Transcript Permo-Carboniferous Glaciation

Permo-Carboniferous
Glaciation
Charlie Boland
11/25/09
ESS433
The Paleozoic
Estimates of Earth’s spin
(geographic) axis relative to
Gondwana from glacial
sediments and
paleomagnetism
A.G. Smith
October 1996
An estimation of Paleozoic ice
centers
• Using a Quaternary analog
• Paleomagnetism
• Glacial sediments
Paleozoic ice ages
• All glaciation centered on Gondwana
• Milankovitch frequencies of early
Paleozoic analogous to Quaternary
• Widespread distribution of glacial
sediments: South America, Africa,
Madagascar, Arabia, India, Antarctica, Australia
• Paleozoic glacial sediments in China
previously from Gondwana
• Some evidence lost to tectonic processes
• Some uncertainty of the extent of ice mass
• Ice sheet assumed to be extensive,
perhaps reaching up to 50°
• Paleo-South pole estimated to be at 30° S,
47° E during maximum glaciation
(uncertainty of 5°)
Permo-Carboniferous Ice Sheet
Using a Quaternary Analog
• Data from most recent glaciation
considered a possible analog for
Paleozoic
• Similar Milankovitch frequencies
• Known pole distribution
• Lowest latitudes ice reached
• Predicts that ice would be unlikely to
extend North of 40°S or deviate over 17°
from South pole
Paleomagnetism
• Iron-bearing minerals leave paleomagnetic
signatures
• Because there are no known hot spots for
Paleozoic Gondwana, all non-stratigraphic
evidence is paleomagnetic
• All data is from averaging VGPs to
determine a geocentric axisymmetric
dipole field for Paleozoic and Quaternary
Selecting Paleomagnetic dataQuaternary
• All poles selected were from the ORACLE
databases
• All poles selected had primary
magnetization the same age as rocks
selected from
• Initial dataset of 313 poles, dropped 75 that
had not been magnetically tested
• Mean pole is 89.3°N, 189°E with α95 of 1.2°
• Mean pole indistinguishable from spin axis
α95
• Standard method for evaluating
paleomagnetic errors
• The value implies that there is a 1/20
chance that the mean pole lies outside a
circle with radius α95 outside the mean
• Measure of precision
Selecting Paleomagnetic dataQuaternary
• All poles selected were from the ORACLE
databases
• All poles selected had primary
magnetization the same age as rocks
selected from
• Initial dataset of 313 poles, dropped 75 that
had not been magnetically tested
• Mean pole is 89.3°N, 189°E with α95 of 1.2°
• Mean pole indistinguishable from spin axis
Paleomagnetic data- Paleozoic
• Gondwanan poles from 490-250 Ma selected
from ORACLE database
• Again, all poles had magnetization ages in
common with the rocks were used
• Poles with uncertainties of over 40 Ma were
excluded
• All demagnetized poles discarded
• All poles with significant error were found to be
from areas of complex tectonic folding or strikeslip faults
Paleozoic Paleomagnetism Data
• Small number of
poles in Paleozoic
data set– sliding
window of 60 Ma
applied
• Mean pole
calculated at 20 m.y.
intervals
Permo-Carboniferous Paleomagnetic Data
Interpretations
• Datasets are compatible with Quaternary
analog provided the ice center migrated
eastwards
• Presumed ice cover of Gondwana as
asymmetric as Pleistocene ice sheets
were
Summary
• Glacigenic sediments indicate the ice
cover varied across the PermoCarboniferous
• Glaciation originated west and migrated
east over time (centroid migrated as well)
• Permo-Carboniferous South pole estimate
falls within 20° of the centroid
• Present-day South pole centroid lies 5°
from magnetic South pole
• The 20° circle around PermoCarboniferous centroid overlaps all α95
values
Reevaluation of the timing and extent
of late Paleozoic glatiation in
Gondwana: Role of the Transantarctic
Mountains
J. Isbell, P. Lenaker, R. Askin, M.
Miller, L. Babcock
2003
Claims
• Early paleogeographic reconstructions assume
Permo-Carboniferous ice sheet was very large
and centered on Transantarctic Mountains
• Weathering profiles show evidence to the
contrary
• Isbell claims no evidence exists that shows the
Transantarctic Mountains were ever close to a
glacial spreading center
• Carboniferous glaciation less widespread than
previously assumed?
Contrasting Evidence
• Cyclothems: alternating sequences of
marine and nonmarine sediments (unique
to Permo-Carboniferous)
• Glacioeustatic fluctuations of 60-200m
• Ice sheet calculated to have volume of 35115x106km3
Previous Assumptions
• Most paleogeographic
reconstructions show
Victoria Land, Darwin
Glacier region and central
Transantarctic Mountains
as buried under centroid
of Paleozoic ice sheet
• Beginnings of glaciation
towards the end of
Mississippian based on
glacial grooves in
deposits along the
Pensacola Mountains
and large unconformity
separating Pennsylvanian
from the Devonian
Conflicting Evidence
• Palynological data found in glacigenic
deposits in the Transantarctic Mountains
supports a Permian age for deposits
• Unconsolidated sediments in Victoria Land
and Darwin glacier region suggest that the
area was not glaciated during
Carboniferous– these sediments would not
have survived multiple future glacial
advances if this were the case