Drawing a Line in the Sand: Identifying and Characterizing Boundaries in the Geological Record
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Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Drawing a line in the sand: identifying and characterizing boundaries in the geological record ALWYNNE B. BEAUDOIN 1 & MARTIN J. HEAD 2 1 Quaternary Environments, Provincial Museum of Alberta, 12845-102nd Avenue, Edmonton, Alberta, T5N OM6, Canada (e-mail: [email protected]) 2 Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK (e-mail: [email protected]) Abstract: The identification and characterization of boundaries is a fundamental activity in geoscience. Spatial and temporal boundaries are rarely sharp but are more usually zones of transition, which may have variable characteristics. The examination of palynological and micropalaeontological data is often crucial for the delineation of geological boundaries, especially for the definition of Global Boundary Stratotype Sections and Points (GSSPs). The sixteen papers in this volume highlight many productive methodological approaches to boundary identification. This essay reviews the theoretical background to boundary identification in geology, and provides the contextual perspective for the subsequent papers. Much of geoscience has to do with classification, guidelines such as the North American Strati- of organisms, of rocks, or of processes. Inevi- graphic Code (North American Commission on tably, classification devolves into an exercise in Stratigraphic Nomenclature 1983), or the Strati- drawing boundaries: putting a limit on what is graphic Procedure (Rawson et al. 2002). The included and what is not. In the geological demarcation of boundaries is generally a con- record, this exercise takes place in three dimen- sequence of the identification of units, rather sions, with space and time interacting to produce than the other way round. Historically, the a plethora of boundaries and boundary defini- identification of boundaries has often arisen tions. Although conventionally, on stratigraphic from the practical exigencies of geological charts, for example, temporal boundaries are mapping: the need to produce visual representa- often shown as sharp lines, in reality they are tions of geology, primarily based on sections or rarely so. Similarly, spatial boundaries on maps surface exposures. Boundaries, therefore, were are seldom as abrupt as they are drawn. Process often synonymous with the limits of mappable boundaries are yet more subtle and usually less units, that were generally field-identified on the clear. Indeed, even to speak of 'a boundary' is basis of lithology. This approach gave rise to the contentious, since boundaries come in many designation of 'body stratotypes', a methodol- forms and types. Boundaries are more often ogy that proved limiting for the subdivision of intervals or spaces of transition, leaving plenty geological time because of gaps in the rock of room for argument over their placement. record (Harland et al. 1990). "erhaps for this reason, some of the more Modern geochronology emphasizes bound- truculent and long-lasting debates in geology aries, especially lower stage boundaries, rather have been over the positioning and identification than units, as fundamental for subdivision and of boundaries. Drawing a line in the sand may correlation (Remane 2003). Under the auspices seem a simple exercise but turns out to be of the International Commission on Stratigra- fraught with complexities! phy (ICS), various subcommissions have been established to examine the placement of parti- ]oundary definitions cularly critical temporal boundaries in the geological record (see list in Gradstein & Ogg In geoscience, the criteria for identifying chron- 2003). Considerable effort has been devoted to ostratigraphic, biostratigraphic and lithostrati- the establishment of Global Boundary Strato- graphic units are outlined in rules set up by type Sections and Points (GSSPs), with more international agreement, such as the Interna- than 40 defined so far (Gradstein et al. undated). tional Stratigraphic Guide (Salvador 1994), or For each boundary, a type section and a specific From: BEAUDOIN, A.B. & HEAD, M.J. (eds) 2004. The Palynology and Micropalaeontology of Boundaries. _Geological Society, London, Special Publications, 230, 1-10. 0305-8719/04/$15 @ The Geological Society of l,nndc)n "~OOt Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 2 A. B. BEAUDOIN & M. J. HEAD point within that section are chosen by interna- which this volume is primarily concerned. tional agreement as a means to define the However, it is worth noting that some geological boundary formally. The type section is therefore boundaries may be a consequence of investiga- a particular section at an identified location (ICS tive boundaries, where research or analyses are 2004). Although parastratotype sections may be constrained by jurisdictional or political limits, established at considerable convenience to local such as those that confine national geological and regional stratigraphers, ultimately all surveys. Establishing boundaries may be more securely identified boundaries must be correla- than an esoteric exercise; boundary definitions table to the GSSP. In practice, almost all GSSPs may have important management implications, are defined from shallow-marine sediments. as in the establishment of ecological reserves, or Macrofossil biostratigraphy is used for many legal consequences, as in the definition of ~SSP definitions, particularly in the Mesozoic continental margins (e.g. Hedberg 1979). (see list in ICS 2004). However, changes in Spatial boundaries differ in type and degree. micropalaeontological indicators - especially Boundaries may be abrupt or gradual, solid or conodonts, calcareous nannofossils, and forami- permeable, permanent or ephemeral, constant or nifera - often underpin these definitions. Micro- fluctuating, stationary or moving, narrow or palaeontology is becoming increasingly broad, relatively straight or highly convoluted. important, particularly for GSSPs in the Cen- Where a boundary is defined by a physical ozoic and Palaeozoic, due to the more contin- landscape expression or environmental disconti- uous nature of microfossil recovery. Since it acts nuity, its character may affect the way in which as an exemplar, the selection of a type section is organisms react to it. Moreover, conditions that often a difficult matter, and candidate sections form a boundary for one type of organism may are minutely scrutinised (see Sikora et al. and have no impact on another. Physical boundaries Mei et al.). The establishment of a GSSP rarely can also act as a filter, only allowing certain ends discussion or extinguishes debate; GSSPs organisms to pass through. For these reasons, may be challenged, re-examined, and defended the identification and characterization of a (Zhang & Barnes a). boundary through biotic indicators may depend on what organism is being examined as a proxy ]oundary types and the sensitivity of that organism to change. Boundaries may also regulate flows of materials Surprisingly, despite all the attention paid to or energy (Cadenesso et al. 2003b). Where a boundaries, there is no cohesive theoretical boundary is defined by a perturbation, the treatment of them in the geological literature, magnitude, extent, and duration of that pertur- although stratigraphic principles have been bation may influence biotic response. Some widely discussed (e.g. Hedberg 1976; Salvador biota may show considerable complacency or 1994; Remane 2003; Walsh 2004). This situation resiliency until critical threshold values are differs from ecology, where there is an emerging crossed. body of literature dealing with boundary theory (see Gosz 1991, and Cadenasso et al. 2003a, 2003b). Because ecology deals with living organ- isms and their environment, this theoretical Analogues from modern ecological and approach is also applicable to palynology and environmental boundaries micropalaeontology. Strayer et al. (2003) distin- guish two major types of boundaries: investiga- The identification of boundaries on the modern tive and tangible boundaries. Investigative landscape provides many examples of these boundaries are those that are imposed by different types. Here, we can examine bound- practical or administrative considerations and aries in the simplest spatial cases, with the often have no physical expression in the land- complication of geological time removed. How- scape. Investigative boundaries can also occur ever, the identification of boundaries across the when language barriers preclude discussion or landscape, and the characterization of transi- access to information. Nikitenko & Miekey, for tions, provides analogies for the 'space for time' example, review an enormous quantity of substitution which is the foundation of the literature from Russia, which has not hitherto 'present is the key to the past' approach to been accessible to people who do not read geoscience. Some boundaries are obvious, the ~,ussian. Spatially speaking, tangible boundaries land-sea transition, for example, and have long are associated with some biotic change, environ- been a focus of research. State-change bound- mental discontinuity, or landscape expression. It aries, such as the water-atmosphere or ice-water is the exploration of tangible boundaries with interfaces, are also clearly marked. Other Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 IDENI'IFYING AND CHARACTERIZING BOUNDARIES 3 boundaries, such as temperature limits,