Geological Survey of Denmark and Greenland Bulletin 42, 2018, 133-147 133 Burial and exhumation history of the Jameson Land Basin, East Greenland, estimated from thermo- chronological data from the Blokelv-1 core Paul F. Green and Peter Japsen Apatite fission-track analysis (AFTA) data in two Upper Jurassic core samples from the 231 m deep Blokelv-1 borehole, Jameson Land, East Greenland, combined with vitrinite reflectance data and regional AFTA data, define three palaeo-thermal episodes. We interpret localised early Eocene (55– 50 Ma) palaeotemperatures as representing localised early Eocene heating related to intrusive activity whereas we interpret late Eocene (40–35 Ma) and late Miocene (c. 10 Ma) palaeotemperatures as representing deeper burial followed by successive episodes of exhumation. For a palaeogeothermal gradient of 30°C/km and likely palaeo-surface temperatures, the late Eocene palaeotemperatures require that the Upper Jurassic marine section in the borehole was buried below a 2750 m thick cover of Upper Jurassic – Eocene rocks prior to the onset of late Eocene exhumation. As these sediments are now near outcrop at c. 200 m above sea level, they have been uplifted by at least 3 km since maximum burial during post-rift thermal subsidence. The results are consistent with estimates of rock uplift on Milne Land since the late Eocene and with interpretation of Ocean Drilling Program (ODP) data off South-East Greenland suggesting that mid-Cenozoic uplift of the margin triggered the marked influx of coarse clastic turbidites during the late Oligocene above a middle Eocene to upper Oligocene hiatus. Keywords: East Greenland, Jameson Land, Upper Jurassic, apatite fission-track analysis, burial, exhumation ___________________________________________________________________________ P.F.G., Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia. E-mail: mail@geotrack.com.au P.J., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark With sedimentary basins offshore East Greenland yet to be drilled, the onshore Jameson Land Basin (Surlyk 2003) provides a window into the nature of potential Jurassic source-rock sequences in offshore basins. Understand- ing the thermal history and maturity development in the onshore sequences can therefore provide unique insights into the prospectivity of the offshore basins. The Jame- son Land Basin has itself been the focus of hydrocarbon exploration by a group of concessionaries with Atlantic Richfield (ARCO) as operator, but in 1990 the group decided not to continue exploration (Christiansen et al. 1992; Mathiesen et al. 2000). One of the main geologi- cal risks that ARCO critically assessed was the thermal maturity of their main target, the Upper Permian car- bonates, since earlier studies had concluded that Tertiary exhumation had removed up to 3 km of Cretaceous sedi- ments and Paleocene–Eocene volcanic rocks across the basin (Christiansen et al. 1992). Currently, Greenland Gas and Oil A/S and Nunaoil A/S hold a hydrocarbon exploration and exploitation licence across much of the Jameson Land Basin (www.govmin.gl). Presented here is a thermal history study of samples obtained from the core from the Blokelv-1 borehole, drilled by GEUS in 2008 to provide detailed informa- tion about the Upper Jurassic stratigraphy and petroleum system of the Jameson Land Basin (Fig. 1). GEUS suc- © GEUS, 2018. Geological Survey of Denmark and Greenland Bulletin 42 , 133–147. Available at: www.geus.dk/bulletin42 mailto:mail@geotrack.com.au http://www.govmin.gl http://www.geus.dk/bulletin42 134134 cessfully drilled and cored the Blokelv-1 borehole in the central part of the basin (ground level 200 m above sea level; a.s.l.), targeting the Upper Jurassic prolific source- rock interval of the Hareelv Formation (Bjerager et al. 2018a, this volume). The 231 m thick, marine succes- sion cored in the Blokelv-1 borehole covers the middle Oxfordian to lower Volgian interval. The core has 100% recovery, and it consists of interlayered organic-rich, lam- inated mudstones, massive sandstones and heterolithic sandstone–mudstone intervals of the Katedralen Mem- ber and massive sandstones of the Sjællandselv Member of the Hareelv Formation. The recovered core is of very high quality and has been subject to an extensive sam- pling and analytical programme designed to investigate aspects of the petroleum geolog y of the Jameson Land Basin and published in nine papers (Ineson & Bojesen- Koefoed (eds) 2018, this volume). The dataset provides an excellent reference for Kimmeridge Clay Formation- equivalent deposits in the North Atlantic area. At first glance, the relatively low-lying landscape of the Jameson Land Basin is in sharp contrast to the high terrains of the volcanic province along Blosseville Kyst to the south of Scoresby Sund, where elevations reach 3.7 km a.s.l. This might initially suggest that Jameson Fig. 1. A: Simplified geology of the Jameson Land Basin and adjacent areas showing the location of the Blokelv-1 borehole (modified after Surlyk 2003); inset shows the location of the study area in East Greenland. BK: Blosseville Kyst. K: Kangerlussuaq. B: Stratigraphic column of the Blokelv-1 borehole showing the stratigraphic level of the two samples used in this study (1, GC1052-1; 2, GC1052-2). Log modified from Bjerager et al. 2018a, this volume). Litho: Lithostratigraphic subdivision. Chrono: Chronostratigraphic subdivision. Sj. Mb: Sjællands- elv Member. TD: Total depth. B C C' 74°N 76°N Milne Land Profile in F ig. 6 S c o r e s b y S u n d 25 km22°W24°W26°W 20°WLiverpool Land A B Permian Carboniferous Pre-Carboniferous Fault Sandstone Mudstone Igneous intrusion GC1052 sample Blokelv-1 71°N 72°N Palaeogene volcanics Triassic Cretaceous JurassicIce K BK J a m e s o n L a n d Chrono.Litho. Volgian O xf or di an m id dl e up pe r lo w er up pe r K im m er id gi an U pp er Ju ra ss ic H ar ee lv F or m at io n K at ed ra le n M em be r Sj. Mb Depth (m) 50 1 2 100 150 200 233.8 TD PJ 1 135 Land has undergone a less complex post-Jurassic history of uplift and erosion compared to regions to the south. However, thermochronological data from the Jurassic sediments of the Jameson Land Basin presented here (see also Mathiesen et al. 2000; Hansen et al. 2001) provide evidence of post-Jurassic burial and exhumation that is surprisingly similar to the region around Kangerlussuaq, to the south (Larsen & Saunders 1998; Brooks 2011; Bonow et al. 2014; Japsen et al. 2014). Thermal history interpretation AFTA data Two sandstone core samples from the lower and upper levels of the Blokelv-1 borehole (Fig. 1B) were processed for apatite fission-track analysis (AFTA), and both sam- ples gave excellent apatite yields. Apatite fission track ages of 50.4 ± 6.2 Ma and 38.5 ± 4.0 Ma in the two sam- ples are much less than the depositional age of the sam- pled units; at depths of less than 250 m (and present-day temperatures less than 20°C), this degree of age reduc- tion immediately shows that the sampled units have been much hotter in the past. Mean track lengths of 12.25 ± 0.22 µm and 11.71 ± 0.23 µm also demonstrate that these samples have been hotter in the past, prior to cooling to present-day temperatures. Full details of the AFTA data are provided in Ap- pendix 1. Quantitative thermal history constraints have been extracted from the data using principles outlined by Green & Duddy (2012) and Green et al. (2013), with re- sults summarised in Table 1. AFTA data from both sam- ples provided highly reliable thermal history constraints. Thermal history interpretation of AFTA data The AFTA data in sample GC1052-1 can be explained in terms of two palaeo-thermal episodes, as detailed in Table 1. In contrast, the AFTA data in sample GC1052- 2 require three palaeo-thermal episodes to explain all aspects of the data, although the precise timing of the earliest episode cannot be defined with confidence. This is because the palaeotemperature of 100–110°C in the second episode produced almost total annealing of all tracks formed up to that time, largely masking the previ- ous history. On the basis of evidence discussed below, we Table 1. Palaeotemperature analysis summary: AFTA and VR data from the Blokelv-1 borehole Sample Mean Present Stratigraphic VR** Maximum Onset Maximum Onset number depth temperature* age (%) palaeotemp.+ of cooling+ palaeotemp.+ of cooling+ GC1052- (m below KB) (°C) (Ma) (°C) (Ma) (°C) (Ma) 1 6 5 159–146 100–105 58–28 70–80 17–5 23.59 159–146 0.52 86 32.78 159–146 0.54 90 68.77 159–146 0.50 83 80.77 159–146 0.55 91 92.74 159–146 0.56 93 104.79 159–146 - - 116.82 159–146 0.56 96 152.75 159–146 0.61 100 164.82 159–146 0.68 113 176.77 159–146 0.62 102 188.77 159–146 0.65 108 200.76 159–146 0.66 109 212.77 159–146 0.78 125 2 219 12 159–146 >110 55–50? 100–110 50–28 80–85 13–3 224.74 159–146 0.74 121 Combined timing (Ma): 55–50 13–5 50–28 * Present temperature estimates based on an assumed surface temperature of 4°C, and an assumed thermal gradient of 30°C/km. ** From Bojesen-Koefoed et al. (2018). + Thermal history interpretation of AFTA data is based on an assumed heating rate of 1°C/Myr and a cooling rate of 10°C/Myr. Quoted ranges for palaeotemperature and onset of cooling correspond to ±95% confidence limits. Conditions shown in italics represent events that cannot be rigorously defined from the AFTA data. PJ table 1 136136 infer that the earlier event in which this sample cooled below 110°C was related to igneous activity in the region (Larsen 2018, this volume), in the interval 55 to 50 Ma. Timing constraints derived from AFTA data in each sample are listed in Table 1, and in Table 2 these con- straints are compared with the timing of three Cenozoic cooling episodes defined from AFTA data in two previ- ous studies of the East Greenland margin: (A) a study of the region north of the Jameson Land Basin (Thomson et al. 1999) and (B) a study in the Kangerlussuaq region to the south of Jameson Land ( Japsen et al. 2014). The similarity in timing of the late Eocene and the late Mio- cene cooling episodes defined in this study and in the previous studies suggests that each represents a regional, synchronous cooling episode across the entire region, and by combining all constraints we arrive at our preferred timing of the onset of cooling in these events, between 40 and 35 Ma and ~10 Ma (Table 2). In addition to the episodes shown in Fig. 2, Japsen et al. (2014) also defined late Oligocene and early Miocene palaeo-thermal epi- sodes in the region around Kangerlussuaq, but these are restricted to that region and do not extend to the Jame- son Land Basin, so are not considered here. We interpret the three events illustrated in Fig. 2 (from Table 2) in the following way: 55–50 Ma event. The timing of the early Eocene (55–50 Ma) event overlaps with that of intensive Palaeogene in- trusive activity and correlates with the age of c. 53 Ma for dykes and sills in the Jameson Land Basin (Hald & Tegner 2000; Larsen 2018, this volume). Palaeotempera- tures associated with this event are 100°C or above and are identified sporadically around the region ( Japsen et al. 2014). On this basis, the palaeotemperatures charac- terising this episode are interpreted to be due either to contact or hydrothermal effects associated with igneous activity. No convincing evidence for any regional Pale- ocene to mid-Eocene cooling (related to exhumation) has been identified for samples in the area around Jame- son Land (Thomson et al. 1999; Japsen et al. 2014), and the geological history recorded south of Jameson Land indicates that subsidence and burial dominated at the Palaeocene–Eocene transition (Brooks 2011; Bonow et al. 2014). 40–35 Ma event. Late Eocene cooling beginning be- tween 40 and 35 Ma was interpreted largely in terms of regional uplift resulting in kilometre-scale exhumation by both Japsen et al. (2014) in the Kangerlussuaq region and Thomson et al. (1999) in the region to the north of Jameson Land. Japsen et al. (2014) interpreted the end- result of this phase of exhumation to have been a regional peneplain, the Upper Planation Surface (UPS) of Bonow et al. (2014). The presence of late Eocene intrusive bodies around Traill Ø (Price et al. 1997) suggests the possibility locally of a significantly elevated basal heat flow in this region at this time. 10 Ma event. Late Miocene cooling beginning at c. 10 Ma was again detected by both Japsen et al. (2014) in AFTA data 55–50 50–28 13–5 Blokelv samples GC1052-1, -2 AFTA data (Study A) 40–30 10–5 AFTA data (Study B) * 40–35 ~10 Age of intrusion (Study C) ~53** (55–51) Preferred regional timing 55–50 40–35 ~10 (early Eocene) (late Eocene) (late Miocene) Dominant mechanism of cooling Cooling after intrusive heating Exhumation Exhumation Table 2. Intervals defining the beginning of episodes of cooling from AFTA data Onset of cooling (Ma) Study A. 11 outcrop samples from northern East Greenland, north of the Jameson Land Basin, 72–74°N (Thomson et al. 1999). Study B. 90 samples from outcrops and drillholes in a regional study of southern East Greenland focussed between 68 and 70°N (Japsen et al. 2014). Study C. Analysis of the igneous intrusions in the Blokelv cored borehole (Larsen 2018, this volume). *Japsen et al. (2014) related an event of cooling with overlapping timing (55–50 Ma) in southern East Greenland to the emplacement of the Kangerlus- suaq Intrusion. **The Blokelv sills are tholeiitic basalts considered to belong to the main group of dykes and sills in the Jameson Land Basin which has been dated at ~53 Ma. The intrusions form part of a 55–51 Ma group of tholeiitic basalt intrusions that were emplaced within the sedimentary basins in East Green- land (Larsen 2018, this volume). PJ table 2 137 the Kangerlussuaq region and Thomson et al. (1999) to the north of Jameson Land. In both cases cooling was interpreted in terms of regional uplift resulting in kilo- metre-scale exhumation. Outcrop samples at low eleva- tions around Kangerlussuaq cooled from peak palaeo- temperatures around 60–70°C at this time. Japsen et al. (2014) interpreted these values to represent burial below the UPS that defines the present-day surface of elevated summits along the Blosseville Kyst. Table 2 illustrates a high degree of consistency between the timing of cooling identified in this study and the dominant regional episodes identified by Thomson et al. (1999) and Japsen et al. (2014). On this basis, the results from sample GC1052-1 are interpreted as representing the two most recent episodes (i.e. late Eocene and late Miocene), while these two episodes as well as the early Eocene episode are recognised in sample GC1052-2. Thermal history interpretation of vitrinite reflectance data Results of vitrinite reflectance (VR) analyses are plot- ted against depth below surface in Fig. 3, together with equivalent VR (VR eq ) values derived from Rock-Eval T max values (Bojesen-Koefoed et al. 2018, this volume). Also shown are the ranges of equivalent VR values (VR eq ) derived from AFTA data in samples GC1052-1 and -2 (defined by the maximum paleotemperature in each sam- ple), together with the VR profile predicted on the basis 0 0255075100125 Time (Ma) 150175200225250 50 100 150 200 250 D ep th (m ) Q GC1052-1 Stratigraphic age Regional cooling episodes: Regional constraints on the onset of cooling Palaeothermal constraints from AFTA sample Triassic Jurassic Cretaceous Cenozoic GC1052-2 55–50 Ma 40–35 Ma ~10 Ma PJ 2 Fig. 2. Timing constraints on cooling episodes derived from AFTA data in two samples from the Blokelv-1 borehole (horizontal bars) plotted against depth. The regional constraints on the onset of cool- ing in three palaeo-thermal episodes (vertical bars) are based on AFTA data in the Blokelv samples and in regional studies (Table 2). Q: Quaternary. Fig. 3. Maturity indicators in samples from the Blokelv-1 borehole plotted against depth; VR values (Table 1) together with equivalent VR values (VReq) derived from Rock-Eval Tmax data and equivalent ranges of VR defined from the AFTA data. Note that the orange ar- row for the deeper sample indicates that the constraints on maturity from the AFTA data only provide a lower limit. The solid sub-verti- cal line shows the profile predicted from the “Default Thermal His- tory”, i.e., the history calculated from the assumption that all units throughout the well are currently at their maximum temperatures since deposition. The horizontal lines indicate the position of thin basaltic sills intruded into the Hareelv Formation (note the maturity effects close to these intrusions). 0 20 40 60 80 120 140 160 180 200 220 240 100 0.2 0.3 0.4 0.5 0.7 1.0 2.0 GC1052-2 GC1052-1 D ep th (m ) Default history Maturity (%Ro) U pp er Ju ra ss ic Sill Sill Sill Measured VR values (Bojesen-Koefoed et al. 2018, this volume) Equivalent VR from AFTA Equivalent VR from Tmax AFTA sample horizon PJ 3 138138 of the Default Thermal History (i.e. the history expected if the section has never been any hotter than it is today). Both VR and VR eq data plot well above the profile pre- dicted by the Default Thermal History, confirming the evidence from AFTA that the sampled units have been hotter than their present-day temperatures at some time since deposition. Mean VR values tend to be slightly lower than the VR eq values derived from the T max data throughout the section. In general, T max values tend to be sensitive to a range of factors and are not used quantita- tively to provide estimates of maximum palaeotempera- ture in the way that VR data are. Note that the T max data define local contact effects due to the three recognised intrusions, which the VR values do not show. Maximum palaeotemperatures derived from the VR values (based on Burnham & Sweeney 1989) are listed in Table 2, and show a progressive downhole increase from 86°C to 125°C through the section intersected in the borehole. This apparent increase of c. 40°C over a depth interval of c. 200 m is equivalent to a thermal gradient of 200°C/km, which is well outside the range of typi- cal sedimentary basin thermal gradients (Allen & Allen 2013). Integration of results from AFTA with the VR data resolves this apparent anomaly, as explained below. Integration of AFTA and VR data, palaeo- temperature profiles and mechanisms of heating and cooling Palaeotemperature constraints from AFTA and the measured VR values are plotted against depth in Fig. 4. Maximum palaeotemperatures derived from VR val- ues at depths between 20 m and 120 m are c. 10–20°C lower than the late Eocene palaeotemperature range of 100–105°C indicated by AFTA data in sample GC1052- 1 at similar depths. In contrast, maximum palaeotem- peratures of c. 125°C derived from the two deepest VR values at depths of 210–225 m are higher than the late Eocene palaeotemperatures derived from AFTA in sam- ple GC1052-2 at a similar depth. They are, however, con- sistent with the early Eocene palaeotemperature defined from AFTA data in this sample. Furthermore, as illustrat- ed in Fig. 4, maximum palaeotemperatures derived from VR values between 150 m and 200 m are consistent with the trend of the late Eocene palaeotemperatures derived from AFTA in samples GC1052-1 and -2. We interpret the mismatch at shallow depths between VR values and the late Eocene palaeotemperature range from AFTA in sample GC1052-1 as due to suppression of reflectance levels at these depths. Suppression is com- monly observed in Upper Jurassic organic-rich mud- stones of the North Atlantic region (Wilkins et al. 1992; Newman 1997). Bjerager et al. (2018b, this volume) and Bojesen-Koefoed et al. (2018, this volume) provided de- tailed discussions of the differences between the organic material above and below c. 100 m in the cored interval. They showed that at shallow depths, amorphous marine 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Temperature (°C) 0 50 100 150 200 250 D ep th (m ) AFTA sample horizon (with sample number) Maximum palaeotemperature from VR Estimates of maximum post-depositional palaeotemperatures from AFTA Sill Sill Sill U pp er Ju ra ss ic GC1052-2 GC1052-1 Local heating? 30°C/km30°C/km Low due to suppression of VR? Thermal gradient (°C/km) 100 80 60 40 20 PJ 4 Fig. 4. Interpreted palaeotemperature profiles describing the pa- laeotemperatures in two episodes derived from AFTA and VR data in the Blokelv-1 borehole. Based on evidence from regional studies of East Greenland (Table 2), we interpret the early Eocene (55–50 Ma) maximum palaeotemperature (green arrow) revealed by AFTA data in sample GC1052-2 to represent localised heating due to in- trusive activity. The late Eocene (40–35 Ma) and late Miocene (c. 10 Ma) palaeotemperatures (red and blue horizontal bars, respec- tively) revealed by AFTA are interpreted to represent the effects of burial, with late Eocene cooling representing the onset of regional, post-Jurassic exhumation, and late Miocene cooling representing the final phase of exhumation. It is thus inferred that the VR val- ues recorded above 120 m in the borehole (yellow datapoints) are anomalously low due to suppression of ref lectance in the source- rock facies (organic-rich mudstones) (Wilkins et al. 1992; Newman 1997). We regard the VR values below 120 m (red datapoints) as re- liable indications of the degree of post-depositional heating. Linear profiles (red and blue lines) represent our preferred interpretation (based on regional data) involving palaeogeothermal gradients of ~30°C for the late Eocene and Miocene episodes. 139 organic material dominate whereas an increased content of terrigenous organic material occurs at depth. As illustrated in Fig. 4, maximum palaeotemperatures from VR data at depths between c. 150 m and 200 m, to- gether with late Eocene palaeotemperatures derived from AFTA in samples GC1052-1 and -2 define a linear pro- file characterised by a palaeogeothermal gradient around 30°C/km. Late Miocene palaeotemperatures defined from AFTA in the two samples are also consistent with a similar palaeogeothermal gradient. This gradient con- trasts markedly with the apparent gradient of c. 200°C/ km defined by the VR data (see above), and is more typi- cal of heating related to deeper burial. Accepting this interpretation, units throughout the borehole underwent a major phase of cooling in the late Eocene (beginning between 40 and 35 Ma), followed by a later phase of cooling in the late Miocene (c. 10 Ma). Most units cooled from maximum post-depositional pal- aeotemperatures in the late Eocene event but locally some horizons reached higher palaeotemperatures in the early Eocene, presumably reflecting the effects of intrusive bodies. This interpretation is consistent with the results reported from the Kangerlussuaq region to the south by Japsen et al. (2014), who regarded the late Miocene and late Eocene cooling episodes as representing successive episodes of exhumation, while early Eocene events were interpreted as local effects associated with igneous intru- sions. Thermal history synthesis On the basis of the discussion presented above, we in- terpret the palaeotemperature constraints derived from AFTA and VR data in the Blokelv-1 borehole as repre- senting the combined effects of deeper burial followed by successive episodes of exhumation in the late Eocene and Miocene, as well as localised early Eocene heating due either to contact heating or hydrothermal effects associ- ated with intrusive activity. The results provided here are highly consistent with regional data and the interpreta- tion presented here is regarded as reliable. Thermal history reconstruction Here we present reconstructed thermal and burial-uplift histories for the Upper Jurassic section intersected in the Blokelv-1 borehole (Fig. 5) based on the results presented above. It should be emphasised that while the preferred reconstruction illustrated here provides a satisfactory ex- planation of the AFTA and VR data from this well, the reconstruction is not unique. Therefore, it is important to appreciate those aspects of the histories that are con- strained by the data, and those that are not. Factors that can be confidently defined in this study (within the limits of analytical uncertainty) include: (a) magnitude of heating at the palaeothermal maximum and the subsequent palaeo thermal peaks and (b) tim- ing of the onset of cooling in each episode. Factors that can be defined when samples are available over a suffi- ciently large range of depths or elevations, but cannot be constrained in this study include: (a) palaeogeothermal gradients during each episode and (b) additional burial during each episode for a specified value of palaeogeo- thermal gradient. Aspects which cannot be uniquely de- fined in any situation include: (a) thermal history prior to the palaeo-thermal maximum and/or the subsequent palaeo-thermal peak, (b) amounts of re-burial between multiple episodes within a single unconformity and (c) detailed style of cooling history from each episode. Figure 5A illustrates a possible thermal history recon- struction for the sedimentary units intersected in the Blokelv-1 borehole, based on the synthesis developed above. Key aspects of this reconstruction are: • Surface temperature of 20°C at 30 Ma and earlier, de- creasing to 10°C at 10 Ma and to a present-day value of 4°C over the last 12 Myr. • Palaeogeothermal gradient of 30°C/km, constant to the present day. • Localised heating within the vicinity of sample GC1052-2 to a palaeotemperature around 120°C, shown at c. 53 Ma but any time between 56 and 45 Ma is allowed by the regional timing constraints on this episode. • An additional 2750 m of section deposited above the Upper Jurassic section intersected in the borehole, be- tween 146 and 35 Ma (resulting in heating of sample GC1052-1 to c. 100°C prior to late Eocene cooling and exhumation). • Subsequent removal of 1150 m (arbitrary) of section between 35 and 30 Ma, followed by deposition of a further 600 m (arbitrary) of section between 30 and 10 Ma. • Removal of the remaining 2200 m of additional sec- tion between 10 Ma and the present day. Note that the amount of re-burial between the two epi- sodes of exhumation cannot be controlled by the data, 140140 and therefore also the amount of section removed in the initial episode beginning at 35 Ma is also not constrained. This reconstruction is considered to provide a reliable de- piction of the post-depositional history of the Upper Ju- rassic section intersected in the Blokelv-1 borehole. Note that in this reconstruction, for the purposes of illustra- tion, the localised heating around sample GC1052-2 at c. 53 Ma is shown as taking place over a duration of 2 Myr, as is the subsequent cooling. However, in reality heat- ing and cooling would have been much more rapid, and for that reason the true maximum palaeotemperatures would have been much higher. Comparison with previous studies Magnitude of exhumation Christiansen et al. (1992) presented the results of a first study of the exhumation of the Jameson Land Basin. They based their estimate on a range of observations including thermal maturity parameters, apatite fission- track parameters, porosity and seismic velocities; in par- ticular they reported VR values up to 0.6% for organic- rich Permian and Jurassic formations at outcrop. They interpreted their data to indicate that between 1.5 and 3 km of overburden had been removed across the basin, 0 20 40 60 80 100 160 140 120 100 80 Time (Ma) 60 40 20 0 120 Te m pe ra tu re (º C ) Possible reconstruction J K Pa PE O M GC1052-1 GC1052-2 ? Possible reconstruction J K Pa PE O M 0 2000 1500 1000 500 2500 3000 Bu ri al d ep th (m ) 160 140 120 100 80 Time (Ma) 60 40 20 0 Regional constraints on the onset of cooling Palaeothermal constraints from AFTA in sample GC1052-1 Palaeothermal constraints from AFTA in sample GC1052-2 Thermal histories for the two sample horizons Approximate burial depths during late Eocene and late Miocene palaeothermal peaks ? A B PJ 5 Fig. 5. Schematic illustration of the preferred thermal (A) and burial (B) history reconstruction for the section intersected in the Blokelv-1 borehole based on the AFTA and VR data. J: Jurassic. K: Cretaceous. Pa: Palaeocene. E: Eocene. O: Oligocene. M: Miocene. P: Pliocene. A: Comparison of palaeothermal constraints from AFTA in two samples (red and blue boxes) with thermal histories of the corresponding sample horizons (red and blue curves). The vertical columns show the preferred timing of three dominant palaeothermal episodes identified in the region (Fig. 2; Table 2). Black line at the top of the diagram defines the assumed palaeosurface temperature. B: Burial and exhumation history corresponding to A; the blue boxes show the approximate depth of burial during the late Eocene and late Miocene palaeothermal peaks. Note that no convincing evidence for regional Paleocene to mid-Eocene exhumation has been identified for samples in the area around Jameson Land (Thomson et al. 1999; Japsen et al. 2014). 141 and that the lost cover likely consisted of more than 1 km of Cretaceous sediments as well as 1–2 km of Palaeogene basalts (primarily across the southern part of the basin). Mathiesen et al. (2000) investigated the denudation history of the Jameson Land Basin using basin modelling constrained by apatite fission-track data. They concluded that the Upper Jurassic sediments that are exposed at the present day across Jameson Land were buried below a 2–3 km thick rock column: (1) a Cretaceous succession that varied from 1.3 km in the south to 0.3 km in the north; (2) a wedge of Palaeogene volcanics with a thickness of >2 km in the south thinning to <0.1 km in the north. According to their calculations, the erosion happened in response to tectonic uplift of c. 1 km. The magnitude of the section removed at the location of the Blokelv-1 borehole defined in this study (c. 2.8 km) thus agrees well with that estimated by Mathiesen et al. (2000) (removal of 2 –3 km of section across Jameson Land). Hansen et al. (2001) studied the late Mesozoic – Ce- nozoic thermal history of the Jameson Land Basin con- strained by apatite and zircon fission-track data of Per- mian to Jurassic sedimentary rocks at outcrop. These authors interpreted their results in terms of regional thermal evolution related to burial leading to tempera- tures close to and in excess of the maximum temperatures of the apatite annealing interval (c. 125°C) followed by cooling mainly due to Cenozoic uplift and erosion. Fur- thermore, basaltic dyke and sill intrusions were found lo- cally to cause resetting of apatite fission-track ages. These results are thus broadly in agreement with the conclu- sions presented here. Our estimate of the magnitude of the section re- moved above the Blokelv-1 borehole (c. 2.8 km) agrees well with the results of Bonow et al. (2014) and Japsen et al. (2014) in their studies of the area between Milne Land and Kangerlussuaq (68–71°N) based on integra- tion of evidence from stratigraphic landscape analysis, thermochronolog y and the stratigraphic record (Fig. 6). These authors argued that the present-day high elevation in East Greenland is the result of three tectonic phases of uplift and erosion during the Cenozoic that followed the eruption of voluminous flood basalts onto a largely horizontal lava plain near sea level at the Paleocene– Eocene transition (Larsen & Saunders 1998; Brooks 2011; Bonow et al. 2014), viz: 1. The late Eocene (c. 35 Ma) phase of uplift and ero- sion led to formation of an Oligo–Miocene erosion surface (peneplain) near sea level, the Upper Plana- tion Surface (UPS). 2. Uplift of this surface in the late Miocene (c. 10 Ma) led to formation of a lower surface (the Lower Pla- nation Surface, LPS) by incision below the uplifted UPS. 3. An early Pliocene uplift phase (c. 5 Ma) led to in- cision of valleys and fjords below the LPS, result- ing in mountain peaks reaching 3.7 km a.s.l. Today, remnants of the UPS are preserved west of Jameson Land near the summits of Milne Land at c. 2 km a.s.l. Bonow et al. (2014) estimated the magnitude of rock uplift at Milne Land to be c. 2.7 km based on Larsen et al.’s (1989) investigation of zeolite isograds (levels of equal thermal alteration; see also Neuhoff et al. 1997), and on their argument that the absence of the shallow and less altered zeolite zones may be explained by the removal of these zones by erosion. For Milne Land, they concluded that a section of about 900 m had been removed above the basalt flows that cover the summits there (assuming a palaeogeothermal gradient of 40°C at the time of zeolite formation). Bonow et al. (2014) then assumed that the palaeo-surface during the formation of the zeolites was near sea level (shortly after the eruption of the volcanics 3 Milne Land Land surface prior to onset of late Eocene denudation relative to present-day sea level Removed section Present-day land area W E Jameson Land ? Liverpool Land 25 km Blokelv borehole 2 El ev at io n (k m a .s. l.) 1 0 PJ 6 Fig. 6. Present-day elevation profile with indication of the section removed since late Eocene maximum burial. Based on Bonow et al. (2014) and Japsen et al. (2014). For location of profile, see Fig. 1. 142142 at the Paleocene–Eocene transition at a time of regional subsidence; Brooks 2011). Consequently, the basalts, now at c. 1800 m a.s.l., have been uplifted 2700 m (i.e. 1800 + 900 m) since the formation of the zeolites in the Eocene. Japsen et al. (2014) found that the UPS that defines the summits of Milne Land at c. 2 km a.s.l. represents the remnants of the Oligo-Miocene peneplain that was formed by erosion to sea level after late Eocene uplift and erosion, and that this peneplain was uplifted to its pre- sent elevation during uplift that began in the late Mio- cene. Assuming that the lost cover of 900 m (estimated from zeolite isograds) had been removed after late Eo- cene maximum burial, the magnitude of rock uplift was about 2.9 km since the late Eocene. These estimates of the magnitude of rock uplift thus match those presented here because the Upper Jurassic marine sediments at outcrop (now at c. 200 m a.s.l.) at the Blokelv-1 location have been uplifted by a minimum of 3.0 km since their maximum burial below a (now lost) cover of c. 2.8 km in the late Eocene. Timing of burial and exhumation With no way of determining the timing of the onset of exhumation, Christiansen et al. (1992) and Mathiesen et al. (2000) assumed that the exhumation of the Jame- son Land Basin accelerated after the Palaeogene volcanic eruptions. The present study, however, clearly shows that the onset of exhumation was in the late Eocene (c. 35 Ma) and documents that the exhumation process took place in at least two stages. These results thus imply that the section of rocks removed across Jameson Land not only included volcanic rocks extruded at the Paleocene– Eocene transition (as assumed by Mathiesen et al. 2000) but also a sedimentary cover deposited during the 20 Myr that followed the volcanic eruptions till the onset of ex- humation in the late Eocene. The geological record from the area south of Jame- son Land confirms the timing of the Cenozoic events of burial and exhumation presented here. The area be- tween Scoresby Sund and Kangerlussuaq subsided after continental breakup at 56 Ma. This is documented by the Igtertivâ Formation which immediately overlies the Main Basalts of Larsen et al. (1989, 2013) and includes volcanic flows interdigitated with marine sediments. The volcanic pile of the Main Basalts is up to 6 km thick and was erupted in less than 1 Myr (Pedersen et al. 1997). Subsidence continued during deposition of the fluvial to shallow-marine Kap Dalton Group (early to mid Lute- tian), which interfingers with and overlies the Igtertivâ Formation in a downfaulted block at Kap Dalton (Larsen et al. 1989, 2005, 2013). Results from ODP Site 918, off SE Greenland (Leg 152, c. 63°N) show that major uplift of the margin oc- curred long after continental break-up. Larsen et al. (1994) reported that the marine, lower Eocene sediments drilled there indicated low sedimentation rates with lim- ited terrigenous influx before a middle Eocene to upper Oligocene hiatus. Larsen et al. (1994) thus argued that mid-Cenozoic uplift of the inner margin triggered the sudden, voluminous influx of coarse clastic turbidites at this ODP Site during the late Oligocene. Uplift phases in the late Miocene and in the Pliocene are consistent with the late Miocene, early Pliocene and middle Pliocene ages of seismic sequence boundaries within the late Neogene and Quaternary deep-sea sedi- mentary succession off SE Greenland (Clausen 1998). Conclusions Three palaeo-thermal episodes affected the Upper Juras- sic sediments penetrated by the Blokelv-1 borehole based on AFTA and VR data combined with regional AFTA data. These episodes are interpreted to be due to the fol- lowing mechanisms: • Early Eocene (55–50 Ma) palaeotemperatures repre- sent localised early Eocene heating related to intrusive activity. • Late Eocene (40–35 Ma) palaeotemperatures repre- sent deeper burial followed by exhumation. • Late Miocene (c. 10 Ma) palaeotemperatures repre- sent deeper burial followed by exhumation. The presence of two elevated planation surfaces in the region that were formed and uplifted after the volcanic eruptions supports the interpretation of the palaeother- mal data in terms of episodic rather than monotonic cooling (Bonow et al. 2014; Japsen et al. 2014). The late Eocene palaeotemperatures require that c. 2800 m of Upper Jurassic – Eocene rocks covered the Upper Jurassic section in the borehole prior to the onset of late Eocene exhumation, assuming a palaeogeothermal gradient of 30°C/km and likely palaeo-surface tempera- tures. This implies that maximum burial in the Jameson Land Basin was achieved long after the rift climax in central East Greenland at the Jurassic–Cretaceous tran- 143 sition (Surlyk 2003) and after the volcanic eruptions that accompanied break-up in the north-east Atlantic at the Paleocene–Eocene transition (Pedersen et al. 1997). As the Upper Jurassic sediments at the location of the Blokelv-1 borehole now crop out at c. 200 m a.s.l., they have been uplifted by at least 3 km since maximum burial during post-rift thermal subsidence. Such a magnitude of rock uplift is comparable with estimates from Milne Land where a regional peneplain (the upper planation surface, UPS) defines the summits at c. 2 km a.s.l. This surface was formed by erosion to sea level (after removal of a rock column of c. 900 m) following late Eocene up- lift and subsequently uplifted to its present elevation dur- ing uplift that began in the late Miocene. Consequently, the rock uplift on Milne Land was c. 2.9 km since the late Eocene. Rock uplift in the order of 3 km since the late Eocene has thus affected a wide area, far beyond the boundaries of the Jameson Land Basin. That strong uplift of the East Greenland margin be- gan at the Eocene–Oligocene transition is supported by interpretation of ODP data off South-East Greenland which suggest that uplift of the margin at this time trig- gered the marked influx of coarse clastic turbidites dur- ing the late Oligocene above a middle Eocene to upper Oligocene hiatus. Acknowledgements We acknowledge the pertinent and constructive com- ments of the referees, Andrew Carter and Andrew G. Whitham. Jette Halskov and Stefan Sølberg are thanked for graphical support. Reference list Allen, P.A. & Allen, J.R. 2013: Basin analysis: Principles and appli- cation to petroleum play assessment, 632 pp. Indianapolis: John Wiley & Sons. 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Organic Geochemistry 18, 629–640. _________________________________________________________________________________________ Manuscript received 16 December 2015; revision accepted 8 February 2018 145 Slide Current Ns Ni Na ρs ρi RATIO U Cl FT Age ref grain no (ppm) (wt%) (Ma) G1124-9 3 1 23 36 4.414E+04 1.015E+06 0.043 8.1 0.04 12.2 ± 12.4 G1124-9 6 14 51 24 9.270E+05 3.377E+06 0.275 27.0 0.03 76.4 ± 23.1 G1124-9 7 0 13 49 0.000E+00 4.216E+05 0.000 3.4 0.14 0.0 ± 36.1 G1124-9 10 29 129 30 1.536E+06 6.833E+06 0.225 54.7 0.01 62.6 ± 13.0 G1124-9 11 18 48 24 1.192E+06 3.178E+06 0.375 25.4 0.06 104.1 ± 28.9 G1124-9 12 6 35 60 1.589E+05 9.270E+05 0.171 7.4 0.05 47.8 ± 21.2 G1124-9 13 27 96 60 7.151E+05 2.543E+06 0.281 20.3 0.02 78.2 ± 17.2 G1124-9 14 6 53 42 2.270E+05 2.005E+06 0.113 16.0 0.03 31.6 ± 13.6 G1124-9 15 54 339 20 4.290E+06 2.693E+07 0.159 215.6 0.11 44.4 ± 6.6 G1124-9 16 4 33 35 1.816E+05 1.498E+06 0.121 12.0 0.00 33.8 ± 17.9 G1124-9 17 1 13 60 2.648E+04 3.443E+05 0.077 2.8 0.08 21.5 ± 22.3 G1124-9 18 9 110 42 3.405E+05 4.162E+06 0.082 33.3 0.03 22.9 ± 8.0 G1124-9 19 5 18 30 2.648E+05 9.534E+05 0.278 7.6 0.02 77.3 ± 39.1 G1124-9 20 19 49 80 3.774E+05 9.733E+05 0.388 7.8 0.02 107.6 ± 29.2 G1124-9 21 4 32 70 9.080E+04 7.264E+05 0.125 5.8 0.02 34.9 ± 18.5 G1124-9 22 31 234 80 6.158E+05 4.648E+06 0.132 37.2 0.03 37.0 ± 7.1 G1124-9 23 11 113 50 3.496E+05 3.591E+06 0.097 28.7 0.03 27.2 ± 8.6 G1124-9 25 4 27 50 1.271E+05 8.581E+05 0.148 6.9 0.02 41.3 ± 22.2 G1124-9 28 1 6 100 1.589E+04 9.534E+04 0.167 0.8 0.07 46.5 ± 50.2 G1124-9 32 25 93 50 7.945E+05 2.956E+06 0.269 23.7 0.21 74.8 ± 17.0 269 1515 4.309E+05 2.427E+06 19.4 For abbreviations, see page 147 Area of basic unit = 6.293E+07 cm2 χ2 = 47.992 with 19 degrees of freedom P(χ2) = 0.0% Age calculated using a zeta of 392.9 ± 7.4 for CN5 glass ρD = 1.424E+06 cm 2 ND = 2241 ρD interpolated between top of can; ρD = 1.388E+06 cm 2 ND = 1092 ρD interpolated between bottom of can; ρD = 1.461E+06 cm 2 ND = 1149 Age Dispersion = 38.770% Ns / Ni = 0.178 ± 0.012 Mean Ratio = 0.176 ± 0.024 Pooled age = 49.5 ± 3.6 Ma Central age = 50.4 ± 6.2 Ma 0 50 100 150 0.0 0.1 0.2 0 -2 +2 76 0 43 21 1.00 0.75 0.50 0.25 0.00 0.0 0.5 1.0 1.5 2.0 wt% Cl wt% Cl 30 20 10 0 0 5 10 15 20 Track length (µm) N Fr ac tio n Fi ss io n- tr ac k ag e (M a) ML: 12.25 µm Std dev: 2.28 FT age: 50.4 Ma Stratigraphic age 107 200 0.25 Radial plot of single grain ages* Distribution of Chlorine (Cl) contents in apatite grains Single grain age vs. weight % Cl for individual apatite grains Distribution of confined track lengths PJ Appendix Sample 1 * See Appendix B in Geotrack Report GC1052 (available online) for details of radial plot construction. Colour datapoints indicate wt% Cl: Dark blue: <0.1%. Green: 0.1-0.2%. Pale blue: 0.2-0.3%. Appendix 1: Analytical details of the AFTA data Sample GC1052-1: Apatite This appendix documents the raw fission-track count data together with radial/compositional graphical plots. The location of the two samples (GC1052-1, GC1052-2) in the Blokelv-1 core is indicated in Fig. 1. 146146 Slide Current Ns Ni Na ρs ρi RATIO U Cl FT Age ref grain no (ppm) (wt%) (Ma) G1124-10 3 15 84 50 4.767E+05 2.670E+06 0.179 21.3 0.23 49.9 ± 14.1 G1124-10 4 5 31 70 1.135E+05 7.037E+05 0.161 5.6 0.20 45.1 ± 21.8 G1124-10 10 7 56 50 2.225E+05 1.780E+06 0.125 14.2 0.01 35.0 ± 14.1 G1124-10 11 9 126 30 4.767E+05 6.674E+06 0.071 53.2 0.01 20.0 ± 6.9 G1124-10 12 3 63 28 1.703E+05 3.575E+06 0.048 28.5 0.02 13.4 ± 7.9 G1124-10 14 41 198 35 1.861E+06 8.990E+06 0.207 71.7 0.08 57.9 ± 10.1 G1124-10 15 14 94 36 6.180E+05 4.149E+06 0.149 33.1 0.01 41.7 ± 12.0 G1124-10 16 1 12 28 5.675E+04 6.810E+05 0.083 5.4 0.02 23.4 ± 24.3 G1124-10 17 9 48 48 2.980E+05 1.589E+06 0.188 12.7 0.10 52.4 ± 19.1 G1124-10 19 5 54 30 2.648E+05 2.860E+06 0.093 22.8 0.03 25.9 ± 12.1 G1124-10 21 16 152 40 6.356E+05 6.038E+06 0.105 48.2 0.04 29.5 ± 7.8 G1124-10 22 7 35 40 2.781E+05 1.390E+06 0.200 11.1 0.02 55.9 ± 23.2 G1124-10 23 5 54 40 1.986E+05 2.145E+06 0.093 17.1 0.14 25.9 ± 12.1 G1124-10 24 15 86 35 6.810E+05 3.905E+06 0.174 31.1 0.00 48.8 ± 13.7 G1124-10 25 10 57 42 3.783E+05 2.157E+06 0.175 17.2 0.00 49.1 ± 16.9 G1124-10 26 8 93 36 3.531E+05 4.105E+06 0.086 32.7 0.02 24.1 ± 8.9 G1124-10 29 7 101 70 1.589E+05 2.293E+06 0.069 18.3 0.00 19.4 ± 7.6 G1124-10 30 21 84 40 8.343E+05 3.337E+06 0.250 26.6 0.01 69.8 ± 17.1 G1124-10 33 0 7 40 0.000E+00 2.781E+05 0.000 2.2 0.02 0.0 ± 74.1 G1124-10 34 17 95 40 6.754E+05 3.774E+06 0.179 30.1 0.04 50.0 ± 13.3 215 1530 4.126E+05 2.936E+06 23.4 For abbreviations, see page 147 Area of basic unit = 6.293E-07 cm2 χ2 = 32.061 with 19 degrees of freedom P(χ2) = 3.1% Age calculated using a zeta of 392.9 ± 7.4 for CN5 glass ρD = 1.429E+06 cm 2 ND = 2241 ρD interpolated between top of can; ρD = 1.388E+06 cm 2 ND = 1092 ρD interpolated between bottom of can; ρD = 1.461E+06 cm 2 ND = 1149 Age Dispersion = 27.489% Ns / Ni = 0.141 ± 0.010 Mean Ratio = 0.132 ± 0.014 Pooled age = 39.3 ± 3.1 Ma Central age = 38.5 ± 4.0 M Sample GC1052-2: Apatite 0 -2 +2 69 0 49 2713 1.00 0.75 0.50 0.25 0.00 0.0 0.5 1.0 1.5 2.0 wt% Cl 40 30 20 10 0 0 5 10 15 20 Track length (µm) PJ Appendix Sample 2 N Fr ac tio n ML: 11.71 µm Std dev: 2.36 FT age: 38.5 Ma 0.0 0.1 0.2 0.25 wt% Cl Fi ss io n- tr ac k ag e (M a) Radial plot of single grain ages* Distribution of Chlorine (Cl) contents in apatite grains Single grain age vs. weight % Cl for individual apatite grains Distribution of confined track lengths * See Appendix B in Geotrack Report GC1052 (available online) for details of radial plot construction. Colour datapoints indicate wt% Cl: Dark blue: <0.1%. Green: 0.1-0.2%. Pale blue: 0.2-0.3%. Purple: >0.3%. Stratigraphic age 0 50 100 150 200 147 Table abbreviations Na = Number of grid squares counted in each grain Ns = Number of spontaneous tracks in Na grid squares Ni = Number of induced tracks in Na grid squares RATIO = Ns/Ni U (ppm) = Uranium content of each grain (= U content of standard glass × ρ i /ρ D ) Cl (wt%) = Weight percent chlorine content of each grain ρ s = Spontaneous track density (ρs) = Ns/ (Na × area of basic unit) ρ i = Induced track density (ρi) = Ni/(Na × area of basic unit) FT age = Fission-track age, calculated using equation B.1 Area of basic unit = Area of one grid square Chi squared = χ2 parameter, used to assess variation of single grain ages within the sample P(chi squared) = Probability of obtaining observed χ 2 value for the relevant number of degrees of freedom, if all grains belong to a single population Age Dispersion = % variation in single grain ages Ns/Ni = Pooled ratio, total spontaneous tracks divided by total induced tracks for all grains Mean ratio = Mean of (Ns/Ni) for individual grains Zeta = Calibration constant, determined empirically for each observer ρ D = Track density (ρ D ) from uranium standard glass (interpolated from values at each end of stack) N D = Total number of tracks counted for determining ρ D Pooled age = Fission track age calculated from pooled ratio N s /N i . Valid only when P(χ2) >5% Central age = Alternative to pooled age when P(χ2) <5%