Download Structural evolution of the Triangle Zone in the central Alberta Foothills

Structural evolution of the Triangle Zone in the central Alberta Foothills:
evidence from along strike variation
Janice LeDrew* (Conoco Canada Limited, 205-5thAveSW, Calgary, Alberta, T2P-2V7),
Don Lawton, Deb Spratt (University of Calgary), and Willem Langenberg (Alberta Geological Survey)
Geometry of structures in the initiation, development and abandonment stages of the Triangle Zone reveal
much about the evolution of the Triangle Zone as well as the fold - thrust belt as a whole. This integrated
study involved mapping of surface geology, interpreting seismic and well data, integrating these data and
palenspastically restoring cross sections. There is a general progression in time of deformation from
hinterland to foreland with shallow structures forming earlier than deeper structures. The stratigraphy
greatly influences the geometry and evolution of these structures. Detachment faults occur in weaker
stratigraphic units separating rock of varying structural competencies, resulting in varied amounts of
shortening, style of deformation, and frequency of faulting and folding. There are four main detachments
in this area: Wapiabi Detachment, Blackstone Detachment, Triassic/Jurassic Detachment and Banff
Detachment.
The Elk River area to the foreland of the Triangle Zone (cross section DD’) is the least deformed and is
interpreted to represent initiation stages of a triangle zone. It consists of a duplex structure in the marine
shale dominated Alberta Group, with roof and sole thrusts at the Wapiabi and Blackstone detachments.
There is also a broad pop-up or bowl like structure in the shallower Coalspur and Paskapoo rocks, which
flattens out in the lower coal of the Coalspur Formation. Interbedded continental, clastic rocks of the
Brazeau Group show some evidence of small-scale deformation on seismic data, but there is no obvious
fault transfer of this displacement up section.
The Lovett River area is a classic triangle zone (cross sections AA’, BB’ and CC’) with the Pedley Thrust
being the upper detachment, or “upper bounding backthrust”. It emerges from the Wapiabi Detachment
and cuts up section through the Brazeau Group to the surface. The relatively incompetent marine shale
dominated Upper Cretaceous Alberta Group forms a duplex or anti-formal stack, with roof and sole
thrusts at the Wapiabi and Blackstone detachments. There are numerous thrust sheets with fault-bend
fold structures, pop-up structures (cross section AA’) as well as a detachment or fault-propagation fold in
the core of the structure. Foreland verging displacement on these structures is interpreted to have reached
the surface via the Pedley Thrust. These structures are outlined on seismic data by the Cardium
Formation, in the middle of the Alberta Group. Several wells have been drilled into the Cardium
sandstone in this area, which are producing commercial quantities of sweet natural gas. The deeper, more
competent, Mississippian carbonate rocks are deformed into a larger fault-bend folded thrust sheet with
fewer faults. There is also proven sour gas in the rocks in this area, which is being developed using
horizontal well technology. The Lower Cretaceous interbedded clastic rocks have moderate competencies
and form intermediate structures that generally mimic the deeper structures. This package is bounded by
the Blackstone and Lower Cretaceous detachments and has a higher frequency of faulting. These
structures also have some folding not accounted for in the fault-bend fold model. Late stage emplacement
of the deeper structures has folded (cross section BB’) and faulted (cross section AA’ and CC’) the
Blackstone Detachment and steepened faults and strata above. It is difficult to trace the displacement
paths of deep thrust faults. Movement on deep faults was partly taken up by faults in the shallow section
immediately above, as well as along the Blackstone Detachment and then onto thrust faults in the Alberta
Group some distance into the foreland, as in the Elk River area. The Mississippian thrust sheet in the
Lovett River area is cut out along strike to the SE by a lateral ramp and is therefore not present on cross
section DD’. This lateral ramp forms a wedge seen on strike oriented seismic data looks similar to a
triangle zone with a backthrust. The Pedley Thrust, which is the upper bounding backthrust of the Lovett
River Triangle Zone, terminates to the SE into an anticline – syncline pair and is also not present on cross
section DD’.
Structural evolution of the Triangle Zone in the central Alberta Foothills
The Brazeau River area is an abandoned or decapitated triangle zone (cross section DD’), which is en
echelon with the Lovett River Triangle Zone roughly along strike to the SE from Lovett. Most of the
deformation in this area is interpreted to have occurred earlier than that in the Lovett River area, partly
because this area has more shortening and is more to the hinterland, lining up more with the Pembina
River structure. The Ancona Thrust, as observed in outcrop and on seismic data, consists of a very
steeply dipping foreland-verging thrust zone that cuts through the Wapiabi Detachment and the upper
bounding backthrust of the pre-existing triangle zone (cross section DD’). The Ancona Thrust is cut by
the Pedley Thrust in the Lovett area (cross section CC’), supporting the interpretation that the structures
in this area formed earlier than the Lovett structure. Advanced deformation resulted in the backthrust
being rotated beyond its critical limit of stability and the triangle zone being decapitated. A younger
upper bounding backthrust is interpreted higher in the stratigraphic section. This fault is observed to cut
up section from the upper Brazeau Group to lower Coalspur Formation along strike to the NW, on
seismic data to the foreland of the Ancona Thrust zone. This deformation is interpreted to have occurred
late in the deformational history of the area, synchronous with the emplacement of the deeper sheets and
the minor deformation to the foreland in the Elk River area (cross section DD’). Natural gas is being
produced in the Blackstone field from the Cardium rocks in this area.
The Pembina River area hinterland of the Lovett River area (cross section BB’) is interpreted to be a
“relict triangle zone”. This structure is similar to the triangle zone at Lovett River. The same
detachments are active in this area and there is a similar duplex in the Alberta group. The Blackstone
Detachment, the Alberta Group duplex, and the upper bounding backthrust, ie the Pembina Thrust, were
all folded by the emplacement of two large Misssissippian thrust sheets below. The Lower Cretaceous
section has a similar intermediate structural style as in Lovett and Blackstone. The Mercoal Thrust is
another backthrust that reached the surface but it is not interpreted to be the upper bounding backthrust of
this relict triangle zone because it has much less displacement than the Pembina Thrust. It is interpreted
to be a minor, late feature that developed as a result of the Pembina Thrust locking up due to being folded.
There is proven gas in the Triassic, Lower Cretaceous and Cardium rocks in this area.
There are two different geometries that occur in this general area in the abandonment stages of triangle
zones. In the Pembina River area the upper bounding backthrust was folded by the late emplacement of
thrust sheets carrying Mississippian strata (cross section BB’). In the Brazeau River area a late stage
thrust fault emerging from the deeper section carrying Mississippian strata cut through the upper
bounding backthrust and reached the surface (cross section DD’). Emplacement of Mississippian thrust
sheets causes uplift, increased folding and faulting, and erosion of triangle zone structures, making them
difficult to recognize and interpret. Percent shortening increases upward across detachments in the
triangle zone, whereas the more mature structures to the hinterland have relatively more shortening in the
deeper section. There is an increase in shortening in the frontal structure along strike from the classic
triangle zone at Lovett to the decapitated triangle zone at Blackstone. Late stage thrusts and backthrusts
are generally steeper relative to bedding than earlier faults.
Structural evolution of the Triangle Zone in the central Alberta Foothills
Structural evolution of the Triangle Zone in the central Alberta Foothills