Methods for improving the chronometric time-scale

Smith, Alan G.

doi:10.1144/gsl.sp.1993.070.01.02

Cited by 4 publications

(3 citation statements)

References 21 publications

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“…It must also discount model studies suggesting that certain sedimentary processes may generate regular or irregular repetitions of facies (autocycles, Burgess, 2006; autostepping, Muto and Steele, 2001) with no external forcing. Model (c) is consistent with the fractal nature of most stratigraphic layering, (Bailey and Smith, 2005, 2008), with the power‐law scaling of hiatuses in the record (Smith, 1993; Sadler, 1999) and with the criticality of Earth’s surface systems (Bak, 1997; Bailey, 1998; Jensen, 1998; Phillips, 1999; Bak and Chen 2001; Hergarten, 2002). Yet it appears to be comprehensively refuted by the claimed ubiquity of cycles that provide evidence for orbital forcing of past sedimentary processes (Hilgen et al.…”

Section: Cyclostratigraphic Time Calibration: Synthesissupporting

confidence: 55%

Cyclostratigraphic reasoning and orbital time calibration

Bailey¹

2009

Terra Nova

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Cyclostratigraphy postulates that the stratigraphic record cyclically encodes the periodic orbital forcing of terrestrial insolation, thus providing a time calibration. Regular cycles are sought using spectral analysis of lithological data series, but there are inherent ambiguities in this method. It may detect more cyclicities than conventional orbital forcing allows, but only those with the closest correspondence to estimated orbital frequencies are used for time calibration. Irregular cycles are subjectively defined in terms of non‐rhythmic repetitions of facies. The calibrations assume that they record the spatially distorted sedimentary effects of orbitally forced periodicity in insolation. The null hypothesis that such non‐rhythmic repetitions are autogenic, rather than orbitally forced, cannot, however, be rejected. Both types of cyclicity conform to an ‘expected universe’ where orbital forcing is reliably and recognisably encoded in the stratigraphic record. Neither form of cyclicity rules out the presence of hiatuses; thus, even if orbital in origin, neither can provide dependably refined, orbitally scaled, time calibration.

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Section: Cyclostratigraphic Time Calibration: Synthesissupporting

confidence: 55%

Cyclostratigraphic reasoning and orbital time calibration

Bailey¹

2009

Terra Nova

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“…rates conform to this pattern (Sadler 1981;Plotnick 1986), implying that stratigraphic hiatuses are fractal, existing at all scales (Smith 1993;Sadler 1999). It follows that accumulation rates derived from the stratigraphic record differ according to the duration of the interval considered.…”

Section: Continuity Completeness and The Geologic Time Scale (Gts)mentioning

confidence: 84%

Strata and time: probing the gaps in our understanding

Smith¹,

Bailey²,

Burgess

et al. 2015

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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...

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“…Although none of these assumptions is likely to be true, they cannot be avoided altogether if a relative scale is needed for interpolation. Unless applied judiciously and explicitly, they surely compromise the interpolation task (Smith, 1993).…”

Section: Traditional Approaches To Paleozoic Timescale Problemsmentioning

confidence: 99%

High-resolution, early Paleozoic (Ordovician-Silurian) time scales

Sadler¹,

Cooper²,

Melchin³

2009

Geological Society of America Bulletin

145

112

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For much of the geologic time scale, resolving power appears to be limited by the duration of biostratigraphic zones and subzones. Yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span. Computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line. An illustrative case history builds an Ordovician and Silurian time scale from the geologic record of the entire graptolite clade-over 1900 species from more than 400 localities and sections worldwide. The same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations. It provides the foundation for time scales and paleobiologic time lines.Using logic similar to graphic correlation, optimizing algorithms search for a composite sequence of species fi rst-and last-appearance events that minimizes the implied shortcomings of all the fi eld observations. The algorithms minimize the number of species coexistences that are implied, but never observed, and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events. After total section thicknesses have been rescaled to help normalize for variation in depositional rate, mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence. The resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events. This relative scale is, in turn, calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence. These dated events are also used to test for linearity of the scaled composite.The fi nal time scale has a potential resolving power of 0.02-0.1 m.y., more than ten times better than can be achieved by traditional zones. Graptolite zones vary widely in duration from as short as 0.1 m.y. to nearly 5.0 m.y . The mean duration of zones or zonal groupings calibrated here is 1.44 m.y. in the Ordovician and 0.91 m.y. in the Silurian. The average uncertainty in locating zone boundaries in a single composite sequence is about one-fourth of the mean zone duration. The variance resulting from differences in time scales developed from differing numbers of fi eld observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method. These differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration. For zone duration, the mean uncertainty amounts to about onehalf of the length of an average zone.

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Methods for improving the chronometric time-scale

Cited by 4 publications

References 21 publications

Cyclostratigraphic reasoning and orbital time calibration

Cyclostratigraphic reasoning and orbital time calibration

Strata and time: probing the gaps in our understanding

High-resolution, early Paleozoic (Ordovician-Silurian) time scales

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