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.
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 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 (<17 µ) 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.
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 '4.8 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
Río 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.