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and are postulated to have been driven
by orbital forcing on eustatic sea level.
Consequently, multiple statistical techniques
have been developed to evaluate
whether patterns of stratigraphic succession
are more consistent with a periodic
sea level signal or are stochastic. Previous
studies focused on development
and/or application of such methods to
test empirical geological records. However,
the character of such records cannot
be known a priori, as deposition,
erosion, and preservation influence the
resultant composition, nor can those records
be readily manipulated to explore
the sensitivity, robustness, and overall
validity of statistical methods. Here we
simulate carbonate layers using computer-modeled
successions generated by
periodic sea level changes. The resulting
stratigraphic records were then evaluated
statistically. Thickness distributions
of simulated lithofacies were compared
to distributions predicted for Poisson
processes, which by definition are not
driven by cyclical sea level changes. Our
results suggest that periodic processes
produce stratigraphic thickness frequencies
that are difficult to distinguish from
random frequencies except under highmagnitude
sea level fluctuations. Similarly,
autocorrelation fails to correctly
recognize cyclic patterns in such simulated
records. Models with highmagnitude
sea level fluctuations (icehouse
conditions) had thickness frequencies
that are suggestive of orbital
forcing, whereas low-magnitude sea
level fluctuations (greenhouse conditions)
appeared independent even
though they were modeled using Milankovitch
orbital forcing. The increasing
evidence based on spectral data from
real rock successions suggests that Milankovitch
drivers are common in both
icehouse and greenhouse periods. Because
statistical approaches are unsuccessful
in recognizing the cyclic driver
of these simulated records, we infer that
it is difficult to disprove independence
from real stratigraphy even when orbital
forcing is controlling the rock composition.
Even in the necessarily simplified
world of computer simulations, the numerous
factors involved in depositing
stratigraphic successions work to complicate
or mask any periodic signal, thus
generating the appearance of stochasticity
in some successions.
2010010556
埋 藏 学 和 风 暴 沉 积 形 成 的 陆 壳 层 : 以
白 垩 纪 西 部 内 陆 盆 地 为 实 例 = Taphonomy
and sedimentology of stormgenerated
continental shell beds: A case
example from the Cretaceous western
interior basin. ( 英 文 ). Roberts, E M;
Tapanila, L; Mijal, B. Journal of Geology,
2009, 116(5): 462-479
An extraordinary continental shell bed
is reported from the Upper Cretaceous
Kaiparowits Formation in southern Utah.
This shell bed, referred to as the Kaiparowits
Blues Ceratopsian shell bed, is
highly unusual among fluvial-estuarine
shell beds for its great thickness, surface
area and shell density and its geometry.
It covers > 850 m(2), ranges from 85 to
280 cm thick, and consists of a series of
10-50-cm-thick low-angle, dipping beds.
The shells are generally undamaged and
articulated (> 75%), commonly with
valves still closed, and strongly oriented
normal to the dip orientation of the shell
layers. The shell bed is interpreted as a
lateral accretion bar set that developed
in a point bar or midchannel bar setting.
Nearly 45% of shells are encrusted by
the brackish-water bryozoan Conopeum
sp., indicating that deposition transpired
within the upper-fluvial to mixedfluvial-marine
part of an estuarine channel
system. At least five unionoid (Unionoidea)
shell morphotypes are present,
representative of both parautochthonous
(intrachannel) and allochthonous (adjacent
quiet-water pond/marsh) taxa. Taphonomic
and sedimentologic investigations
suggest that rapid winnowing and
amalgamation of live and recently dead
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