This paper presents an explorationist's view of the geological evolution of the Mackenzie Basin during the Tertiary.

Gulf Canada Resources, Inc. has been involved in petroleum exploration in the Mackenzie delta/Beaufort Sea area since its inception in the 1960s. During this period there has been a need to incorporate all available geological and geophysical data in order to guide exploration strategy. This paper represents the latest update of this continual effort, which was performed primarily by geologists and geophysicists in Gulf Canada's exploration and research departments.

REGIONAL SETTING

The Beaufort Sea is in the southern part of the Arctic Ocean, extending from the western edge of the Canadian Arctic Islands to Point Barrow in Alaska. Figure 1 outlines the physiography of the southern Beaufort Sea between Banks Island and Prudhoe Bay. Within the Canadian portion of the Beaufort Sea the shaded area highlights the Mackenzie Basin, which is filled with great thicknesses of Tertiary and Mesozoic sediments. The Mackenzie Basin covers about 35,000 sq mi and is a sedimentary depocenter bounded by the Cordillera to the southwest and by a major fault system to the southeast. Seaward, the Mackenzie Basin extends to approximately the present-day shelf edge. The Mackenzie delta, the focal point of this basin, can be subdivided into two parts; the larger, offshore portion underl ing the continental shelf and a smaller, onshore part occupied by the Arctic coastal plain including the present-day Mackenzie delta. The areas on the coastal plain underlain by Tertiary sediments are indicted in a coarse stipple on Figure 1.

METHODOLOGY

The maps and cross sections in this paper result from integration of well information and seismic data. Approximately 150 wells have been drilled to date in the region; 20 were drilled offshore and the remainder on land. Seismic data cover the area in a grid averaging 1 to 2 mi between lines.

Paleontological age determinations are the primary tool in establishing the stratigraphic subdivision of the study area, and are useful in determining the depositional environment of any given time slice. These data, combined with lithologic and seismic data, develop a basic geologic model for the Mackenzie Basin.

POST CRETACEOUS ISOPACH

This paper deals with sedimentation in the Mackenzie Basin only during the Tertiary. References to the basin's formation are in Young, Myhr, and Yorath

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(1976) and Jones (1980).

Figure 2 shows the total thickness of post-Cretaceous, mainly Tertiary sediments, contoured at 5,000 ft intervals. A maximum thickness of over 30,000 ft is centered directly north of the present Mackenzie delta. Areas of relatively thin Tertiary sediments to the north and west are attributed to a series of diapiric shale ridges which are discussed later. The map of the study area (Figure 2) falls within the black frame shown on the regional map (Figure 1). More notable fields, such as Parsons Lake and Taglu, and wells such as Ukalerk and Kopanoar, are shown on this and succeeding figures. The present-day Mackenzie Delta is highlighted as a hachured area on Figure 2.

Figure 3 shows a generalized north-south cross section through the central part of the Mackenzie Basin. Paleozoic and Mesozoic rocks, which define the basin floor, are shown in heavy hachured pattern near the base of the section. Within the Tertiary basin five deltaic cycles are identified and the depositional facies are coded for each cycle. The vertical exaggeration of the cross section is about 15:1.

STRATIGRAPHY/DEPOSITIONAL HISTORY

The columnar section of Figure 4 displays the informal names and ages of the various deltaic sequences. The ages derive from paleontological and palynological age determinations by Gulf, which are not always in accordance with the age determination cited in the literature (compare Young, Myhr, and Yorath, 1976; Lerand, 1973; Staplin, 1976).

The first Tertiary deltaic cycle deposited in the basin is the Paleocene Moose Channel Formation. Following a transgression indicated by wide-spread thick deposits of shale, the late Paleocene to Eocene "Taglu Delta" sequence (Reindeer Fm., Mountjoy, 1967) built out into the basin. This sequence today lies mainly beneath the Mackenzie Delta while the three younger deltas lie beneath the present Beaufort Sea shelf. The

Fig. 1. Index map. The coarsely stipled area indicates the portion of the coastal plain underlain by Tertiary sediments. Heavy frame indicates area of study.

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Fig. 2. Post-Cretaceous sediment thicknesses in the Mackenzie delta region. Present-day delta is indicated by cross-lining.

Fig. 3. Generalized cross section of the Mackenzie delta. 1-Paleocene Moose Channel delta; 2-Eocene Taglu delta; 3-O-ligocene Pullen delta; 4-Akpak delta and; 5-Beaufort delta.

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third deltaic sequence, the Oligocene "Pullen delta" (informal name used in the oil industry; Hea et al, 1980) is separated from the "Taglu delta" by thick widespread shale and is overlain by the "Akpak Sequence" (informal name used internally in Gulf) which formed during late Oligocene and early Miocene times. In the early to middle Miocene most of the basin was tectonically unstable, resulting in widespread faulting and erosion. A major unconformity formed at this time cuts progressively older rocks in a southward direction. The last (compare Hea et al, 1980) deltaic pulse, the Miocene to Plio/Pleistocene "Beaufort sequence," rests on the truncated older deltaic sequences as shown on Figure 3.

DEPOSITIONAL MODEL

Figure 5 depicts the depositional features on the shelf and slope in an area influenced by deltaic sedimentation. Sediments carried to the depocenter are deposited on the delta plain and in the nearshore marine environment (in this study the delta plain facies are defined as sediments containing more than 35% net sand). The delta plain facies is coded as open circles on the illustrations and is dominated by channel pointbar sands and lake deposits with abundant coals. Sediments containing 15 to 35% net sand are dominated by deposits originating in distributary mouthbars, offshore bars, and shallow bays. They are referred to as the delta front facies and are coded in a stippled pattern on the illustrations.

Sediments with less than 15% net sand form toward the basin from the delta front. Typically these sediments have less than 5% sand and are dominated by silt and marine clay of the prodelta facies. Within the prodelta facies is a zone of sandy sediments which are interpreted to have been deposited by turbidity currents and slumps at the continental slope base. The net sand percentage is usually 5 to 10% in these deep water sediments, which are coded in a hachured pattern on the illustrations. As the delta progrades into the basin the sedimentary facies recording these depositional

Fig. 4. Ages and informal names of delta sequences.

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environments form a continuous vertical sequence. In a typical vertical column the facies succession begins with prodelta shales, including turbidites, progressing upward into delta front then delta plain sediments. The only sandy sediments found beyond the shelf edge are turbidites.

PALEOGEOGRAPHY; PALEOCENE MOOSE CHANNEL FORMATION

The first deltaic cycle deposited in the Mackenzie Basin during the Tertiary is the Moose Channel Formation. This unit overlies shales and silts of late Cretaceous and early Paleocene age, and is restricted to the southwestern part of the basin. Figure 6 and succeeding maps of the five delta sequences are isopachs of the gross sand interval, contoured in feet. The map includes the lowest to the uppermost sand in the delta sequence and excludes the thick shale section between the individual deltaic cycles. Within this interval the net sand percentage varies according to the depositional environment. Note that the gross sand intervals for the shelf and slope derived sediments are contoured independently.

Two large indentations in the erosional zero edge to the south are caused by erosion over structural highs (Figure 6). It is believed that the Blow High to the west is partly syndepositional, since it is the result of a series of diapiric ridges extending in a northwest to southeast direction. The Kipnik High in the lower centre of Figure 6 is most likely a postdepositional tectonic feature.

In the middle to late Paleocene, marine shales were deposited over the entire area either representing an intermission in clastic material influx from the source areas or a rapid subsidence of the basin. These shales are overlain by the late Paleocene to Eocene Taglu delta.

EOCENE TAGLU DELTA SEQUENCE

The Taglu delta prograded into the basin late in the Paleocene and at about the end of the Eocene had reached its maximum areal extent along the shelf edge at the heavy dashed line marking the transition between the shelf and slope (Figure 7). To date this deltaic sequence is the major exploration target in the southwestern portion of the basin. The depocenter of this sequence is located near the Taglu field, for which the unit is named. Note that the depocenter is displaced basinward from the Moose Channel depocenter, and that the gross sand interval reaches approximately 6000 ft thick, about twice the thickness of the previous unit. The depositional strike trends approximately northwest to southeast and parallels a series of syndepositional diapiric ridges located under the outer sh lf and slope areas (compare to Figure 12)

Fig. 5. Schematic drawing showing inferred depositional environments.,

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Fig. 6. Gross sand intervals in shelf and slope facies, Moose Channel delta sequence.

Fig. 7. Gross sand intervals in shelf and slope facies, Taglu delta sequence.

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and changes to parallel the northeast trending coastline east of Taglu.

In the turbidite component of the Taglu delta sequence, approximately 3000 ft of gross sand interval have been penetrated under the Taglu shelf edge. The Kipnik High is postdepositional with respect to this deltaic sequence. The Taglu delta sequence terminated toward the end of the Eocene by a thick sequence of marine shales deposited in late Eocene and early Oligocene time.

OLIGOCENE PULLEN DELTA SEQUENCE

Following transgression, renewed deltaic deposition began early in the Oligocene. Informally this deltaic sequence is called the Pullen delta, since its depocenter is located near Pullen Island off the northern tip of the Mackenzie delta. In this area the gross sand interval reaches a maximum 8400 ft thick, making the Pullen delta sequence the thickest deltaic unit in the Mackenzie basin. This delta is almost exclusively confined to the modern Beaufort Sea shelf and the northernmost part of the Mackenzie delta.

During the Oligocene, the Pullen delta prograded into the basin in a north to northeasterly direction, and at its climax it pushed the shelf edge about 50 mi offshore from the previous Eocene shelf edge. During the Paleocene and Eocene the shelf edge had a northwest to southeast orientation, but during the Oligocene the shelf edge migrated counterclockwise to an east to west orientation. This is indicated by the trend of the contours on Figure 8 and is especially evident in a series of syndepositional diapiric ridges trending roughly east to west through the Kopanoar area (Figure 12).

Delta plain sediments in the area near the tip of the modern Mackenzie delta dominate the deltaic deposition. The net sand content and the thickness decrease towards the Pullen shelf edge in a concentric pattern away from the depocenter.

The turbiditic sands found within the prodelta shales have been penetrated in several wells. In the Kopanoar discovery well these turbidite sands yielded oil at a calculated rate of 12,000 barrels/day. In the Ukalerk well, which is close to the Pullen shelf edge, sands of turbiditic origin have been encountered within the prodelta shales below the relatively thin shelf sediments.

The turbiditic sediments are more than 4000 ft thick just north of Ukalerk, and generally trend parallel with the shelf edge and the coastline to the southeast.

Turbidites in the Kopanoar area show thinning over east to west orientated diapiric ridges, indicating that diapirism was syndepositional. The area northeast of Ukalerk lacks diapirs, but has a system of compound mounds which presumably represent slumped material associated with deep sea fans. None of these mounds has been drilled as yet; hence, their origin

Fig. 8. Gross sand intervals in shelf and slope facies, Pullen delta sequence.

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remains uncertain. The Pullen delta sequence is the major offshore exploration target and hosts all the significant hydrocarbons discovered to date in that area.

LATE OLIGOCENE/EARLY MIOCENE AKPAK DELTA SEQUENCE

The cross section in Figure 3 illustrates that another transgression terminated the Pullen delta, causing marine shales to be deposited on top of the delta sequence.

In some wells sands are present at the top of this transgressive shale, indicating yet another delta cycle. Paleontological information indicated a late Oligocene to early Miocene age for this sequence, which we informally call the Akpak delta (Figure 9). The map of this unit is largely based on seismic information, indicating the late Oligocene to early Miocene deltaic cycle reaches a maximum gross sand interval of about 3000 ft. The position of the shelf edge is interpreted from very pronounced, large-scale foresets on seismic data (compare Figure 13).

The Akpak sequence is a minor deltaic cycle extending the shelf edge a short distance beyond the Pullen shelf edge. The limited size of the sequence seems to result from its short lifespan near the Oligocene/Miocene boundary.

In the early Miocene the whole Beaufort region experienced widespread normal faulting and erosion.

Figures 3 and 4 show a regional unconformity, indicated in a heavy black line, cutting progressively older units in a southern direction. This unconformity can be traced over the entire southern Beaufort Sea region and represents one of the better seismic markers in the area. The unconformity is smooth and only disturbed by subsequent deposition. Since it erodes the tops of tilted fault blocks to the south, it is clear that extensive faulting and uplift occured prior to or during erosion.

MIOCENE -- PLEISTOCENE BEAUFORT SEQUENCE

After a period of tectonic instability and erosion, active deposition began again in mid-Miocene times and continued through to the Plio/Pleistocene. This time span is represented by what is proposed as the Beaufort formation (Hea et al, 1980).

The Beaufort deltaic sequence (Figure 10) apparently prograded in a northwesterly direction. The sedimentary package reaches thicknesses of some 10,000 ft, but a gross sand interval greater than 5000 ft has not been observed. The depocenter for the unit is located 40 to 50 miles shoreward of the Plio/Pleistocene shelf edge. Turbidites are not prominent in this unit, but they are found in some wells in the outer shelf area.

Fig. 9. Gross sand intervals in shelf and slope facies, Akpak delta sequence.

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SHELF EDGE PROGRESSION

The map on Figure 11 summarizes the shelf edge position at various times during the Tertiary. At the end of the Cretaceous, the shelf edge was located along the recent shorelines to the southwest and the southeast. During the Tertiary, the shelf edge was displaced into the Mackenzie basin through deltaic deposition in five cycles. Its position at the termination of each cycle is shown on the figure. The arrow follows the general direction of transport and displays a distinct counterclockwise rotation with time. Deposition during the Tertiary added an area of about 35,000 sq mi to the continental shelf in the Mackenzie basin area with sediment thicknesses reaching a maximum of about 30,000 ft.

TIMING OF TECTONIC FEATURES

Figure 12 shows the major tectonic features of the study area, where main tectonic elements consist of faults and diapirs. The diapirs form long ridges parallel with the depositional strike at the time of their formation. The are thought to be cored by Cretaceous shales, and were formed in the continental slope area as the deltaic cycles prograded into the basin. As the deltaic deposition moved further into the basin, ridges formed roughly parallel with the shelf edge. In some cases the shelf sediments prograde over diapirs formed at an earlier stage, which is the case for the Paleocene and Eocene deltaic cycles in the southwestern part of the study area.

Comparing Figure 11 and 12 shows a strong similarity in the trends of diapiric ridges and shelf edges for the various stages. This supports the interpretation that sedimentation began in the southwestern part of the basin and migrated counterclockwise with time.

The only major break in the Tertiary sequence occurred in the early Miocene. The first phase of this event was normal faulting, which is present in most of the basin. The faults indicated on Figure 12 are only the most prominent of the multitude of faults dating back to this time. Down-to-the-basin throw dominates, but some area display faults with throw in the opposite direction. The overall impression is one of extensional tectonics accompanied by uplift and erosion.

TERTIARY STRATIGRAPHY/STRUCTURE

The seismic line on Figure 13 is about 60 mi long, runs north to south in the western part of the study area, and is distal to the main depocenters. This particular line was chosen because it crosses two sets of diapiric ridges of Eocene and Oligocene age respectively, and three wells are located on or near it.

Fig. 10. Gross sand intervals in shelf and slope facies, Beaufort delta sequence.

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Fig. 11. Location of present-day and paleo-shelf edges.

Fig. 12. Timing of diapiric activity and faulting. Diapirism coincided with seaward progression of deposition.

Fig. 13. Seismic section through Taglu, Pullen, Akpak, and Beaufort delta sequence showing seismic character associated with delta facies, prodelta shales, and diapirs. Location of seismic line is shown in Figure 12.

Oversized Image :click to view

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The Nektoralik well is located on the present-day shelf edge on the extreme left; two other wells, Tarsiut and North Netserk, are at the southern end of the section.

The slanted hachure represents mainly shaly, supposedly Cretaceous, sediments, that form cores of the diapirs present in the section. The vertically hachured portions represent the gross sand interval for the individual deltaic cycles discussed previously. The unhachurred part of the seismic line consists of Tertiary shales, which lie between deltaic sequences.

The Eocene Taglu delta has a thick package of distal delta front shelf sediments of North Netserk. Further north at the Tarsiut well the unit consists of fairly dirty sand which have been interpreted as turbidites. The Taglu gross sand interval gradually wedges out to the north, indicating a southerly source. The Oligocene Pullen delta sequence is quite thin in the area because it is distal to its depocenter located some 60 mi east. At Tarsiut and North Netserk the sequence develops in an outer shelf facies, but it is turbiditic in the Nektoralik well to the north. (The age determination used here is different from the one published by Jones, Brache, and Lentin, 1980). The transition from shelf to turbiditic sediments occurs in about the middle of the seismic profile, showing that the shelf sediments respond only very mildly, if at all, to the underlying diapirs. This suggests that the diapirs to the south were not active in the Oligocene. To the north the Pullen sediments thin over the crests of diapirs, which accordingly are interpreted as syndepositional. This line of evidence is used to show the diachronous nature of diapirism, which becomes progressively younger in a northward direction, although the diapirs are all derived from the same, possibly Cretaceous, material.

The Akpak sequence is quite heavily eroded in the section's southern end. The heavy black line on top of the Akpak sequence is the regional early Miocene unconformity. Erosion occurred after the extensional faulting as the unconformity cuts cleanly across the faults, and is fairly smooth and undisturbed. In the left center, the unconformity suddenly changes slope which represents the shelf edge position at the termination of the Akpak sequence.

Sediments above the unconformity range in age from mid-Miocene to Pleistocene, and, apart from a relatively narrow band at the top of the section, consist of mainly prodelta shales.

On the left side of the seismic section the recent shelf edge stands out quite sharply, and it is seen that the present-day bathymetry is mimicked in sediments below. The uppermost reflectors tend to be horizontal, while the deeper reflectors dip north, parallel with the present-day continental slope. The junction of these two differently inclined segments defines the position of the shelf edge. The shelf edge migrates progressively deeper and shifts to the south as you follow the section to the right from the present-day shelf edge. The seismic line illustrates the progradation of the shelf edge in the post Oligocene time, and similar evidence is found in older parts of the section.