Experimental Stratigraphy Under Precisely Controlled and Monitored Conditions of Sediment Supply, Subsidence, Base-Level Variation, and Transport Mechanics

field is the ultimate repository of information, but exposure is limited, and it is often difficult to constrain key governing variables independently. We have developed a novel experi- Experimental mental basin—nicknamed Jurassic Tank—that allows us to produce experimental stratigraphy under precisely controlled and monitored conditions of sediment supply, subsidence, base-level variation, and transport mechanics. The unique feature of the basin is a fully programmable subsiding floor. Stratigraphy In the first application of the system, we looked for evidence of decoupling (out-of-phase behavior) between shoreline and base level, as has been predicted by some recent stratigraphic models. We found little support for this idea, but the results demonstrate the potential that experiments have for complementing field and theoretical studies of the filling of Chris Paola, St. Anthony Falls Laboratory, University of sedimentary basins. Minnesota, Minneapolis, MN 55414, USA, and Department of Geology & Geophysics, University of Before you read this, try solving the problem posed in Figure 1. Minnesota, Minneapolis, MN 55455-0219, USA INTRODUCTION Jim Mullin and Chris Ellis, St. Anthony Falls Laboratory, The central goal of sedimentary geology is to interpret the University of Minnesota, Minneapolis, MN 55414, USA history of Earth’s surface from sedimentary rocks. We develop competing hypotheses, debate, discuss, and compare, but un- David C. Mohrig, Department of Earth, Atmospheric and like areas of science that deal in accessible time and space Planetary Sciences, Massachusetts Institute of Technology, scales, in sedimentary geology it is often difficult to determine Cambridge, MA 02139, USA unambiguously who is right. The ultimate source of truth—the stratigraphic record itself—is like a fragmentary manuscript John B. Swenson, Department of Geological Sciences and written in a long-forgotten language. Deposits are imperfectly Large Lakes Observatory, University of Minnesota, Duluth, exposed and hard to date, seismic images are highly filtered MN 55812, USA and expensive, and the precise sequence of events that pro- duced real-world stratigraphy usually cannot be determined Gary Parker, St. Anthony Falls Laboratory, University of independently. Trying to understand the language of sedi- Minnesota, Minneapolis, MN 55414 USA ments using rocks alone would be like trying to understand Russian by opening War and Peace to the middle and staring Tom Hickson, Department of Geology, University of St. at the pages. Thomas, St. Paul, MN 55105, USA Sedimentary geologists have long recognized this and Paul L. Heller, Department of Geology & Geophysics, sought Rosetta stones for the stratigraphic record through University of Wyoming, Laramie, WY 82071, USA Lincoln Pratson, Earth and Ocean Sciences, Duke University, Durham, NC 27708-0230, USA James Syvitski, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, USA Ben Sheets and Nikki Strong, St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414, USA, and Department of Geology & Geophysics, University of Minnesota, Minneapolis, MN 55455-0219, USA ABSTRACT Stratigraphy has been a descriptive science for most of its Figure 1. Can you interpret this panel? It shows a section of basin sediment taken parallel to transport (i.e., a dip section), with flow from history. Recently, thanks to the development of the mechanis- right to left. Darker material is lighter and hence more mobile. Distal tic view of Earth embodied in plate tectonics and to improve- part of deposit was formed under water, and proximal part is fluvial. ments in our understanding of sediment dynamics, the strati- Break between light and dark material is a good indicator of shoreline graphic community has developed a first generation of position. The challenge: Deduce history of sediment supply, subsi- quantitative models for the filling of basins and the formation dence, and base level for this section using only information above of stratigraphic patterns. How do we test such models? The and geometry of the preserved deposits. Answer is given in Figure 5. 4 JULY 2001, GSA TODAY studies of modern environments and apply our newly minted models to processes. Most of our understanding of the stratigraphic record—after all, an- sedimentary lithofacies, for instance, cient basins are one setting where it comes from synthesis of the mainly hori- is very hard to check our model results zontal information we have from mod- independently! ern depositional environments with the mainly vertical information provided by The Rationale for the eXperimental ancient deposits. A particularly fruitful EarthScape Facility line of research has clarified the origin In most sciences, carefully controlled of sedimentary structures (e.g., cross- experiments are the preferred means of stratification) in terms of bed forms and testing theoretical models. For a variety other relatively generic features of sedi- of reasons, experimentation has not ment-depositing flows (Allen, 1984; played much of a role in stratigraphic Middleton and Southard, 1984). This re- science, but the experimental approach search, carried out in both field and lab- is well developed in other areas of sedi- oratory, has taught us much about the ment dynamics, particularly in civil engi- alphabet of the language of sediments. neering and geomorphology. One of the The paragraphs and chapters of the main logistical hurdles to experimental sedimentary narrative are written in the stratigraphy is the necessity of including form of larger-scale sequences of sedi- tectonic effects such as subsidence and mentary facies. A basic tenet in stratigra- uplift. We have addressed this by build- phy is that patterns in these sequences ing a large experimental basin that in- are controlled by three main indepen- corporates a unique, flexible subsiding dent variables: sea level, subsidence floor to simulate the development of Figure 2. Schematic diagram of subsidence (rate and distribution), and sediment sedimentary basins under a wide variety mechanism used in eXperimental EarthScape (XES) subsiding-floor experimental basin. supply (e.g., Sloss, 1962). To this trinity of subsidence conditions. This new experimental facility (the eXperimental Pulses of water shot through narrow tubes we should add a fourth variable group knock gravel out of pipe, causing subsidence EarthScape or XES basin) can be used to that controls the efficacy of the transport of gravel surface. system (e.g., water supply for rivers, study the formation of stratigraphy un- wave climate or tidal range for the der completely controlled conditions of Formally, we use theory to link continental shelf). The first attempts to base-level change, subsidence, sediment experiments and field cases. Once a the- understand how changes in these inde- supply, and transport—the same influ- ory has had a good workout in a con- pendent variables are recorded strati- ences that control natural basin stratigra- trolled system, we can be more confident graphically were descriptive. However, phy. The experimental system includes about using it to scale the experimental the physical mechanisms that distribute the most fundamental physical pro- results to the field, to evaluate effects sediment (not to mention biological cesses associated with basin filling— that cannot be scaled down to experi- and chemical processes) are complex river, wave, current, and mass-flow sediments, and to model cases where we enough that it is difficult to model ment transport—and it allows the cannot check the answer independently. stratigraphy using descriptive methods boundaries between transport environ- Stratigraphic experiments are especially alone. Two major developments have ments (e.g., the shoreline) to evolve on well suited for testing formal “inversion” allowed us to create a first generation of their own. The resulting data sets docu- models for reconstructing variables like physically based, quantitative strati- ment spatial and temporal changes in sea level and sediment supply directly graphic models (Cross, 1990; Harbaugh sediment budgets, morphodynamics, from the stratigraphic record (Lessenger et al., 1999; Paola, 2000; Slingerland et and stratigraphic response. and Cross, 1996), and for evaluating al., 1994): (1) development of quantita- The main advantages of experimental how unique such reconstructions are tive models of the mechanics of basin stratigraphy are that boundary condi- (Heller et al., 1993). subsidence, an outgrowth of plate-tec- tions can be controlled, processes with At a more informal level, experiments tonic theory; and (2) improvements in natural analogs that occur over long help build intuition. There is nothing our understanding of how sediment- time scales can be thoroughly docu- like watching a transport system evolve transport systems work. By coupling mented, the resultant deposits can be in front of you and then dissecting it to subsidence and transport, we produce dissected at high resolution and visual- see how the depositional filter has ren- theoretical models of stratigraphy. ized in three dimensions, and transport dered it in stratigraphy. We manipulate (Because of the complexity of the equa- processes can be directly related to de- only boundary conditions. Within the tion systems involved, quantitative strati- positional products. On the other hand, basin, the transport systems organize graphic models are nearly always nu- experimental systems leave some

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