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Figure Captions
 Figure
1. Geologists entered Argentina's Rio Iruya canyon, northwestern
Argentina (Andean foreland), to collect a magnetostratigraphic section;
results showed that the rocks in the wall were deposited 5-4 Ma..
 Figure
2 -Magnetostratigraphic study of the Quebrada la Porcelana section,
northwestern Argentina (Andean foreland). Column A is the stratigraphic
column with detail provided for exposed Neogene strata. Column B is a
plot of VGP latitude vs. stratigraphic level of sample sites. Column C
is correlated with the GMPTS (column D) of Cande and Kent (1995) to
determine reversal ages.
 Figure
3 - Sediment accumulation history for the Quebrada la Porcelana area
based on magnetostratigraphic dating of Neogene overburden strata: Using
outcrop thicknesses, the Los Monos Formation passed through the
generation depth (assumed to be 4 km; red arrows) during the 4.8-4.1 Ma
interval (green arrows). Growth strata deposition commenced about 5.2 Ma
(purple arrows).
Knowledge
of basin evolution rates provides insight into the timing of hydrocarbon
generation, facies migration, and  structural trap formation. In marine
environments, fossils often furnish excellent geochronometry from which
relatively precise rate calculations are possible, but the near lack of
well-constrained fossils in most continental environments confounds our
ability to establish the temporal dimension of basin-filling
(overburden) strata in which basin history is recorded (Figure
1).
Magnetostratigraphy
can provide a relatively precise chronology in strata independent of
fossil content. The technique correlates magnetic reversals found in a
stratigraphic column with reversal ages derived from sea-floor magnetic
stripes. Magnetostratigraphic geochronometry works best in fine-grained.
Neogene, siliciclastic strata, but it can be used effectively in rocks
as old as Middle Jurassic. In rocks that predate the oldest modern
sea-floor, magnetic reversal patterns can still be used as correlative
tools.
Siliciclastic
rocks are desirable because they are more likely to possess sufficient
magnetic mineral contents, but successful studies exist from chemical
and biochemical sedimentary environments. Fine grain sizes are necessary
because only single-domain magnetic minerals consistently align with the
ambient magnetic field during deposition. Siltstones and mudstones are
most effective, but poorly sorted fine- to medium-grained sandstones can
yield a consistent signal. Young rocks are most favorable because the
Global Magnetic Polarity Time Scale (GMPTS) is more precisely
constrained-and because there is less likelihood that these strata were
overprinted by remagnetization events that mask the original Natural
Remanent Magnetization (NRM).
Field
Techniques
Oriented
samples are collected throughout a stratigraphic section. A minimum of
three samples is collected from each site for statistical purposes.
Sampling is usually accomplished using a coring drill. Hand-sampling
techniques also work but are more labor-intensive. Because precise rock
ages and deposition rates are intangible at the outset, initial sampling
intervals are judged largely on regional experience and intuition. A
general rule in continental environments is that sections proximal to
their source sustain larger intervals than distal sections.
In
Argentine Andean foreland basins, stratigraphic sample spacings of 15-40
meters are common, whereas in the Himalayan foreland of Pakistan intervals
of 5-10 meters are more typical. Spacings between adjacent sites usually
express considerable variation determined by the availability of
fine-grained strata.
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Laboratory
Analysis
Samples
are first analyzed to determine their outcrop NRM. A cryogenic
magnetometer is usually the instrument of choice, but spinner
magnetometers still play important roles in many laboratories. The NRM
consists of two components: a "stable" Detrital Remanent
Magnetization (DRM) and a variable Viscous Remanent Magnetization (VRM).
The
VRM may change polarity during magnetic reversals; the DRM does not. The
VRM may or may not be stronger than the DRM, so it is essential that it be
removed to determine the true orientation of the DRM. VRM removal is
usually accomplished by either Thermal Demagnetization or Alternating
Field Demagnetization. Both techniques effectively randomize the VRM
component allowing the stable DRM to dominate.
Confident
magnetic cleaning is the greatest obstacle to downhole
magnetostratigraphic analysis. Demagnetized sample orientations from each
site are averaged and tested for statistical significance. Sites that pass
are designated Class I. Class II sites have only two surviving samples,
but both exhibit the same polarity. These are used only to support
adjacent sites of the same polarity.
Data
Interpretation
Upon
completion of laboratory analysis, the latitude of the Virtual Geomagnetic
Pole (VGP) is calculated for each site. This parameter places the North
Magnetic Pole in either the northern (normal) or southern (reversed)
hemisphere. VGP latitudes are plotted vs. sample stratigraphic levels.
This information is abstracted to the standard black and white column in
which black designates normal and white signifies reversed polarity.
Reversal boundaries are placed halfway between adjacent sites of unlike
polarity (Figure
2).
The
local paleomagnetic column is correlated with the GMPTS (Figure
2).
Because of the binary nature of polarity zones (normal or reversed), it is
essential that the local column be independently calibrated with either an
isotopic age or a well-constrained fossil to avoid correlation errors due
to variable sediment accumulation rates.
Sediment
accumulation (basin subsidence) history (rate) is derived by plotting
reversal ages vs. their stratigraphic levels (Figure
3). Variation in
accumulation rate is often due to tectonism in mountain belts, but climate
and eustacy may also be important contributors. Relatively precise dating
of internal and cross-cutting features of the sedimentary pile also arise
from the magnetostratigraphy. These can include constraining sediment
source area changes, depositional hiati, facies changes, and ages of
faulting and folding. Where strong seismic reflectors crop out, they can
be dated and carried into the subsurface to provide an intrinsic
chronometry for seismic sections.
Case
Study
Perhaps
the most interesting application of these data is an estimate of ages of
hydrocarbon maturation/migration. Subsidence of source strata through the
generation window can be modeled using the ages of the overburden beds.
Paleomagnetic
results from the 4,650 meter-thick Neogene Quebrada la Porcelana section
in the Sierra de Ramos of northwestern Argentina illustrate this
application. The base of the paleomagnetic section is situated ~ 1,700
meters above the base of the 300 meter-thick Los Monos source horizon.
Magnetostratigraphic chronology suggests that growth strata derived from
rising anticlinal structures accumulated between 5.2 Ma and the top of the
section (< 1 Ma). Assuming a generation depth of four kilometers and
using outcrop thicknesses, the base of the Los Monos Formation probably
attained generating depths ~ 8 Ma. A backstripped sedimentary column would
suggest that generation depth may actually have been reached at about the
same time the growth strata began to accumulate. Using either data set
suggests that local trapping structures were available during initial
generation and migration.
Similar
analysis in the 7.5 km-thick Rio Iruya section, ~35 km to the west,
revealed that generation depths were attained two-three million years
before local trapping structures formed. Ongoing magnetostratigraphic
research continues to reveal the chronology of basin evolution in other
parts of the Argentine Andean foreland. In conjunction with existing
geological and geophysical information, these data are unveiling an
impressive diachronism in structural development and hydrocarbon
generation across the region.
Cande,
S.C., and D.V. Kent, 1995, Revised calibration of the geomagnetic polarity
timescale for the late Cretaceous and Cenozoic: Journal of Geophysical
Research, v. 100, p. 6093-6095.
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