ABSTRACTS FROM SOME RECENT PAPERS:


1996 Wilkinson, B.H., Diedrich, N.W., and Drummond, C.N., Facies successions in peritidal carbonate sequences:  Journal of Sedimentary Research, v. 66, P. 1065-1078.

ABSTRACT

    High-frequency stratigraphic order in epicratonic sections is increasingly attributed to the widespread influence of Milankovitch-band climate forcing and attendant eustatic sealevel change on patterns of limestone and dolostone accumulation throughout much of the Proterozoic and Phanerozoic record.  However, the actual existence of upward-shallowing lithofacies associations has rarely been explicitly demonstrated and, in contrast to such perceptions of periodic and global accumulation, many carbonate sequences can just as readily be interpreted as largely constituting unordered assemblages of various peritidal lithologies.  Examination of published data from several long epicratonic sequences indicates that their division into shallowing cycles is a rather subjective exercise.  Inference of repeated shoaling has commonly relied more heavily on the stratigraphic recurrence of particular units interpreted as representing extremely shallow to exposure conditions than on any documented tendency for groups of lithofacies to actually constitute upward-shallowing associations.  Moreover, cycle definition via such picking of cycle "tops" results not only in a varied assemblage of overlying substitutable "base" and mid-cycle lithologies, but also leads to the designation of cycles that contain a relatively small number of stratal elements; most reported peritidal cycles contain only two stratal elements (mean = 2.25 lithofacies/shallowing cycle; n = 627) and thus are indistinguishable from sequences of randomly stacked peritidal units.
    Comparison of data on thicknesses and numbers of stratal elements in real-world "cyclic" and model chaotic sequences demonstrates that most Proterozoic and Phanerozoic sections exhibit little more meter-scale ordering of component units than would commonly be present in sequences of randomly associated peritidal lithofacies.  On the basis of these considerations, we conclude that meter-scale cyclicity in many if not most epicratonic sequences is more apparent than real, that perceptions of repeated and eustatically driven platform flooding are largely incorrect, and that a substantial component of presumed meter-scale stratigraphic order in peritidal carbonates reflects little more than the random migration of various sedimentary subenvironments over specific platform localities during long-term accumulation of peritidal carbonate.

1997 Wilkinson, B.H., Drummond, C.N., Diedrich, N.W., and Rothman, E.D., Biological mediation of stochastic peritidal carbonate accumulation:  Geology, v. 25, p. 847-850.

ABSTRACT

    Exponential thickness frequencies of peritidal carbonate units in the Lower Ordovician Kindblade and West Spring Creek formations at Ardmore, Oklahoma are readily interpreted in a context of probabilities of upsection transition from one lithology to another.  These largely reflect Poisson (random) processes of deposition from suspended load, traction load, and microbialitic accumulation.  Although grainy to muddy particulate and cyanobacterial elements exhibit nearly equal ranges of unit thickness, carbonate generation and/or entrapment via algally mediated processes was less likely to lapse, and therefore led to lower probability of transition to some other sediment type.  The mean thickness of microbially bound units is roughly double those from the physical transport and deposition of particulate material.  Greater persistence of algal accumulation probably related to intrinsically higher biologically induced rates of carbonate precipitation and/or binding by cyanobacteria.
    Stratigraphic intervals between successive occurrences of suspended load, traction load, and microbial units are also closely approximated by exponential frequency distributions for which regression slopes define probabilities of upsection recurrence of a particular sediment type.  Values for grainy and algal carbonates are similar, and are nearly twice that of muddy suspended-load units.  Although biological processes resulted in significantly lower transition probabilities for thrombolitic bioherms and cryptalgal laminites, spatial dominance of carbonate mud across the region led to higher rates of stratigraphic recurrence and a volumetric dominance of muddy lithologies in the Ardmore sequence.
    Poissonian distributions of unit stratigraphic duration and recurrence suggest a significant component of haphazard variation in the type and amount of accumulated carbonate sediment.  If deposition was influenced by extrabasinal forcing, such control must have been nearly random in both secular and spatial dimensions of water depth change.  Stratigraphic durations and recurrences in this sequence more closely reflect the inherently stochastic nature of carbonate accumulation in epicratonic platformal settings than any influence of rhythmic eustatic forcing.


1997 Wilkinson, B.H., Drummond, C.N., Rothman, E.D., and Diedrich, Stratal order peritidal carbonate sequences:  Journal of Sedimentary Research, V. 67, P. 1068-1078.

ABSTRACT

    Speculation on the depositional origins and geological significance of meter-scale cycles in peritidal carbonates is becoming an increasingly prominent facet of sequence stratigraphic theory, the understanding of which bears directly on their appropriateness as chronostratigraphic entities as well as their usefulness as records of periodic extra-basinal forcing during sediment accumulation.  In spite of the generally wide acceptance of the stratigraphic importance and interpretational significance of meter-scale peritidal cycles, little has been done to quantitatively document the stratigraphic nature of regularly-recurring lithologic associations, or to verify the predominance of such cyclicity in shallow water limestone/dolostone sequences.
    In order to determine the statistical extents and stratigraphic scales of stratal order in such sequences, we have examined several long sections of peritidal carbonate both with respect to the presence or absence of Markovian lithologic transitions, and with respect to the "upward-shallowing" character of lithofacies associations.  In contrast to common wisdom, these measures of stratal order suggest that any lithologic manifestation of meter-scale peritidal cyclicity is relatively uncommon.  All of the several sequences deemed "cyclic" via qualitative inspection in fact contain relative few intervals of demonstrable lithologic order, and even fewer exhibit any tendency for contained units to shallow upsection.  In reality, most shallow water carbonate sequences exhibit little more stratal order than would be apparent in random sequences of peritidal lithologies.
    On the basis of these considerations, we suggest that discrimination of meter-scale cyclicity in epicratonic carbonates is perhaps more perceptional artifact than stratigraphic reality.  Imminent and future efforts intended to fruitfully evaluate the importance of intra- versus extra-basinal processes of sedimentation in shallow low-latitude settings should perhaps eschew more generic perceptions of periodic paleoclimatic forcing in favor of a less regimented view toward the importance of stochastic processes of carbonate accumulation.

1998 Fuks, K.H., and Wilkinson, B.H., Quaternary sedimentation in two northwestern Michigan estuaries:  Journal of Great Lakes Research, v. 24, p. 822-837.

ABSTRACT

    Changing levels of the Laurentide Great Lakes over the past 14.5 ka have strongly influenced development of surrounding coasts.  One of the most striking geomorphic features inherited from earlier levels is the presence of several dozen coast-normal flooded river valleys that occur along the eastern margin of the lake basin.  Westward flowing rivers deeply cut through preexisting terraced fluvial deposits and Pleistocene glacial outwash during the Chippewa lowstands between 10 and 8 ka BP.  Fresh water estuaries were formed as these valleys were flooded during the post-late Chippewa transgression, and as rivers graded their fluvial terraces to Nipissing levels.  More recent (~5 ka BP) attainment of somewhat lower lake levels has been accompanied by regressive progradation of most estuarine deltas.
    Two freshwater estuaries, Manistee Lake and Pentwater Lake, in northwest Michigan were examined in detail.  Both are partially filled with post-late Chippewa sediment comprising terrigenous and organic fluvial, deltaic, and estuary-center facies that comprise an almost complete record of Holocene sedimentation in these settings.  Stratigraphic successions in either estuary consist of a transgressive-regressive fluvial-deltaic-estuarine sequence deposited over the past 10,000 years.  A typical section consists of basal fluvial-deltaic sand, thick (>20 m) transgressive dark brown lake mud (gyttja), regressive interbedded prodelta sand and mud often containing abundant pebble to cobble size wood debris, and highstand fluvial-deltaic sand of modern deltas.  These sequences represent change in Lake Michigan water level during the post-late Chippewa transgression, the Nipissing highstand and stillstand, and the post-Nipissing regression.
    Radiocarbon dates of allochthonous organic material in Manistee Lake indicate an abrupt decrease in accumulation of terrigenous components at about 3.1 ka, and relatively invariant sedimentation rates at about 4.5 m/ka since that time.


1998 Wilkinson, B.H., Diedrich, N.W., Drummond, C.N., and Rothman, E.D., Michigan hockey, meteoric precipitation, and carbonate accumulation on peritidal carbonate platforms:  Bulletin of the Geological Society of America, v. 110, p. 1075-1093.

ABSTRACT

    On Saturday afternoon of March 30th, 1996, the University of Michigan hockey team won the 1996 NCAA Division I national championship.  Durations between Wolverine goals, between opponent goals, and between total goals during the preceding 40-game regular season each define an exponential distribution in which duration frequency only depends on number of shots on goal and probabilities of success.  Compared to opponent scores, UM between-goal duration frequencies define a trend with a steeper slope (UM shot better) and a higher intercept (UM took more shots).
    Over much of the preceding 100 years, meteoric precipitation on Ann Arbor occurred during 11,949 days.  Time durations of the 6,401 precipitation episodes that occurred over this interval, as well as durations of contiguous days of precipitation and contiguous days of drought, each define an exponential distribution in which duration frequency is largely defined by total interval length (35,101 days) and probability of precipitation (34%).
    Road-cuts near Wytheville, Virginia, yield spectacular exposures of a 303.7 meter-thick section of peritidal carbonate in the Middle to Upper Cambrian Elbrook and Conococheague Formations.  Stratigraphic durations (thicknesses) of the 527 lithologic units within this sequence, of the 265 "cyclic" lithofacies associations that can be designated over this interval, and of stratigraphic intervals between recurrences of like lithofacies, also define exponential distributions wherein frequency of stratigraphic recurrence is only dependent on the thickness and abundance of designated stratal elements.
    Frequency of goal scoring, and frequency and/or magnitude of meteoric precipitation can be described in terms of random independent processes at short time scales.  Similarly, exponential distributions of lithologic and "cyclic" thickness frequencies at Wytheville Virginia (as well as in most other epicratonic peritidal sequences) indicate that meter-scale variation in carbonate deposition was predominantly controlled by stochastic (Poisson) processes that were largely unrelated to recurrent intra- or extra-basinal forcing and/or to periodic (rhythmic) sediment accumulation.

1999 Wilkinson, B.H., Rothman, E.D., Drummond, C.N., and Diedrich, N.W., Poisson processes on Holocene carbonate platforms and in Phanerozoic peritidal sequences:  Journal of Sedimentary Research, in press, 30 ms. pages

ABSTRACT

    Cambro-Ordovician carbonate lithofacies units in the Elbrook and Conococheague formations exposed at Wytheville, Virginia, as well as those in many other Phanerozoic peritidal sequences, exhibit exponential thickness frequency distributions.  That is, occurrence frequency decreases exponentially with linear increase in unit thickness.  Such distributions are characteristic of waiting times between independent Poisson events.  This relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions.  Abundances of different lithologies, both as net stratigraphic thickness and as number of occurrences, also decreases exponentially among successively rare sediment types, with each lithology being about 60% as plentiful as the next more abundant rock type.  Relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all Wytheville units of 0.48 m, a relation indicating that thickness distribution is independent of facies type.
    Similar relations are apparent for the horizontal extent of carbonate sediment bodies from the Holocene Florida/Bahama platform.  Areal extents of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution.  Although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior, such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this Holocene surface.  Lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies, with each being about 70% as extensive the next more abundant sediment type.  Relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2.2 x 103 km2 for all Florida/Bahama lithotopes.
    We consider several numerical models of stochastic carbonate accumulation; although not demonstrably unique, scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits.  Such simulations of Poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers.
 To the as yet unknown degree that peritidal lithofacies area and thickness are correlated, data from Paleozoic and Holocene platforms suggest that carbonate units should exhibit length/height ratios of approximately 105.  Given the decimeter scale over which facies are designated in most Paleozoic peritidal successions, these relations predict mean lateral extents on the order of several tens of kilometers, a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in Paleozoic carbonate sequences.


1999 Diedrich, N.W., and Wilkinson, B.H., Depositional cyclicity in the lower Devonian Helderberg Group of New York State:  Journal of Geology, 25 ms pages, submitted.

ABSTRACT

    The Helderberg Group of New York state consists of a wide range of shallow water carbonate lithologies, and contains one of the finest and most complete stratigraphic records of Lower Devonian earth history anywhere in the world.  As has been the case for numerous other peritidal carbonate successions, a hypothesis has been developed for the Helderberg Group suggesting that "the stratigraphic record consists of small-scale (meter-scale) shallowing-upward cycles", for which "glacial eustasy driven by orbital perturbations" is the preferred mechanism of their formation (Goodwin and Anderson, 1985).
    The spatial and temporal rhythm of carbonate accumulation in the Helderberg Group has been evaluated by examining thickness frequency distributions of individual sedimentation units in flasered ribbon rock and of individual lithofacies elements.  The form of these distributions provides important insight into magnitudes and temporal recurrences of Lower Devonian depositional processes that ultimately controlled stratal thicknesses.  In both cases, observed thickness frequencies are closely approximated by the exponential gamma distribution, wherein flaser and lithofacies thicknesses are only dependent on net sequence length and number of identified stratal elements present.  These are the thicknesses frequencies that would be expected if lithologic change were a Poisson process; that is, if horizons of lithologic change occur more or less randomly throughout various Helderberg Group sections.  At a scale of individual bedding units and individual lithofacies, Helderberg Group deposition was primarily a stochastic rather than a deterministic process.  Such patterns of Poisson lithologic variation strongly suggest a near-complete absence of regularly-recurrent depositional forcing during accumulation of the Helderberg Group.
    Distributions of thicknesses of supposedly upward-shallowing lithofacies associations, and of magnitudes of sea level rise inferred from amounts of lithofacies change across shallowing cycle boundaries, exhibit nearly identical log-normal patterns of thickness/magnitude recurrence in Helderberg Group sections.  These are also readily interpretable in a context of largely Poisson processes of carbonate accumulation.  If upward-shallowing runs were to be designated in any random sequence of peritidal lithofacies elements, resultant "associations" (be they ascending or descending) should comprise generally log-normal thickness frequencies that would be virtually identical to those inferred for the Helderberg Group.
    On the basis of these considerations, we conclude that bedding unit and lithofacies thicknesses in the Helderberg Group largely reflect the more or less random migration of the Lower Devonian facies mosaic across New York State during sediment accumulation.  Stratigraphic extents of upward-shallowing lithofacies associations and magnitudes of apparent punctuated deepening across shallowing lithofacies "cycles" are little different than those anticipated for the chance grouping of stratal units randomly drawn from an exponential population of such elements.  If any sea level control is manifest in patterns of sediment accumulation in this Lower Devonian succession, such changes must have been nearly random with respect to both secular recurrences and eustatic magnitudes of sea level change.