Thermochronology, denudation and variations in palaeosurface temperature: a case study from the...

Basin Research (1999) 11, 191–204

Thermochronology, denudation and variations inpalaeosurface temperature: a case study fromthe North Slope foreland basin, AlaskaP. B. O’Sullivan*Victorian Institute of Earth and Planetary Sciences,Department of Earth Sciences, La Trobe University,Bundoora, Victoria, 3083, Australia

ABSTRACT

Integration of vitrinite reflectance (Ro) and apatite fission track (AFT) data from wellsequences can provide a direct estimate of the geothermal gradient at the time of maximumpalaeotemperatures and the time at which sequences began to cool from maximumpalaeotemperatures. These values, plus an understanding of the effects of cooling in responseto long-term climatic changes, are particularly important when estimating the amount ofdenudation experienced by the sequences during cooling from maximum palaeotemperatures.In this case study, AFT data have been generated for subsurface samples from eight wellsdrilled within the North Slope foreland basin of northern Alaska in an effort to study thethermal history of the basin. The combination of Ro and AFT data establish that maximumpalaeotemperatures were attained within the North Slope foreland basin prior to coolingbeginning in the Palaeocene. Furthermore, they indicate that palaeogeothermal gradients whencooling began were close to the present-day values, and that Cenozoic surface cooling resultedin a significant amount of ‘apparent’ denudation. These results suggest that heatingthroughout the basin was largely due to deeper burial, and that cooling was due to bothremoval of section by denudation and a drop in the mean annual surface temperature.

analysis outlined by Bray et al. (1992) and Duddy et al.INTRODUCTION(1991, 1994), based on the integration of apatite fission

Thermal history is an essential component in the evalu- track (AFT) and vitrinite reflectance (Ro) data sets, mayation of the tectonic evolution of sedimentary basins and substantially reduce problems associated with theserepresents the relationship between subsurface tempera- uncertainties. The Ro method measures the maturitytures and duration of heating of a rock through geological (coalification rank) of sedimentary organic material (e.g.time (e.g. Corrigan, 1991; Duddy et al., 1991). Most Lopatin, 1971; Dow, 1977; Tissot & Welte, 1978; Waples,approaches to assessing the thermal history of rocks 1980; Burnham & Sweeney, 1989). The AFT method isproceed from assuming a variation of heat flow through based on the resetting of geochronological clocks due totime on theoretical grounds. As a consequence, numerous the tendency for fission tracks in apatite to totally annealassumptions have to be made as to the thermal conduc- upon heating to temperatures greater than #110 °C (e.g.tivity of the stratigraphic section and its variation through Naeser, 1979; Gleadow et al., 1986; Green et al., 1989a,b).time. This can lead to the introduction of large errors in While both methods provide estimates of maximumestimated heat flow by incorrect assumption of the palaeotemperatures and therefore maximum palaeogeo-thermal conductivity, due primarily to the effects in the thermal gradients experienced by rocks in a sedimentarysubsurface of factors such as porosity, compaction and basin, AFT data also provide direct estimates of whendiagenesis (Corrigan, 1991; Duddy et al., 1991). In the rocks began to cool from these maximum tempera-uplifted regions this is further complicated as the lith- tures (e.g. Gleadow et al., 1986; Green et al., 1989a,b;ology and porosity, and hence the thermal conductivity Duddy et al., 1991, 1994). When combined, theseof the eroded section, are unknown. methods yield direct estimates of the time at which

Alternatively, use of the method of thermal history cooling from maximum palaeotemperatures began andthe prevailing palaeogeothermal gradient at that time,both of which are particularly important when constrain-* Present address: School of Earth Sciences, University of

Melbourne, Parkville, Victoria 3052, Australia. ing amounts of denudation.

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However, another factor that is not commonly This is demonstrated by showing the significance of thefollowing factors: (1) the mean annual surface tempera-addressed is a variation of the surface temperature since

cooling began. For instance, in a study of the thermal ture at the time the sequence was exposed to maximumpalaeotemperatures; (2) the prevailing geothermal gradi-history of the North Slope foreland basin in northern

Alaska (Fig. 1), it was found that episodic cooling from ent at the time of maximum palaeotemperatures; and (3)the time at which cooling from maximum palaeotempera-maximum palaeotemperatures, due primarily to denu-

dation, began in the Palaeocene (O’Sullivan, 1996). tures began. Together, these factors help us to understandthe effects that a change in mean annual surface tempera-Importantly, this cooling began prior to a significant

decrease of #15–20 °C in the mean annual surface ture has on estimates of denudation.temperature which occurred in response to a majorclimatic change from temperate to arctic conditionsduring the Middle to Late Cenozoic (Clark, 1982, 1990). ESTIMATING DENUDATION USINGFurthermore, the results of this earlier study showed PALAEOGEOTHERMAL GRADIENTSthat the subsurface stratigraphy must also have experi-enced cooling in response to the decrease in surface Estimating the amount of removed section using thermal

data requires either detailed knowledge of the lithologiestemperature (O’Sullivan, 1993; O’Sullivan & Brown,1998). Therefore, estimated denudation in areas like the of the removed section, or assumptions about either the

thermal conductivity and heat flow variation or theNorth Slope foreland basin, which have experiencedsignificant variations in surface temperatures since the palaeogeothermal gradient through the missing section.

In the absence of this information, a straight-line projec-subsurface section started to cool from maximum tem-peratures, must incorporate any variations in surface tion of the palaeogeothermal gradient to an appropriate

palaeosurface temperature value will provide a reasonabletemperature. As a case study, results from the NorthSlope foreland basin presented here illustrate the impor- estimate of the amount of removed section (Duddy et al.,

1994). This, however, assumes that the lithologiestance of utilizing both Ro and AFT data when evaluatingthe thermal history of such an area, and when estimating removed were similar to those in the preserved section,

and the primary source of heat was basal heat flow.the amount of denudation that area has experienced.

Fig. 1. Location of the North Slope foreland basin study area showing locations of major geographical features, wells and otherspecific features mentioned in the text. The extents of major recorded cooling/denudation events at #60±4 Ma and#25±3 Ma determined from previous AFT data (O’Sullivan, 1993, 1996) are shown by thick black lines. ANWR: ArcticNational Wildlife Refuge; NPRA: National Petroleum Reserve Alaska.

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Denudation/cooling of the North Slope foreland basin

behaviour of both Ro and AFT parameters throughMaximum palaeotemperaturestemperature and time. Therefore, even though the uncer-tainty associated with estimation of palaeotemperaturesFollowing the method proposed by Bray et al. (1992)

and Duddy et al. (1991, 1994), maximum palaeotempera- from Ro data is not as well known, it is probably ofsimilar order to that of AFT results (±10 °C; e.g. Duddytures are first determined from AFT and Ro results from

samples collected down a vertical section or well. Both et al., 1991, 1994; Bray et al., 1992).AFT and Ro can be considered as maximum palaeotem-perature recorders (e.g. Lopatin, 1971; Waples, 1980; Maximum and present-day geothermalGreen et al., 1989b; Duddy et al., 1991, 1994; Bray et al., gradients1992). In theory, a vertical Ro profile is also a record ofthe palaeogeothermal gradient (Duddy et al., 1991). After maximum palaeotemperatures are determined for

samples from a vertical sequence in a well, plotting theseMaximum palaeotemperatures from both AFT and Rodata are determined following the methodology presented values relative to depth provides a maximum palaeotem-

perature profile (Fig. 2). From this profile, the prevailingpreviously by Duddy et al. (1991, 1994).Maximum palaeotemperature estimates from the AFT palaeogeothermal gradient at the time of maximum

palaeotemperatures can be estimated. However, there aredata are constrained using the understanding of AFTsystem response described by Green et al. (1989b). This two major factors that limit the degree to which the

palaeogeothermal gradient can be constrained (Duddyunderstanding is based on an empirical kinetic descriptionof laboratory annealing data in Durango apatite (Laslett et al., 1991). The first is the depth range over which

maximum palaeotemperature values are collected. Ifet al., 1987). More recently, Gallagher (1995) has auto-mated this procedure to give a forward modelling maximum temperature values are only determined over

a narrow depth range then the difference in palaeotem-approach, which combines a Monte-Carlo simulation ofnumerous possible thermal histories with statistical test- peratures is quite small, introducing considerable uncer-

tainty into estimating the palaeogeothermal gradienting of the outcome against the observed fission trackmeasurements. The kinetic response of the AFT system (Fig. 3). Therefore, values must be derived from as large

a section as possible in order to best constrain theis now reasonably well understood and can be used toprovide estimates of maximum palaeotemperatures up to maximum palaeogeothermal gradient.

The second factor is the difference between maximum#110 °C with an uncertainty of the order of ±10 °C(Green et al., 1989b). palaeotemperatures and present temperatures (net cool-

ing). If the degree of net cooling is small, for instance ofEstimates of maximum palaeotemperatures from Rodata are determined using the model proposed by the order of #10 °C, which is similar to the uncertainty

in absolute palaeotemperature estimation, only broadBurnham & Sweeney (1989; their equation 2). Resultsusing this model are in close agreement between observed limits can be established on the palaeogeothermal gradi-

ent. However, as the degree of net cooling increases,and predicted Ro values when modelled through proposedthermal histories determined using AFT data. This gives the control on the palaeogeothermal gradient also

increases.confidence in the kinetic models used to describe the

Fig. 2. Temperature vs. depth plots showing end-member models to explain cooling using maximum temperature estimatesderived from AFT and Ro data. In (A) the palaeogeothermal gradient was greater than the present-day gradient, but not offset atthe surface. This indicates that a drop in the geothermal gradient (change in heat flow) was responsible for cooling to present-day temperature conditions. In (B) the palaeogeothermal gradient is similar to the present-day gradient, but offset towards highermaximum temperature values. This indicates that there has not been a change in heat flow and that cooling must have occurredin response to denudation. In (C) the section has experienced cooling in response to both a change in the geothermal gradientand a change in denudation.

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present surface temperature is used, the amount ofcooling which has occurred since maximum palaeotem-peratures were reached will be over estimated. Therefore,when calculating the amount of denudation in regionswhich have recently experienced significant changes inclimate, it is important to know the time at which theclimate changed, the time when cooling from maximumpalaeotemperatures began and the surface temperature atthat time.

In the case in Fig. 4B, the amount of denudationdetermined using the change in temperature would beover-estimated by a value introduced here as the ‘appar-ent’ denudation. ‘Apparent’ denudation is defined as thatthickness of section that would seem to have beenremoved in response to denudation, but in fact somecooling occurred in response to a significant decrease inthe mean annual surface temperature without associatederosion. In the case presented in Fig. 4B, in which themaximum palaeogeothermal gradient is similar to thepresent-day gradient, the value of ‘apparent’ denudationFig. 3. Plot illustrating how the constraints oncan be estimated by dividing the amount of coolingpalaeogeothermal gradients increase with the number of(difference between palaeo- and present-day mean annualmaximum palaeotemperature estimates per section.surface temperatures) by the geothermal gradient. Thepalaeo-surface temperature is that which was present atthe time when the subsurface section started to cool fromIn contrast to the problems in estimating palaeogeo-

thermal gradients, present-day gradients are relatively maximum temperatures.easy to constrain. This is due primarily to the fact thatboth temperature and depth measurements are routinely

NORTH SLOPE FORELAND BASIN:carried out during drilling. Therefore, after allowing forcorrection due to drilling activity in the temperature A CASE STUDYmeasurements, the present-day geothermal gradient can Geological settingusually be estimated within 1 or 2 °C km−1 (Duddyet al., 1991). Northern Alaska contains two principal geological fea-

tures, the Brooks Range fold and thrust belt and theNorth Slope foreland basin (Fig. 1). The fold and thrustEstimating denudationbelt is exposed in rugged, linear mountain ranges, ridgesand hills that rise to over 3000 m in the east, but areIn a well section that has been hotter in the past, the

plot of maximum palaeogeothermal gradient can be com- progressively lower in maximum altitude and relieftowards the west (Wahrhaftig, 1965). The fold and thrustpared with the present-day geothermal gradient, allowing

interpretation of the cause of the high palaeotempera- belt generally trends east–west throughout northernAlaska, but bends east-north-east in eastern Alaska andtures, and the cause of the subsequent cooling to present

temperature conditions (Fig. 2). Figure 4A presents a then curves to the south-east in north-western Canada.The North Slope foreland basin underlies the foothillsmore detailed example in which heating and subsequent

cooling were caused solely by deep burial followed by of the Brooks Range, a series of east-trending rollinghills and ridges, and the Arctic Coastal Plain, charac-denudation with no change in mean annual surface

temperature or basal heat flow. In this case, the present terized by marshland with little or no relief. The northernboundary of the basin approximately coincides with thegradient would remain the same as the maximum palaeo-

geothermal gradient, but the palaeotemperature profile north shore of Alaska and the Beaufort Sea, where a riftshoulder, now a subsurface passive high termed thetrend would be translated towards higher temperatures.

Therefore, the amount of removed section can be esti- Barrow arch, separates the foreland basin to the southfrom the Canada basin to the north.mated by dividing the amount of cooling by the geother-

mal gradient. Starting during the Late Jurassic, collision with anoceanic island arc to the south produced the BrooksFigure 4B introduces a further complication in that

the mean annual surface temperature has decreased Range orogen and the North Slope foreland basin (Mull,1982; Mayfield et al., 1988; Moore et al., 1994). Sincesignificantly since maximum palaeotemperatures were

reached (e.g. the North Slope foreland basin). If the the Late Jurassic, the Brookian orogeny has continued tocomplicate the tectonic history of northern Alaska (Mull,surface temperature at the time the section began to cool

from maximum temperatures is not known, and thus the 1982; Mayfield et al., 1988; Moore et al., 1994; O’Sullivan



Fig. 4. Hypothetical case to illustrate estimation of palaeogeothermal gradients and the cause of heating and subsequent coolingfrom palaeotemperature profiles. In each case a palaeotemperature profile is shown which is similar to the present-day profile,suggesting that heat flow has not changed since maximum palaeotemperatures were reached. However, the maximum profile isoffset to higher temperatures, indicating that the section has been hotter in the past. Therefore, heating in the past was causedby burial prior to cooling by denudation, and the amount of removed section can be estimated by dividing the amount of coolingby the geothermal gradient. (A) The simple case where mean annual surface temperatures have not changed since the time atwhich the section reached maximum palaeotemperatures and all cooling has resulted from denudation. (B) The case where themean annual surface temperature has decreased significantly since the time at which the section reached maximumpalaeotemperatures. In this case a component of ‘apparent’ denudation is introduced.

et al., 1997). During the early stages of the orogeny, in (AAPG & USGS, 1976; Blanchard & Tailleur, 1982).The organic maturation of rocks at the surface generallythe Late Jurassic and Neocomian, the arctic Alaska

passive margin was subducted beneath the oceanic island decreases northward across the North Slope, a reflectionof denudation in the Brooks Range orogenic belt andarc to the south, resulting in the emplacement of relatively

far-travelled thrust sheets and the obduction of ophiolite subsidence and sedimentation in the adjacent forelandbasin (Bird & Molenaar, 1992; Howell et al., 1992).(e.g. Roeder & Mull, 1978; Patton & Box, 1989; Miller

& Hudson, 1991). This was followed by extensional upliftof the core of the Brooks Range during the mid- Interpretation of AFT dataCretaceous (Mayfield et al., 1988; Moore et al., 1994).Younger deformation occurred during the Late Modern use of AFT analyses along with the above

methods has allowed details of the thermal history ofCretaceous to Early Tertiary in response to furthercompression (Bird & Molenaar, 1992; Moore et al., 1994). northern Alaska to be resolved, leading to an improved

understanding of the development of the foreland basinThis is characterized by gentle long-wavelength foldingof Lower Cretaceous and younger strata in the foreland (e.g. O’Sullivan et al., 1993, 1995, 1997; O’Sullivan,

1993, 1996; Murphy et al., 1994; Blythe et al., 1996,basin, and emplacement of thrust faults displaying rela-tively smaller amounts of shortening in the northern 1997). For example, AFT data from outcrop locali-

ties and wells indicate that most sedimentary rocksmountain front than that of the earlier contraction.within the North Slope foreland basin experiencedmaximum palaeotemperatures due to burial during theNorth Slope foreland basin thermal historyMaastrichtian to Early Palaeocene (O’Sullivan, 1996).Subsequently, these rocks have experienced a minimumPrevious knowledge of the North Slope thermal history

was based primarily on present-day thermal profiles, of two regional episodes of kilometre-scale denudationduring the Tertiary, the effects of which extended acrossmeasurements of organic maturation (mainly Ro, TAI

and CAI) and geothermal conditions calculated from the central and western North Slope foreland basin(Fig. 1) (O’Sullivan, 1996). The first occurred during thethese data. The present-day North Slope thermal regime

is characterized by variable geothermal gradients; higher Palaeocene (#60±4 Ma; all AFT errors are ±2s),probably recording the later Brookian orogenic phasegeothermal gradients are generally found in wells located

close to the present-day coastline and lower gradients in discussed previously. The second occurred as recently asthe late Oligocene at #25±3 Ma.wells located in the foothills north of the Brooks Range


P. B. O’Sullivan

Detailed modelling of AFT data from eight wells River suggest rapid cooling occurred at some timebetween #20 and 10 Ma (Fig. 6) (O’Sullivan, 1993,located in the central and western North Slope foreland

basin (Fig. 1) has been presented by O’Sullivan (1993, 1996). In contrast to the previous six wells, the preservedstratigraphy within these wells suggests that little or no1996). Location and stratigraphic information, analytical

results including single-grain age plots and track length section was removed during the late Oligocene toMiocene. Therefore, it is proposed that the recordeddistributions, and interpretative results from the model-

ling of the data generated from the foreland basin have cooling was primarily the result of a major change inclimatic conditions during the late Oligocene to Miocenebeen presented previously in O’Sullivan (1993; 1996) and

O’Sullivan & Brown (1998). As most of that information (O’Sullivan & Brown, 1998).From the available information it is unlikely that theis not pertinent to this paper, only representative results

from the wells are summarized here. late Oligocene deformation episode and the late Oligoceneto Miocene climatic change are related. Furthermore,The results from six of these wells suggest that the

sampled rocks experienced maximum palaeotemperatures due to the errors involved, it is not possible to separateout the effects of these two cooling events from onein response to burial during the Late Cretaceous to mid-

Palaeocene (Fig. 5). Subsequently, much of the basin another purely by modelling the AFT data.experienced multiple episodes of rapid cooling. Two ofthese occurred due to denudation; the first during the Middle to Late Cenozoic climatic changemid-Palaeocene at #60±4 Ma, and the second duringthe late Oligocene at #25±3 Ma (O’Sullivan, 1993, Besides the effects of denudation, the Tertiary thermal

history of the North Slope foreland basin is further1996). For reasons discussed below, a third episode ofcooling is proposed to have occurred in the late Oligocene complicated because the region has experienced a signifi-

cant change in climate, from temperate conditions at theto Miocene in response to a major change in climaticconditions. end of the Cretaceous to present-day arctic conditions.

This change has resulted in a decrease in the meanData from two wells located along the present-daynorthern coastline between Prudhoe Bay and the Canning annual surface temperature at some time in the Tertiary

from #+5 °C in the Late Maastrichtian (Spicer, 1987),to present-day values between #−4 °C along the north-ern flank of the Brooks Range, and #−12 °C along thenorthern coastline (Lachenbruch et al., 1988). The timingof this change in climate is poorly constrained primarilydue to a lack of an adequate data base (Clark, 1990).However, Clark (1982, 1990) proposed the transition ofthe Arctic Ocean from ice-free conditions to arctic

Fig. 5. Proposed depositional history for Husky Seabee #1 wellFig. 6. Proposed depositional history for the Alaska State C-1based on AFT, Ro and stratigraphic parameters (modified after

O’Sullivan, 1996). This presents a representative depositional well based on AFT, Ro and stratigraphic parameters (modifiedafter O’Sullivan, 1996; O’Sullivan & Brown, 1998). Thishistory within the North Slope foreland basin, but south of its

northern margin. Results suggest that maximum presents a representative depositional history along the north-eastern margin of the North Slope foreland basin. Solid linespalaeotemperatures were reached in the early Palaeocene prior

to two episodes of rapid cooling in the Tertiary. Symbols: represent true burial history, whereas dashed lines follow thethermal history in relation to prevailing down-holeBHT, bottom-hole temperature; T.D., total depth to bottom of

well. See text for details. temperatures. See text for details.



conditions occurred during the Middle to Late Cenozoic. change experienced by the North Slope during theMiddle to Late Cenozoic (scale of #106 years) is likelyBy the late Miocene, conditions essentially the same as

exist today were present (Clark, 1982). to have influenced the distribution of temperatures atdepths much greater than 500 m.This change in climate in the Miocene has resulted in

at least two possible complications affecting the Tertiary To test whether a decrease in the mean surface tem-perature of the order of #15–20 °C was sufficient tothermal history of the basin: (1) the formation of a thick

layer of permafrost to estimated depths of between #200 cause similar amounts of cooling at depths of severalkilometres within the crust, O’Sullivan & Brown (1998)and 650 m (Lachenbruch et al., 1988), and (2) a rapid

increase in the depth of isotherms near the surface in formulated a simple thermal model of the lithospherethat could be described in terms of the mathematicalresponse to the decrease in the mean annual surface

temperature. equations describing the transfer of heat (e.g. Carslaw &Jaeger, 1959, section 12.6, equation 10). The results oftheir calculations are presented graphically in Fig. 7.Formation of permafrost. Permafrost formed in response

to the change in climate from temperate to arctic con- These plots show that a decrease in temperature at thesurface of #20 °C results in a rapid (in geological time)ditions. Many consider permafrost to be simply material

containing ice. However, Muller (1947) best defines it decrease in shallow-level (<5 km) subsurface tempera-tures. For example, after only 0.2 Myr, temperatures atby its ‘negative centigrade’ temperature, thus suggesting

that the base of the permafrost is located at the position #2 km depth will have decreased between #11 and12 °C, and after 5 Myr, temperatures at #3 km depthof the 0 °C isotherm. Furthermore, since ice is a better

conductor than water, it is commonly expected that the will have decreased between #17 and 18 °C (Fig. 7).Therefore, in response to the drop in mean annualsubsurface geothermal gradient increases at the base of

the permafrost (Gretener, 1981). Original observations surface temperature at some time between #20 and10 Ma, the subsurface isotherms in time would havefrom wells drilled in Prudhoe Bay seemed to confirm

this prediction with geothermal gradients through the propagated downward, resulting in cooling of the strati-graphic section unrelated to erosion (O’Sullivan &permafrost (<10 °C km−1) being much lower than the

values present below the ice (>20 °C km−1). However, Brown, 1998).Lachenbruch et al. (1982) showed that the low gradientsthrough the permafrost were due to drilling operations Maximum palaeogeothermal gradientsdisturbing the measured heat flow and that very rapidly(in terms of years) the gradient reaches equilibrium and To determine maximum palaeogeothermal gradients from

the preserved sections, estimated maximum palaeotem-returns to values close to or equivalent to the gradientbelow the permafrost. peratures were calculated from the Ro and AFT data

using the methodologies discussed earlier. O’SullivanThe depth to the base of the permafrost is thereforerelated to the mean annual surface temperature and the (1993, 1996) provided the original AFT data and esti-

mated palaeotemperatures from modelled time–tempera-geothermal gradient (Gretener, 1981; Lachenbruch et al.,1988). A decrease in the mean annual surface temperature ture histories from each well. Original Ro data for the

majority of the wells were compiled from Johnsson et al.without a noticeable change in the heat flow would resultin an increase in the thickness of the permafrost with (1992). Vitrinite data from the Prudhoe Bay J-1 were

provided by W. Harris (personal communication 1990),little or no change in the geothermal gradient. Therefore,unless there has been a very recent alteration to the and from the Alaska State C-1 well by C. Ando (personal

communication 1990). Estimated maximum palaeotem-stratigraphic section (e.g. by stripping of the insulatingsoil layer) the presence of permafrost is not a complication peratures were then plotted against present depth in each

of the wells. Representative results are presented in Figswhen determining the present and palaeogeothermalgradients for the North Slope foreland basin. Any change 8 & 9.

As explained earlier, the slope of the fitted linearin the surface conditions simply results in a relativechange in the position of the 0 °C isotherm and in the relationship between maximum palaeotemperature and

present depth provides a direct estimate of the maximumthickness of the permafrost.palaeogeothermal gradient (maximum palaeotemperatureprofile) prior to cooling. With one possible exception,Subsurface response to a decrease in mean annual surface

temperature. It is well recognized that short-term climatic the ‘least-squares’ best-fit estimates of the maximumpalaeogeothermal gradient for each well, derived usingchanges such as glacial retreat from higher latitudes will

affect the geothermal gradient at shallow depths (e.g. the maximum palaeotemperature profiles, suggest themaximum palaeogeothermal gradients for each well areBeck, 1977; Gretener, 1981). The depth to which events

like the retreat of glaciers (scale of #103 years) will affect very similar to the present-day geothermal gradients.Furthermore, data from the Alaska State C-1 well suggestthe geothermal gradient is of the order of #500 m (Beck,

1977). However, it is significant that Beck (1977) also that rocks in the well are presently close to maximumtemperatures (Fig. 8).concluded that longer-term events would affect tempera-

tures at greater depths. Therefore, the long-term climatic The one well that exhibits a nonlinear palaeogeother-


P. B. O’Sullivan

Fig. 7. (a) Plot showing the change in temperature (DT) at depths (Z) less than 5 km through time (Dt) between 0 and 5 Myr,after a decrease in the mean annual surface temperature of #20 °C (modified after O’Sullivan & Brown, 1998). The plot showsthat after 0.2 Myr the temperature at #2 km depth will have decreased by #11 °C and after 5 Myr the temperature at #3 kmdepth will have decreased by #17 °C. (b) Plot of the temperature change vs. time (for a range of depths between 0.5 and 5 km).The plot shows that a decrease in surface temperature of 20 °C will cause a 17 °C decrease in temperature at a depth of 3 kmafter #5 Myr. Calculations assume an initial geothermal gradient of #30 °C km−1 and a thickness of the slab of #30 km. Seetext for details.

Fig. 8. Estimated maximum palaeotemperature profiles for the Prudhoe Bay J-1 and the Alaska State C-1 wells, derived from Roand AFT plotted against sample depth. Also shown are the temperature profiles present at the time the section began to coolfrom maximum palaeotemperatures. Cooling recorded in these two wells has been primarily in response to the Cenozoic decreasein mean annual surface temperature. Vitrinite reflectance data for the Prudhoe Bay J-1 were provided by W. Harris (personalcommunication 1990), and from the Alaska State C-1 by C. Ando (personal communication 1990).

mal gradient is the Inigok Test #1. Results from this those nearer the surface have been exposed to anestimated gradient of #27 °C km−1. It is important towell suggest that rocks at depths >2.5 km have been

exposed to an estimated gradient of #60 °C km−1 while note that the maximum near-surface palaeogeothermal



Fig. 9. Estimated maximum palaeotemperature profiles for the Inigok #1, Seabee #1, Lisburne #1 and Tunalik #1 wells, derived fromRo and AFT plotted against sample depth. Also shown are the temperature profiles present at the time the section began to cool frommaximum palaeotemperatures. The offset between maximum temperature and present temperature profiles in both wells is the result ofboth denudation and ‘apparent’ denudation. Vitrinite reflectance data for all wells from Johnsson et al. (1992).


P. B. O’Sullivan

gradient is similar to the present-day gradient of denudation started, not all cooling experienced by therocks from maximum palaeotemperatures has been the#29 °C km−1 (Fig. 9). There are two possible expla-

nations for this nonlinear gradient: (1) there is a break result of denudation. If the mean annual surface tempera-ture at the time cooling started was not known, calcu-in slope across a unconformity or structural discontinuity,

or (2) basal heat flow was nonlinear and palaeotempera- lations of the amount of denudation would be overestimated by an amount equal to the ‘apparent’tures at depth were increased due to the presence of hot

fluids or an unknown plutonic source. However, any denudation.Values used to calculate the amount of denudation andfurther explanation on the cause of the elevated values

at depth are beyond the scope of this discussion since ‘apparent’ denudation for the wells from the North Slopeforeland basin, including the maximum palaeogeothermalthis study is concerned only with the amount of section

which has been removed from the present-day surface. gradient and the present surface temperature, are summa-rized in Figs 8 & 9 and Table 1. For example, withinAltogether, these data indicate that the recorded maxi-

mum palaeogeothermal gradients across the North Slope the Seabee #1 well (Fig. 9), the estimated maximumpalaeotemperature experienced by rocks now at the sur-foreland basin, at least those near the surface, are very

similar to present-day gradients. This suggests that basal face equals #65–70 °C, and the present mean annualsurface temperature equals #−8 °C (Lachenbruch et al.,heat flow in the basin has changed little since the

Palaeocene when rocks throughout much of the basin 1988). Using the calculated geothermal gradient of#30 °C km−1, the Seabee #1 has experienced #2.1 kmbegan to cool in response to kilometre-scale denudation.

However, in most cases the maximum palaeotemperature of denudation and #0.5 km of ‘apparent’ denudation(Fig. 10). For most other wells, using the geothermalprofiles are offset towards higher temperatures as a result

of previous exposure to elevated temperatures. Since gradients listed in Table 1, the calculated values ofdenudation and ‘apparent’ denudation, respectively,basin heat flow values have not changed much since

maximum palaeotemperatures were reached, but the data equal: #5.6 km and #0.4 km (Kemik #2 well); #1.7 kmand #0.6 km (Inigok #1 well); #1.2 km and #0.4 kmsuggest the rocks in many of the wells experienced higher

temperatures in the past, this indicates section has been (Walakpa #1 well); #2.9 km and #0.4 km (Lisburne #1well); #1.3 km and #0.4 km (Tunalik #1 well); andremoved from the present-day surface as a result of

denudation. #0.3 km and #0.5 km (Prudhoe Bay J-1) (Fig. 10). Inthe case of the Alaska State C-1 well, no denudation hasoccurred; however, #0.6 km of ‘apparent’ denudationTotal amounts of denudation and ‘apparent’must have occurred in response to the decrease in thedenudationmean annual surface temperature from #5 °C to#−11 °C during the Late Tertiary.Previously, the estimated total amount of Cretaceous and

Tertiary denudation across the North Slope forelandbasin, based on the position of the 0.6% and 2.0% Ro Implications for the regional geologyisograds, ranges between #3 and 5 km within the foot-hills along the southern flank of the basin (Howell et al., Throughout the foreland basin, results discussed above

suggest that the sedimentary rocks within the basin were1992), to #500 m in the Inigok #1 well in the centre ofthe basin (Bird & Molenaar, 1992, their figure 17A), to exposed to maximum palaeotemperatures in the Late

Cretaceous to mid-Palaeocene, prior to cooling in#1 km in the region of Pt. Barrow in the north-westpart of the basin (Bird & Molenaar, 1992). Wells located response to denudation starting at #60±4 Ma (e.g.

O’Sullivan, 1993, 1996). Subsequently, the southernon the north-eastern coastline east of Prudhoe Bay havesupposedly experienced little or no denudation during part of the foreland basin experienced a second episode

of denudation in the late Oligocene at #25±3 Mathe Late Cretaceous and Tertiary (Bird & Molenaar,1987). (O’Sullivan, 1993, 1996). Recognition of this later episode

indicates that structures (large-scale folds and thrustUsing the maximum palaeotemperature estimates fromthe AFT and Ro instead of the isograds, the amount of faults) recognized along the southern flank of the foreland

basin could have formed initially or have been reactivatedremoved section can be estimated by dividing the degreeof cooling by the geothermal gradient. The degree of as recently as the late Oligocene. Evidence of Tertiary

denudation located in widespread regions far from areascooling is calculated by determining the maximum palaeo-temperature value at the present surface and subtracting with recognized episodes of Tertiary uplift (e.g. the

NEBR) indicates that tectonism was more active andthe mean annual surface temperature at the time coolingfrom maximum palaeotemperatures began. As determined regional than previously suspected.

Previous estimates of denudation across the forelandfrom the AFT data, cooling from maximum palaeotem-peratures occurred in the Palaeocene. The proposed mean basin have been largely based on regional studies (e.g.

Bird & Molenaar, 1992; Howell et al., 1992) which haveannual surface temperature in the Maastrichtian, justprior to the Palaeocene episode of cooling, was #5 °C provided little or no control on when actual denudation

occurred, and apparently presumed that denudation was(Spicer, 1987). Because there has been a noticeabledecrease in the mean annual surface temperature since responsible for all recorded cooling (e.g. Mayfield et al.,



Table 1. Amount of Tertiary uplift and erosion: North Slope foreland basin.

Surface Lt. Cenozoic Max. temp. Present Maximum ‘Apparent’ Totaltemp. cooling at surface gradient gradient uplift uplift

Well (°C) (°C) (°C) (°C km−1) (°C km−1) (km) (km)

Southern Flank (Foothills)Kemik #2 −7 −12 160 29 29 0.4 5.3Lisburne #1 −6 −11 90 29 30 0.4 2.9

Central Foreland BasinSeabee #1 −9 −14 70 30 30 0.5 2.1Inigok #1 −10 −15 50 28 27 0.6 1.7Tunalik #1 −10 −15 55 37 36 0.4 1.3

Northern Flank (Coastline)Walakpa #1 −12 −17 50 39 38 0.4 1.2Prud. Bay J-1 −11 −16 15 32 33 0.5 0.3AK. State C-1 −11 −16 5 27 27 0.6 0.0

Surface temperatures estimated from trends presented in Lachenbruch et al. (1988). Late Cenozoic cooling equals the change in mean annualsurface temperature since the foreland basin experienced temperatures of #5 °C in the Late Cretaceous (Spicer, 1987). Value of total uplift anderosion does not include the ‘apparent’ uplift and erosion.

Fig. 10. Estimated denudation of the North Slope foreland basin study area. See text for explanation.

1988; Bird & Molenaar, 1992; Howell et al., 1992). Along Palaeocene; these values are consistent with those men-tioned in Bird & Molenaar (1987, 1992).the southern flank of the foreland basin in the foothills,

Ro and AFT data summarized here indicate that the area However, in the central part of the foreland basin, Roand AFT data suggest that the area has experiencedhas experienced between #3 and 5 km of denudation

since the Palaeocene (Fig. 10, Table 1), values that are between #1 and 2 km of denudation since the Palaeocene(Fig. 10, Table 1), a larger amount of denudation thanconsistent with the total amount of Cretaceous and

Tertiary denudation proposed by Howell et al. (1992). the #500 m proposed for the Inigok #1 by Bird &Molenaar (1992, their figure 17A). It is likely that theAlong the northern flank of the basin (Barrow arch), Ro

and AFT data suggest that the area has experienced difference in the estimated values is a function of themodel proposed for the cooling. Howell et al. (1992) andbetween #0 and 1 km of denudation since the


P. B. O’Sullivan

Bird & Molenaar (1992) propose that isostatic movement and that cooling was due to both removal of section bydenudation and by a drop in the mean annual surfacewithin the crust was the most likely cause of Cretaceous

and Tertiary denudation. In contrast, O’Sullivan (1993, temperature of #10–20 °C.The data and interpretations from the North Slope1996) proposed that the majority of Cretaceous and

Tertiary denudation has been driven by compressional foreland basin illustrate the importance of understandingseveral factors when evaluating the thermal history andtectonics during the Palaeocene and late Oligocene.

A comparison between present-day geothermal gradi- the amount of denudation. These factors include knowl-edge of: (1) the mean annual surface temperature at theents and the palaeogradients present at the time the

region experienced maximum palaeotemperatures sug- time maximum palaeotemperatures were reached, (2) thegeothermal gradient at the time maximum palaeotempera-gests that basin heat flow has remained constant since

cooling from maximum palaeotemperatures began in the tures were reached and (3) the time at which coolingfrom maximum palaeotemperatures occurred. This studyPalaeocene (Figs 8 & 9). This is consistent with the

results of Deming et al. (1992), who concluded from also illustrates that long-term changes in mean annualsurface temperatures can impact subsurface cooling his-analysis of present-day thermal gradients and heat flow

calculations in the National Petroleum Reserve-Alaska tories and must be considered when estimatingdenudation.(NPRA) that the observed thermal pattern has persisted

for tens of millions of years.Finally, the impact of the drop in mean annual surface

ACKNOWLEDGMENTStemperature has gone unnoticed across the foreland basin.Because it is assumed that the basal heat flow has Support was provided by a grant from the Mineralsremained constant since the basin was exposed to maxi- Management Service. Sample irradiations at the HIFARmum palaeotemperatures in the Palaeocene, then all reactor were provided with the assistance of an AINSEsubsequent cooling within the basin has been related to grant to the La Trobe Fission Track Research Group.either denudation or drop in mean annual surface tem- The State of Alaska Division of Geological andperature. It is obvious from the values listed in Table 1 Geophysical Surveys provided well samples. Professorthat denudation is the dominant cause for cooling within A. J. W. Gleadow and Dr P. F. Green provided helpfulthe basin; however, it is important to note that the entire reviews of the original interpretations. I gratefullyNorth Slope foreland basin has also experienced #0.5 km acknowledge comments on early drafts of this manuscriptof ‘apparent’ denudation. Thus, future estimates of denu- by D. Issler, K. Bird, D. Deming, M. Mitchell and J. M.dation for the North Slope foreland basin should consider Murphy. K. Gallagher and T. Moore provided the finalMiddle to Late Cenozoic climatic cooling and the surface helpful suggestions.temperature at the time when cooling from maximumpalaeotemperatures began so that estimates of denudation

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