Exercise Three - Objectives & Data
access to the Adobe pdf file containing the un-interpreted seismic
sections of the exercise, click othe appropriate image to download
B/W PDF files:
the 2D seismic data is downloaded it will be seen that most of the
lines are divided in two. These pairs of lines can be taped together,
e.g.: 12-81 & 12a-81; 16-81a & 16-81b; 27-81a & 27-81b;
and 37-81a & 37-81b. Note that 26-74 is complete! Thus this
data set can be considered to consist of five intersecting lines
which can be cross-tied and interpreted.
Below a number of objectives
for this exercise are listed. Start with 1) and work through 2)
to 4). Problem 3) may be considered as a dangerous call, but without
tight biostratigraphic data the use of the Sea Level chart may be
a best first pass as a tool for dating the stratigraphic section!!!
the sequence stratigraphy of each of the lines and the clinoformed
sequences that form the Lower Cretaceous sedimentary section by:
the termination of seismic reflectors at discontinuity surfaces.
Establishing which of these surfaces form the sequence boundaries.
Numbering the sequence boundaries (there are around
21 or so).
Outlining and labeling the geometries of the sequences
(the Nanushuk Group and Torok Formation
clinoforms) and the system tracts they enclose (Figure
Correlate sequence boundaries from line to line; tie these at
the cross lines.
map and study the sedimentary architecture of the clinoforms
and infer the origins of these features. For instance do the clinoform
geometries show evidence of delta switching, slumping and migration
related to the shifting of the source areas in the SW? Was there
a variable rate of sediment supply to the area of study? Is there
evidence of widespread changes in base level? To answer these:
a. First map the location of the crests of the
distinct clinoformed margins
(Figures 11 & 12).
b. Map the varying thickness and widths of the
clinoforms from one or two sequences (isochron maps) and make overlay
maps of the distribution of the systems tracts.
c. What is the cause of the clinoforms? High rates
of sedimentation relative to lower rates of subsidence and periodic
changes in base level, or point source switching? If point source
migration were responsible, would it be possible to correlate the
same clinoforms to different lines?
if the maps suggest parallel progradation of the coastal margin
or show evidence of delta migration?
the direction of progradation or delta migration?
Identify downslope slumps (Figure 10)
and see if they can be tied from line to line for any specific
relative times. Is there evidence that they were produced during changes in base level?
g. Identify and locate basin floor fans, and
slope fans (Figure 10). Is there
evidence that they were produced during
changes in base level?
h. Identify and locate onlapping wedges of sediment
(Figure 07). Is there evidence
that they were produced during changes
in base level?
the crests of clinoformed margins that have been eroded or truncated
and see if they can be tied from line to line so relating them
to specific time intervals (Figure 07). Is there evidence that they were produced
during changes in base level?
j. Do the major eroded clinoform margins (i.e.
basin crests) match the downslope slumps, and associated basin
floor fans, and slope fans? If they do, how do you explain this
indicated above poor biostratigraphic contol means it is difficult
to determine the ages of the individual
clinoforms, never the less Palmer (1983) and Bird and Molenaar (1991)
have been able to infer that the eastern most part margin of the
progradational Torok Formation and Nanushuk Group are Cenomanian
in age. The Haq and others (1987) chart for the Cenomanian/Albian
has nine major changes in sea level, at least three of which might
be expected to produce major second order type 1 unconformities.
There were marked sea level lows, particularly the major type 1
unconformities and associated lows at 112 my, 109 my, 107.5 my,
98 my, 96 my and 94 my, that suggest these may have been large enough
to produce erosional events at the crests of the clinoforms displayed
by the seismic sections. Below the exercise proposes examination
of the erosional events on the seismic sections to see if these
events can be tied to the changes in base level predicted on the
Haq and others (1987) chart. If this can be done, is it possible
to establish their relative timing? Is
there a match between the onlapping and downlapping geometries of
the clinoformed section and the Haq et al 1987 sea level curves:
a. Identify and count the 2nd and 3rd
order "type one" unconformities and their related sequence boundaries
on the seismic lines.
chronostratigraphic charts for each of the lines to separate and
analyze the changes in base level responsible for the sequences.
The chronostratigraphic charts can be created by using a spread
sheet on which the widths each of the numbered sequences is recorded
and then display these widths as a chronostratigraphic chart
these be matched to a number of the 2nd and 3rd
order events on the Haq et al 1987 sea level chart?
tectonic setting of the Collville Basin is that of a foreland basin.
Changes in local base level related to local changes tectonic behavior
would be expected to produce local changes in accommodation. To
this end the internal geometries of the clinoforms should be examined
to see if a presence or an absence of small-scale tectonic control
on the area can be detected.
Similarly there was an overwhelming sediment supply from the folded
and thrust Brooks Range to the South, a deepening in the foreland
basin to the SW suggesting regional tectonic subsidence with the
development of extensive accommodation, while there was reduced
accommodation and thinning over on the adjacent cratonic fragment
to the NE over the Barrow Arch.
To examine the magnitude of these tectonic effects measure the thickness
of the updip Nannushuk shelf
there any local variations in the thickness of onlapping shelf
sediments that could be ascribed to a local tectonic signal?
a local tectonic signal have produced any of the downslope slumping?
there any regional variations in the thickness of onlapping shelf
sediments or downslope downlapping sediments that could be ascribed
to a regional tectonic signal? Are these in phase with the 2nd
and 3rd order events on the Haq et al 1987 sea level
chart or are these separate events?
any evidence of a regional tilt to the sedimentary fill penecontemperaneous
with its deposition or is it post deposition?
on the image designated as linked to the solution and match the
output with your interpretation. For a fuller description of this
solution, click on the link to the Final
Report on this exercise.