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however, is a thick, remarkably complete
sequence of Oligocene through
early Miocene strata ( the John Day
Formation) potentially amenable to addressing
these shortcomings and long
known to harbor one of the richest records
of mid-Tertiary mammals in North
America. Since Prothero and Rensberger's
first magnetostratigraphic study of
the John Day Formation in 1985, new
advances in geochronology, together
with a more comprehensive suite of paleomagnetic
sections keyed to new radioisotopic
and biostratigraphic data,
have greatly enhanced chronostratigraphic
precision. In our attempt to refine
John Day chronostratigraphy, we
sampled nearly 300 sites for magnetostratigraphy
over a 500-m-thick interval
and used several radioisotopically dated
volcanic tuffs for our correlation with
the geomagnetic polarity timescale.
Many of the rocks analyzed showed unusual
magnetic behavior, possibly due to
the known zeolitization in this region,
thereby precluding an abundance of
class 1 polarity determinations. Nevertheless,
preliminary results indicate that
the Turtle Cove Member stratigraphically
upward through the lower Kimberly
Member extends from late chron
C12n through C7n. 1r, or from about
30.6 to 24.1 Ma. Intensive radioisotopic
and magnetostratigraphic characterization
of these strata provides a framework
by which the associated biostratigraphy
is assessed for biochronological significance
relative to fossiliferous successions
of the Great Plains, in turn resulting
in reassessment of Arikareean subbiochron
(Ar1-Ar4) boundaries. We present
a revision of those boundaries that
differs from their traditional timing as a
hypothesis for testing in other locations.
2010010017
热 障 和 坡 栖 动 物 群 的 毁 灭 = Thermal
barriers and the fate of perched faunas.
( 英 文 ). Stanley S M. Geology, 2010,
38(1): 31-34
Thermal barriers provide an explanation
for the geologically sudden extinction
of benthic faunas of epeiric seas
when these seas disappeared by contracting
to the open ocean. Biotic interactions
could not have caused the sudden extinctions,
and neither could reduction of
the regional area of seafloor because
substantial areas of shallow seafloor remained
along neighboring continental
shelves when epeiric seas drained. Instead,
temperature contrasts must have
been responsible. Epeiric seas had
strongly seasonal climates, and when
some receded to continental margins,
many of their species would have encountered
waters that failed to provide
the maximum or minimum temperature
required for reproduction. When epeiric
seas receded poleward, equatorward, or
into Coriolis-driven currents, many species
faced lethal temperatures. The history
of the Jurassic Sundance Sea provides
a striking example of the fate of a
warm-adapted fauna driven westward
into an area dominated by a cool, Coriolis-driven
current.
2010010018
加 拿 大 纽 芬 兰 5.65 亿 年 的 错 点 组 中 埃
迪 卡 拉 生 物 群 自 主 移 动 的 首 个 证 据 =
First evidence for locomotion in the
Ediacara biota from the 565 Ma Mistaken
Point Formation, Newfoundland.
( 英 文 ). Liu A G; Mcllroy D; Brasier M
D. Geology, 2010, 38(2): 123-126
Evidence for locomotion in the Precambrian
fossil record is scant. Reliable
Ediacaran trace fossils are all younger
than 560 Ma, and consist of relatively
simple horizontal burrows and trails
from shallow-water deposits. Here we
describe an assemblage of macroscopic
locomotory traces from deep-water environments
at Mistaken Point, southeastern
Newfoundland, Canada, dated to ca.
565 Ma. These trails extend the record
of complex trace fossils back into the
earliest Avalonian biota. Our new evidence
for large motile organisms on the
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