Estimating palaeo-water depth from the physical rock record
Introduction
Water depth (or bathymetry) is a fundamental parameter of any subaquatic sedimentary environment. In conventional terms, depth implies the vertical interval at a point between two specific interfaces or discontinuities: one being the sediment–water and the other being the water–atmosphere interface (Allen, 1967). Attempts to reconstruct palaeo-water depths from the fossil rock record are usually referred to as “palaeo-bathymetry”. The term “bathymetry” is derived from the Greek “βαθυς”, deep, and “μετρον”, measure. In other words, bathymetry is the study of underwater depth, of the third dimension of lake or ocean floors, whereas palaeo-bathymetry aims at establishing past water depths from the geological record.
Early techniques for measuring water depths used pre-measured heavy rope or cable lowered over a ship's side. Today, the bathymetry of oceans and lakes is commonly quantified by means of an echosounder (sonar) mounted beneath or over the side of a vessel, “pinging” a beam of sound downward at the seafloor. The amount of time it takes for the sound to travel through the water, bounce off the seafloor, and return to the sounder tells the equipment how far down the seafloor is. Modern multibeam echosounders, mounted on research vessels (Krastel et al., 2001, Roberts et al., 2005), are featuring dozens of very narrow adjacent beams arranged in a fan-like swath and provide very high resolution topography maps and the resulting seafloor bathymetry. Alternatively, a radar altimeter mounted on an orbiting spacecraft (Sandwell et al., 2006) can efficiently measure slight variations in ocean surface height and hence seafloor topography and bathymetry.
Geologists, however, dealing with ancient limnic or oceanic deposits, their coastal settings and ancient epicratonic seas cannot make use of such techniques and depend on estimates based on a series of features recognized in the rock record. These features, although cornerstones in palaeo-bathymetric interpretation, are subject to limitations as proxies for palaeo-water depth. Various methods reconstructing palaeo-water depth have been proposed but none is universally applicable (Allen, 1967, Hallam, 1967, Eicher, 1969, Benedict and Walker, 1978, Sundquist, 1982, Clifton, 1988, Brett et al., 1993, ten Veen and Kleinspehn, 2000, Della Porta et al., 2002a, Della Porta et al., 2002b, Immenhauser and Scott, 2002, John et al., 2004, Ryan et al., 2007). Occasionally, a geological situation is encountered in which a direct quantification of absolute palaeo-waterdepth is possible. In this case, the gross geometrical and altitudinal relationship between two exposures of the same formation, or between different formations, is visible in outcrop or can be inferred from subsurface data (Newell et al., 1953, Swann et al., 1965, Shelton, 1965, Soreghan and Giles, 1999, Bahamonde et al., 2004, van der Kooij et al., 2007). This situation is, however, the exception rather than the rule. Most workers use (and combine) sedimentological, lithological, taphonomical, biological or chemical evidence and their typically association with specific water depths as observed in modern marine or lacustrine environments and apply these findings to fossil case settings. Here, the focus is on sedimentological parameters.
I wish to make clear that this paper does not address larger issues of palaeo-bathymetry in general but rather serves the reader by providing a general overview and introduction to palaeo-waterdepth hindcasting from the physical rock record. Consequently, the aim of this paper is to provide a practical and relatively jargon-free guide for non-specialistic (field) geologists, as well as sedimentologists, sequence stratigraphers and those concerned with palaeo-environmental analysis and backstripping studies of sedimentary successions. The paper is organized as follows: the first part (2 Ancient seas and modern analogues, 3 Waves and wave theory, currents, storm hydrodynamics, effective wave base, preservation potential of bedforms and threshold limits of sediment transport) introduces to those hydrodynamic processes that relevant for the interpretation of sediment transport under waves and currents and the formation of related bedforms. This is essential reading for those concerned with palaeo-water depth estimates. The second part (Section 4) summarizes the bathymetry of present-day neritic carbonate and siliciclastic settings in a brief manner. The third part (Section 5) describes and discusses diagnostic facies and features that are of significance for palaeo-water depth reconstruction. A tentative separation in (i) relative, (ii) semi-quantitative and (iii) quantitative palaeo-water depth indicators is proposed. The later ones, and particularly reconstructions of wave climates from vortex ripples, are given priority as they allow for a rather rigorous hindcasting of past water depths. An overview of bathymetric ranges for depth-indicative facies and features within their relative error bars is shown in Table 1. Extensive lists of references for further reading are presented in each section.
Section snippets
Ancient seas and modern analogues
Two major types of marine water bodies are commonly separated: (1) Open oceanic (pelagic) seas, covering oceanic lithosphere, and (2) epicontinental seas, covering continental lithosphere (see references in Immenhauser et al. (2008). Epicontinental seas today (and through geologic time) again fall into two broad categories: (2a) Epeiric–neritic seas that are open or semi-enclosed basins extending into the interiors of continents. In the modern world, these include for example the Yellow Sea,
Water waves and currents
Surface waves are features that occur in the upper layer of the oceans. They usually result from distant or local winds or geological effects and may travel thousands of kilometres before striking land. Waves range in size from small surface ripples to tsunamis (Bahlburg and Weiss, 2007, Holthuijsen, 2007, Xu et al., 2007). Here the focus is on wind waves having periods of between about 1 and 20 s. Surface wind waves are caused by gravity acting on surface disturbances related to wind stress. As
Bathymetry of shoalwater carbonate environments
Schlager (2005) proposes a subdivision of marine carbonate depositional settings in T (tropical), C (coolwater) and M (mound) factories. Here the focus is mainly on the bathymetry of tropical and subtropical carbonate settings. Modern carbonate margins are highly variable with complex morphologies that differ in several aspects from their siliciclastic counterparts but similarities exist as well (Bathurst, 1980, Hine and Mullins, 1983, Quinn and Matthews, 1990, Pomar and Hallock, 2008). Tucker
Palaeo-waterdepth hindcasting from the physical rock record
This section describes diagnostic facies associations and physical structures in the rock record that are of significance for palaeo-water depth estimates. Here an approach has been chosen that subdivides these features in (i) relative, (ii) semi-quantitative and (iii) quantitative indicators of palaeo-water depths. All of these features and their relative error bars are shown in overview in Table 1.
Conclusions
Water depth and its change with time is a fundamental parameter in palaeo-environmental reconstruction, albeit one that is notoriously difficult to obtain from the rock record. The ancient epeiric seas of the Palaeozoic and Mesozoic, inundating vast cratonic areas, have few counterparts in the modern glacial world and the application of observations from modern oceans to the rock record of the Precambrian world is even more controversial. Consequently, the choice of the appropriate modern
Acknowledgements
The following are greatly acknowledged: E. van Bentum, J. Franik and S. Hahn assisted with the drafting of several of the illustrations and aided in compiling relevant literature; H. Bahlburg suggested references for tsunami deposits; T. Reisner provided well-constrained ripple data and their mathematical treatment from Het Hilgelo lake in the Netherlands; J. Renner advised the author with regard to mathematical aspects of hydrodynamics; K.-F. Dämrich, S. Mai and C. Paesler kindly provided
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