequence stratigraphic models for deep-water settings assume that cycles of relative sea-level change and sedimentation have equal frequencies, zero phase shift, and moderate amplitudes (Figs. 4A–4D). As a result, each deep-water sequence is required to correlate to a single lowstand interval, with the greatest amount of sediment delivered during the maximum rate of relative sea-level fall (Weimer, 1990; Posamentier and Kolla, 2003). To address alternative scenarios, we introduce a conceptual framework termed stratigraphic aliasing (see Fig. DR1 in the GSA Data Repository1), which makes use of varied waveforms to describe input parameters and to facilitate multiple working hypotheses. Although aliasing generally refers to the misidentification of signal frequencies due to sampling biases, our use of the term “stratigraphic aliasing” is aimed at objectively comparing (and in the time domain) the cycle frequency, phase, and amplitude of relative sea-level change to those of sedimentation. For utility, we define frequency as the number of sedimentary cycles deposited within one cycle of relative sea-level change; frequencies can be equal to, less than, or greater than one and correspond to equivalent, longer, or shorter sedimentation periods. Phase relates to the timing of sediment delivery with respect to changes in relative sea level and can be used to describe lags and leads in accumulation. We describe deposition occurring at phase shifts of 0° (falls; conventional model expectations), 90° (lowstands), 180° (rises), and 270° (highstands). Amplitude is the rate of accumulation during one cycle of relative sea-level change; its variance describes increased and decreased flux.

“Sadler Effect”

In addition to stratigraphic aliasing, we considered the preservation of Mississippi Fan sequences. Because sequences (i.e., strata bounded above and below by sequence boundaries; see Figs. 2A and 2B) are marked by repetitive changes in accumulation rates, they can be classified as cycles. Cycles (i.e., sequences) can be used to assess the completeness of the depositional record by comparing their amplitude (i.e., sedimentation rate) to their frequency (i.e., time span).

Sadler (1981) observed that the number and duration of nondepositional events (hiatuses) cause accumulation rates to obey an inverse power-law relationship with respect to the span of measurement. Although this technique (i.e., plotting a function and its reciprocal, which results in a negative correlation) has received some scrutiny (see Anders et al., 1987), it has become a standard practice in quantifying the completeness of the stratigraphic record. The trend, which is observed both in Mississippi Fan sequences and in globally sampled turbidites (Fig. DR2), manifests as cycles with lower average sedimentation rates (lower amplitude) at longer durations (lower frequency). Higher rates of accumulation are necessarily sustained only over shorter intervals than are lower rates (see Sadler and Jerolmack, 2014; Tipper, 2015; Paola et al., 2018), implying that the “Sadler effect” operates also in the frequency domain.

DISCUSSION

Assessment of the applicability of sequence stratigraphic concepts relies on the number of sequences satisfying the conditions set forth by well-accepted models. According to these assumptions, the 17 preserved deep-water sequences of the Mississippi Fan require an equal number of relative sea-level cycles, with the bulk of sediment delivered during each lowstand interval. However, our analysis shows that only one sequence (comprising <2% of the past 5.5 Myr) coincides with relative sea-level change in the manner prescribed by conventional assumptions (i.e., equal frequencies, zero phase shift, and moderate amplitudes; see Figs. 3A and 3B).

Of the 17 deep-water sequences, older cycles (numbers 1–9; 5.5–0.7 Ma) are characterized by lower frequencies (longer periods) and lower amplitudes (lower accumulation rates), with the opposite pattern observed for younger ones (numbers 11–17; 0.6–0 Ma). In the former, phase (i.e., onset of sediment delivery) is impossible to determine because sequences span multiple relative sea-level cycles. In the latter, because multiple sequences develop between falls and rises, phase is interpreted to be between 0° and 180°. The only deposit that matches the period of coeval relative sea-level change is sequence number 10 (0.6–0.7 Myr); that deposit spans the transition between lower and higher average accumulation rates and has moderate amplitude (i.e., a sedimentation rate of 105 cm/Myr). However, phase could not be evaluated because sampling was insufficient to determine timing of sediment delivery. Remarkably, sequence number 10 contains the largest number of deep-water channels of any Mississippi Fan accumulation (see Fig. 2C), even when compared with other Mississippi Fan sequences. The finding that deep-water systems are the most well preserved when conventional sequence stratigraphic conditions are met has significant implications in sediment routing and source-to-sink analyses.

The two modes of deposition for the Mississippi Fan (i.e., lower average sedimentation rates in older sediments, and an opposite trend in younger successions) suggest that stratigraphic aliasing is a more appropriate framework of interpretation than sequence stratigraphy. As such, we find that older Mississippi Fan successions accumulated under markedly different conditions than younger ones. Although younger accumulations were deposited during large-scale climatic and tectonic changes (e.g., North American glaciation [see Aharon, 2003], continental drainage reorganizations [see Bentley et al., 2016], and salt-related deformation [see Tripsanas et al., 2007]), they responded with shorter periodicities. We attribute these short-lived stratigraphic responses not to relative sea-level change, but to the liberation of onshore sediment during glacial advance and retreat, and to the subsequent transport through continental drainages and around regional and local salt-related structures. Based on the climatic and tectonic forces that overprinted the effects of relative sea-level change on the Mississippi Fan, we question the very concept of the deep-water depositional sequence model at its type locality. While these results represent only one location, they are surprising enough to warrant further investigation. We therefore suggest that future research uses similar methods (e.g., stratigraphic aliasing) to rigorously and critically evaluate fundamental sequence stratigraphic assumptions and long-held conventions.

CONCLUSIONS

We used the deep-water Mississippi Fan in the central Gulf of Mexico to test objectively the effect of changes in accommodation on the stratigraphic evolution of 17 seismic sequences, which developed during the past 5.5 Myr. Although this locality was pivotal to the creation of sequence stratigraphic concepts, we find that interactions between purported drivers and sedimentation display temporal mismatches in frequency, phase, and amplitude, a concept we refer to as stratigraphic aliasing. By showing that Mississippi Fan sequences have lower average sedimentation rates in older sediments, and an opposite trend in younger successions, we conclude that sequence stratigraphic models present an oversimplified and stationary view of deep-water depositional systems. Our results call into question the concept of accommodation-driven reciprocal sedimentation, highlight the necessity of reevaluating type localities, and suggest that stratigraphic aliasing can be a more objective framework than well-accepted sequence stratigraphic conventions.

ACKNOWLEDGMENTS

We thank Chevron for allowing publication, and J.V. Bent, M.L. Amaru, and F. Lin for their support. We thank reviewers P.A. Burgess and S.M. Hubbard for thoughtful comments.

1GSA Data Repository item 2019202, methods involving aliasing and log-log plot of accumulation rate vs. time span, is available online at http://www.geosociety.org/datarepository/2019/, or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY license.