Exercise Three - Intoduction to the Stratigraphy of the NPRA
is the third of the series of exercises tied to the interpretation
of seismic data
objective of this exercise is to interpret the sequence
stratigraphy of a seismic data set from the northeast
corner of the National Petroleum Reserve of Alaska (NPRA)
just south of Harrison Bay on the coast of the Beaufort
Sea. This exercise includes a study of the geometric relationship
of sequences, delta migration, local and regional tectonics,
and eustatic signal preserved in the rock record. This
data set consists of a grid of five regional seismic lines
that cover a linear elongated belt paralleling the Arctic
coast (Figure 01). These lines are part of the R8 seismic
data shot and processed for USGS and available in the public domain.
geological setting of the exercise is the Colville Basin,
which is part of the foreland basin lying to the north of
the Brooks Range in Alaska. This basin was initiated
in the Jurassic but fill with siliciclastic sediments really
accelerated through the Cretaceous and Tertiary.
section (Figure 02) and the associated seismic data set
contain very clear examples of prograding clinoforms that
provide a unique opportunity for seismic analysis. These
clinoforms overlie a set of prominent high amplitude "double
reflectors" at around 1.4 to 1.55 seconds (Figure
||On the basis of
the biostratigraphy, and the earlier work by Palmer (1983)
and Bird and Molenaar (1991), these reflector doublets are
interpreted to represent the Barremian aged Pebble Shale.
The slopes of the clinoforms in this northeastern portion
of the basin are recognized as the Torok Formation, while
horizontal onlapping reflectors landward of the crest of the
Torok Formation clinoforms are the Nanushuk Group (Figure
chart of the Lower Cretaceous sedimentary fill of the NPRA compiled
by Palmer (1983) and Bird and Molenaar (1991) interprets the Nanushuk
Group and Torok Formation as lithostratigraphic units that transgressed
time (Figure 04).
data for the area is poor but, combined with lithostratigraphy,
they suggest that the Nanushuk (a sand prone unit), and the
Torok (a silt prone unit) young towards the north and east
so that the eastern portion of the prograding sedimentary
wedge can be assumed to be Cenomanian in age while to the
SW they are Aptian.
and third order eustatic signals on the Haq et al 1987
chart (Figures 05 and 06) should be evident in the Cretaceous
section of the NPRA used in the exercises. In contrast
some of the higher-frequency (4th order?) sequences
may be localized and related to delta migration. Based
on observations of the seismic stratigraphy of the area
if the recorded changes of base are the products of eustasy
then it should be possible to use the sea level curve
developed by Haq, et al, 1987 to "preliminarily" date
some of the sequences.
well data and the seismic data suggests that the Nanushuk Group
accumulated in shallow deltaic and fluvial settings, while the
Torok Formation collected in delta slope and basin floor fan settings.
Individual sequences are interpreted to contain evidence of slumping,
basin floor fans, slope fans, and onlapping high stand wedges
(Figure 07), and the exercise will examine this. The clinoforms
can be mapped and the exercise should confirm that they become
younger to the north and east. The internal
geometries of these clinoforms suggest an absence of small-scale
tectonic control on the area. The exercises outlined below ask
that this conclusion be confirmed.
the Cretaceous, the Colville Basin had a
relatively stable tectonic history. There was a slow and constant
rate of subsidence, and a concomitant relatively steady overall
influx of clastic sediments, though there were short-term local
variations in these rates of accumulation. A geological cross
section of the NPRA indicates that to the south and west the Lower
Cretaceous sedimentary fill of this foreland basin subsided some
6000 meters while to the north and east, in the vicinity of the
Barrow Arch, it has subsided no more than 2000 meters (Figure
|| The extensive
tectonic subsidence of the foreland basin developed the accommodation
of the sedimentary fill of the Colville Basin. The clinoformed
sedimentary fill derived from the folded and over thrust Brooks
Range to the south and west overwhelmed the reduced accommodation
on the adjacent cratonic fragment beneath the Barrow Arch
to the north. The exercise seeks confirmation for this conclusion.
the Torok Formation is thickest in the foreland basin and
thins towards the north while the Nanushuk Group is thickest
to the west and thins to the east (Figures 08 & 09) suggesting,
and supported by the orientation of the clinoforms, that the
basin filled from west to east.
|Part of the exercise
will be to use the seismic data set to map the position of
the intersection of the Nanushuk Group with the crests of
the Torok Formation clinoforms. This should establish if
in fact the paleo-shoreline was oriented northwest southeast.
Another part of the exercise will examine the variation in
the thickness and distribution of the clinoforms in order
to establish how variable the rate of sediment supply from
the Aptian through the Cenomanian was?
the shape and geometries (Figure 10) might be related to delta
switching, and migration tied to the shifting of the source
areas from the SW.