Strata and time: probing the gaps in our understanding

Abstract

Sedimentary strata are the paramount source of geohistorical information. The 'frozen accidents' of individual deposits preserve evidence of past physical, chemical and biological processes at the Earth's surface, while the spatial relationships between strata (especially superposition) yield successions of events through time. There is, however, no one-to-one relationship between strata and time, and the interpretation of the stratigraphic record depends on an understanding of its limitations. Stratigraphic continuity and completeness are unattainable ideals, and it is the departures from those ideals -the often cryptic gaps in the record -that provide both its characteristic texture and the principal challenge to its analysis. The existence of gaps is clearly demonstrated by consideration of accumulation rates, but identifying and quantifying them in the field is far more difficult, as is assessing their impact on the degree to which the stratigraphic record represents the environments and processes of the past. These issues can be tackled in a variety of ways, from empirical considerations based on classical field observations, to new ways of analysing data, to the generation and analysis of very large numbers of synthetic datasets. The range of approaches to the fundamental questions of the relationship between strata and time continues to expand and to challenge long-established practices and conventions.Superposed sedimentary strata are the most accessible routes into deep time, and acceptance of their historical significance was a major scientific breakthrough. Given that the study of strata has been undertaken in something like its modern form for over two centuries, stratigraphy as a scientific discipline might be expected to have stabilized, as perhaps is indicated by stratigraphy textbooks suggesting that the subject is widely regarded as boring. Yet if there is a problem with stratigraphy, it is the converse: its development is increasingly punctuated by paradigm shifts triggered by new theories (evolution; global tectonics; eustasy; orbital forcing of climate change) and technological breakthroughs (digital computing; continuous seismic profiling; isotopic methods in chronology and palaeoclimatology). With this accelerating progress, it has become increasingly clear that the stratigraphic record yields only snapshots of Earth's past surface processes -the 'frozen accidents' that give the record its character and its enduring fascination. 'Time is missing from sedimentary sequences on all scales . . . This discontinuity gives recorded planetary (geological) time a different architecture to human time' (Paola, C. 2003. Floods of record. Nature, 425, 459).Strata and Time: Probing the Gaps in our Understanding was the title of the Geological Society's William Smith Meeting for 2012. Its aim was to explore the relationship between the preserved sedimentary rock record and the passage of geological time, identifying, evaluating and updating the models that lie behind current stratigraphic method...

Geological Evidence of Planet‐Wide Groundwater System on Mars

“…More recently, Kemp and Sadler () demonstrated that the number and duration of intervals of nondeposition or erosion increases over time. In stratigraphy it is also now recognized that the duration of the gaps, the distribution of layer thicknesses and the sedimentation rate have fractal properties ( Bailey & Smith, ; Plotnick, ; Sadler, ; Schlager, ; D. G. Smith et al, ). Furthermore, a recent Martian model proposed by Horvath and Andrews‐Hanna () predicts the presence of numerous small lakes around Gale crater even if there are no sediments preserved within them or other geological evidence that supports past groundwater upwelling events.…”

“…More recently, Kemp and Sadler (2014) demonstrated that the number and duration of intervals of nondeposition or erosion increases over time. In stratigraphy it is also now recognized that the duration of the gaps, the distribution of layer thicknesses and the sedimentation rate have fractal properties ( Bailey & Smith, 2005;Plotnick, 1986;Sadler, 1999;Schlager, 2004;D. G. Smith et al, 2015).…”

A new approach to quantifying stratigraphical resolution: application to global stratotypes

“…GSSPs represent the benchmarks for definition of all of the principal boundaries of the geological time-scale (e.g. Smith et al 2014). Each GSSP is a specific stratigraphical horizon in a particular section that has been chosen to formally define the basal boundary of any chronostratigraphical unit at the stage level or higher.…”

“…Davies et al 2011;Melchin et al 2012) and are in need of restudy. The base of the Silurian System has been formally redefined in terms of its biostratigraphical definition (Rong et al 2008), and several of the other Silurian GSSPs are currently under restudy (Smith et al 2014).…”

“…The criteria for selection of GSSPs were outlined by Hedberg (1976), and, more recently, by Remane et al (1996) and Smith et al (2014). In addition to criteria pertaining to continuity of sedimentation and facies, lack of deformation or metamorphism, good exposure and accessibility, the key attributes of a good GSSP candidate section relate to the presence of a diverse and well-preserved fossil biota and as many other stratigraphical attributes as possible (e.g.…”