The thermal history of the eastern Officer Basin (South Australia)

25 Pages·2002·1.2 MB·English

Tectonophysics349(2002)251–275
www.elsevier.com/locate/tecto
The thermal history of the eastern Officer Basin
(South Australia): evidence from apatite fission
track analysis and organic maturity data
Peter R. Tingatea,*, Ian R. Duddyb,1
aNationalCentreforPetroleumGeologyandGeophysics,UniversityofAdelaide,Adelaide,SouthAustralia5005,Australia
bGeotrackInternationalPty.Ltd.,37MelvilleRoad,BrunswickWest,Victoria3055,Australia
Received3November2000;accepted27July2001
Abstract
TheeasternOfficerBasininSouthAustraliacontainsaNeoproterozoictoDevoniansuccessionoverlainbyrelativelythin
(<500m)Permian,MesozoicandTertiarydeposits.Withinthebasinfill,thereareseveralmajorunconformitiesrepresenting
uncertain amounts of erosion. Three of these surfaces are associated with regional deformational events. Regional
unconformities formed between 560 and 540 Ma (Petermann Ranges Orogeny), approximately 510–490 Ma (Delamerian
Orogeny), 370–300 Ma (Alice Springs Orogeny), 260–150 Ma; and 95–40 Ma. AFTAR results from 13 samples of
Neoproterozoic,CambrianandPermiansedimentaryrocksinfivewells(Giles-1,Manya-2,-5and-6andLakeMauriceWest-1)
showclearevidenceforanumberofdistinctthermalepisodes.Resultsfromallsamplesareconsistentwithcoolingfromthe
most recent thermal episode beginning at some time between 70 and 20 Ma (Maastrichtian–Miocene). AFTA results from
Giles-1indicateatleasttwopre-Cretaceousthermalepisodeswithcoolingbeginningbetween350and250Ma(Carboniferous–
Permian) and between 210 and 110 Ma (Late Triassic–Albian). Results from Manya-2, -5 and -6 and Lake Maurice West-1
show evidence for at least one earlier higher temperature event, with cooling from elevated paleotemperatures beginning
between270and200Ma(LatePermiantoLateTriassic).Theseepisodescanbecorrelatedwithothercooling/erosionalevents
outside the study area, and the AFTA-derived paleotemperatures are consistent with kilometre-scale erosion for each of the
episodesidentified.IntegrationoftheAFTAdatawithorganicthermalmaturationindicators(MPI)intheManyaandGiles-1
wellssuggeststhattheCambrianandNeoproterozoicsuccessionsinthenorthernpartofthestudyareareachedpeakmaturation
priortothePermian,whilelimiteddatafromLakeMauriceWest-1allowspeakmaturationtohaveoccurredasyoungasthe
LatePermiantoLateTriassicthermalepisoderevealedbyAFTA.Theapproachoutlinedinthisstudyisrelevanttoallancient
basinsasitemphasisestheimportanceofunderstandingeventsassociatedwithneighbouringregions.Thethermalhistoryofthe
OfficerBasin,aswithmostotherancientbasins,hasbeenstronglyaffectedbysignificanttectoniceventsthroughoutitshistory,
eventhoughyoungerdepositsarenotpreservedinthebasinitself.Therecognitionoftheseyoungerevents,andtheimplications
oftheseeventsforthedepositionalhistory,isimportantasitallowsidentificationofthebestregionsforpreservationofearly
generated hydrocarbons, and in some cases, suggests areas where generation of hydrocarbons could have occurred more
recently than previously thought.D2002Elsevier ScienceB.V.All rights reserved.
Keywords:AFTA;Fissiontrack;OfficerBasin;Thermalhistory;Hydrocarbonprospectivity
* Correspondingauthor.Fax:[phone].
E-mailaddresses:[email protected](P.R.Tingate),[email protected](I.R.Duddy).
1 Fax: [phone].
0040-1951/02/$-seefrontmatterD2002ElsevierScienceB.V.Allrightsreserved.
PII:S0040-1951(02)00056-2
252 P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275
1. Introduction proterozoic to mid-Paleozoic in age. Overlying the
OfficerBasinarethin(<500m)Permian(Arckaringa
TheOfficerBasinisarelativelyunexploredregion Basin), Cretaceous (Eromanga Basin) and Tertiary
in southern central Australia with some oil shows successions, also separated by unconformities.
(Fig. 1) but no economic hydrocarbon accumulations Duetothepre-DevonianageofmuchoftheOfficer
(O’Neil, 1997). The basin contains Neoproterozoic– Basin, true vitrinite is restricted to relatively minor
Paleozoic sedimentary rocks separated by regional occurrences in limited well intersections of Permian
unconformities that represent uncertain amounts of and Mesozoic sediments. As a result, AFTAR (Geo-
erosion. These surfaces range from the latest Neo- track, 1994; Tingate, 1994) and organic maturation
Fig.1.Pre-TertiarysubcropgeologicalmapoftheOfficerBasin(modifiedafterMorton,1997).A–AVshowsthelineofsectioninFig.3.
P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275 253
studies (McKirdy and Michaelsen, 1994) were com- Centralian Superbasin (Walter et al., 1995) that cov-
missioned by Mines and Energy South Australia to eredmuchofAustralia.Thisbasinphaseculminatedin
helpconstrainthethermalhistoryoftheregionandthe minor erosion at 780 to 760 Ma. Stage 2 consists of
timing of petroleum generation. The thermal history depositionwithinaN–Scompressionalbasinbuilding
mayalsobeusefulinhelpingunderstanditsgeological to the Petermann Ranges Orogeny in the latest Neo-
development from part of a continent-scale intracra- proterozoic (560 to 545 Ma) with significant N–S
tonic sag to a smaller structurally controlled basin shortening-induced folding and thrusting (Figs. 2 and
(HoskinsandLemon,1995;LindsayandLeven,1996). 3).Stage3consistsofrenewedsedimentationfollowed
The AFTA data were originally collected and byanotherdeformationalevent,theDelamerianOrog-
interpreted without accompanying apatite chemical eny (510 to 490 Ma). At this time, further thrusting
compositions (Geotrack, 1994; Tingate, 1994; Grave- occurred and reactivated Stage 2 structures. Stage 4
stock and Hill, 1997; Tingate and Duddy, 2000). In consists of Ordovician to Devonian sedimentation
this study, chlorine has been determined in all apatite ending again in N–S shortening associated with the
grainsinwhichagesandtracklengthsweremeasured AliceSpringsOrogeny(370to300Ma).Reactivation
and the AFTA data reinterpreted using a multi-com- ofolderthrustsalsooccurredinthisevent,withmostof
positionalkineticdescriptionof apatite annealingthat thedeformationprobablyoccurringinthelateDevon-
provides estimates of paleotemperatures and time at iantomid-Carboniferous(360to320Ma),byanalogy
F95% confidence limits (Green et al., 1996). Since withtheAmadeusBasin(Shaw,1991).
theapatitesdisplayvariationinchemicalcomposition The eastern Officer Basin contains all the required
within and between samples the resulting thermal petroleum systemelements (MagoonandDow,1994)
history has been significantly revised, illustrating the but large uncertainty exists as to the timing of hydro-
importanceofincorporatingthechemistryofanalysed carbon. Hoskins and Lemon (1995) not only sug-
apatites into the thermal history interpretation of gested that the Petermann Ranges Orogeny (560 to
fission track data. 545 Ma) was the major control on existing basin
morphology but also concluded that the Petermann
Ranges, Delamerian and Alice Springs Orogenies
2. Regional geology were all capable of causing structural traps and plac-
ing source rocks in the oil window. Gravestock and
The Officer Basin is an intracratonic basin located Hill(1997)producedburialhistorymodelssuggesting
in South and Western Australia, covering an area of that the three events listed above differed in impor-
approximately 350,000 km2 (Fig. 1) and containing tance depending on the location in the basin.
up to 10 km of Neoproterozoic to Late Devonian
sedimentary rocks (Fig. 2). The eastern part of the
Officer Basin is bounded by crystalline basement: to 3. Thermal history reconstruction methodology
the north by the Musgrave Block, to the southeast by
theGawler Craton andtothesouth bytheCoompana 3.1. AFTA data
Block.
The Officer Basin succession contains shallow The interpretation of the thermal history from
marine,aeolian,fluvial,lacustrineandglacialdeposits AFTA data in this study is based upon the annealing
(Fig. 2). Gravestock (1997) sub-divided the regional behaviour of spontaneous fission tracks in geological
depositional history into 11 sequences (Fig. 2). For environments (Gleadow et al., 1983; Green et al.,
furtherstratigraphicinformation,thereaderisreferred 1989a) and length data from geological samples
toMoussavi-HaramiandGravestock(1995)andMor- (Gleadow et al., 1986), together with descriptions of
ton (1997). induced fission track annealing in laboratory experi-
Hoskins and Lemon (1995) have summarised the ments (Greenetal., 1986; Laslettetal.,1987; Duddy
development of the eastern Officer Basin into four et al., 1988) that have been extended to geological
stages(Fig.3).Stage1consistsoftheinitiationofthe situations (Green et al., 1989b). The laboratory an-
OfficerBasin as partofa largersag basin,termedthe nealing studies have concentrated on fission track
254 P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275
Fig.2.StratigraphiccolumnfortheOfficerBasinandcoversuccessions(modifiedafterGravestockandMorton,1997).Stratigraphicsequences
andrelativesea-levelinformationcomesfromGravestock(1997).
P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275 255
Fig.3.SchematicPrecambriantoCarboniferoushistory,OfficerBasin(modifiedafterHoskinsandLemon,1995).
lengthmeasurementsandtheirrelationtofissiontrack annealingcomesfromGreenetal.(1985,1986,1996),
density (and hence age) and have been described by Sieber(1986)andTingate(1990).
Laslettetal.(1984)andGreen(1988).Informationon The measured fission track age and track length
the effect of apatite chemical composition on track data for each well sample were grouped based on the
256 P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275
chlorinecontentofeachapatitegraininwhichagesand andbycomparingpredictedandmeasuredparameters
lengths were measured. Interpretation proceeded by the range of temperature–time conditions, which are
assessingwhetherthefissiontrackageandtracklength compatible with the data within F95% confidence
parametersdeterminedforeachchlorinecompositional limitscanbedefined.Thusforeachsample,wedefine
groupineachsamplecouldhavebeenproducedifthe the time and temperature conditions of the two dom-
sample has never been hotter than its present temper- inant thermal episodes required to explain the meas-
atureatanytimesincedeposition.Tomakethisassess- ured data. In cases where a sample actually has been
ment, a ‘‘Default Thermal History’’ was defined for subjected to more than two thermal episodes, the
each sample, derived from the preserved sedimentary maximum temperature episode and the most recent
section in the well and assuming no erosion occurred episodearethetwoepisodesdefinedinourapproach,
within the sedimentary succession, combined with andnoformalquantitativeconstraintscanbeplacedon
constant values for paleogeothermal gradient and the intervening episode from the AFTA results alone.
paleo-surface temperature which are adopted from However, integration of results from individual sam-
present-day values. The AFTA parameters expected ples in a vertical depth section, can allow such inter-
onthebasisofthisDefaultThermalHistorywerethen mediatethermaleventstoberevealedandquantifiedto
predicted using a multi-compositional kinetic model provideamorecompletequantitativedescriptionofthe
for fission track annealing in apatite as described by thermalhistoryasintheapproachtakenhere.
Greenetal.(1996). More detail regarding the methods used in this
If the measured AFTA data show a greater degree study is given in Gibson and Stuwe (2000). From the
of fission track annealing (in terms of either fission interpretationofthefissiontrack data,constraints can
track age reduction or track length reduction) than be placed upon the thermal history, and for most
expected on the basis of this history, the sample must situations a number of different thermal histories can
havebeenhotteratsometimeinthepast.Inthiscase, beconstructedfromtheseconstraints.Thechoiceofa
the AFTA data are analysed to provide estimates of particular style of thermal history for a region is a
the magnitude of the maximum paleotemperature in matter of geological interpretation.
that sample, and the timing of cooling from the
thermal maximum. 3.2. Vitrinite reflectance equivalent data
As AFTA data provide no information on the
approach to a thermal maximum, they cannot inde- Where present, vitrinite reflectance or vitrinite
pendently constrain the heating rate and a value must reflectance equivalent (VRE) data were converted
therefore be assumed in order to interpret the data. to a maximum paleotemperature using the EasyRo
Theresultingpaleotemperatureestimatesaretherefore algorithm of Burnham and Sweeney (1989). The
conditional on this assumed value. AFTA data can paleotemperature obtained was calculated assuming
providegoodcontrolonthehistoryaftercoolingfrom 1 jC/Ma for heating and10 jC/Ma for cooling. VRE
maximum paleotemperatures through the lengths of data were selected from those compiled in Grave-
tracks formed during this period (e.g. Green et al., stock and Hill (1997). The major source of the VRE
1989b). On this basis, data from each sample are information was methylphenanthrene Index (MPI)
interpreted here in terms of two episodes of heating data produced by McKirdy and Michaelsen (1994)
andcooling,astheinherentspreadofthetracklength and Kamali (1995). Only values derived from core
data is such that it is usually not possible to reveal samples without oil staining using the calculation
morethanoneadditionalepisodebetweenthethermal method of Radke and Welte (1983) were used for
maximum and the present-day. In this study, heating thermal history analysis. The other source of VRE
and cooling rates of 1 jC/Ma for heating and 10 jC/ data was fluorescence alteration of multiple macerals
Ma for cooling have been assumed during each (FAMM) (Michaelsen et al., 1997; Wilkins et al.,
episode. 1994). Some Officer Basin Rock Eval data is also
The timing of the onset of cooling and the peak presented in Gravestock and Morton (1997) for
paleotemperatures during the two episodes are varied Giles-1 and Manya-6. Rock Eval T data was not
max
systematically using a forward modelling approach, usedforsettingmaturity levelsasthey arecommonly
P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275 257
influencedbyboththermalhistoryandorganicmatter finedtracklengthmeasurements(Table2).Thesedata
type (Sherwood and Russell, 1996). In Manya-6, the form the basis for reliable thermal history interpreta-
values appear to be low, related to the organic matter tions.
type and a mineral matrix effect (McKirdy et al.,
1984). 4.2. Present temperatures
In application of any technique involving estima-
4. Thermal history results tion of paleotemperatures, it is critical to control the
present temperature profile since estimation of max-
4.1. Sample details imum paleotemperatures proceeds from trying to
determine how much of the observed effect could be
Fourteen samples were collected from core for explained by the magnitude of present temperatures.
processing and thirteen provided sufficient apatite Unfortunately, no temperature data were available for
for AFTA, with apatite yields ranging from fair to thewellsanalysedinthisstudyandonlytwopresent-
mostly excellent (Table 1). Most apatite grains range day geothermal gradient estimates are available from
from sub-rounded to very rounded in shape with the South Australian portion of the basin as a whole:
anhedral grains dominant in samples GC531-3 and - 14 and 24 jC/km (Gravestock and Hill, 1997). For
6 (Cadney Park Formation and Relief Sandstone, this study, a present-day geothermal gradient of 25
respectively), while euhedral grains were found in jC/km has been assumed for all wells, and this has
sample GC531-7 (Murnaroo Fm). The numerical beencombinedwithapresent-daysurfacetemperature
AFTA data are generally regarded as of excellent of25jCincalculatingthecurrenttemperatureofeach
quality with the majority of samples providing the AFTAsample.Whilethisassumptionintroducessome
target 20 grains for age determination and 100 con- uncertaintyintothethermalhistoryreconstruction,the
Table1
Sampledata,easternOfficerBasin
Samplenumber Depth Formation Stratigraphicage Apatiteyield Presenttemperature
(m) (Ma) (jC)a
Manya-2
GC531-1 245.5–247.7 MtToondinaFm 270–260 poor 31
GC531-2 492.9–494.3 BoorthannaFm 290–280 excellent 37
GC531-3 510–516 CadneyParkFm 524–522 excellent 38
Manya-6
GC531-4 448.2–448.8 CadneyParkFm 524–522 good 36
GC531-5 1699.6–1701.2 ReliefSst 545–524 excellent 68
Manya-5
GC531-6 455.1–455.5 ReliefSst 545–524 excellent 36
GC531-7 459–459.6 MurnarooFm 640–600 excellent 37
GC531-8 1054.5–1055.8 TarlinaSst 650–640 excellent 51
LakeMauriceWest-1(SMD5001)
GC531-9 217.7–218.7 ReliefSst 545–524 excellent 30
GC531-10 488.2–488.8 MurnarooFm 640–600 excellent 37
Giles-1
GC531-11 416.6–416.9 ReliefSst 545–524 excellent 35
C531-12 422.3–422.6 TananaFm 585–575 excellent 36
GC531-13 1063.4–1063.8 MeramangyeFm 640–615 excellent 51
a Presenttemperaturecalculatedassumingacurrentsurfacetemperatureof25jCandageothermalgradientof25jC/km.
2
5
8
Table2
Apatitefissiontrackanalyticalresults
Sampleno. No.of RhoD(cid:1)106 RhoS(cid:1)106 RhoI (cid:1)106 U P(v2) Agedispersion Fissiontrack Meantrack S.D. No.of
andwell grains (ND) (Ns) (Ni) (ppm) (%) (%) age(Ma) length(Am) (Am) tracks
Manya-2
P
GC531-1 9 1.300(1991) 2.797(264) 3.496(330) 31 41 2 201.2F17.6 11.59F0.34 1.47 19 .R
GC531-2 20 1.295(1991) 2.871(840) 2.560(749) 23 16 12 279.2F16.2 11.76F0.15 1.54 106 .T
GC531-3 20 1.290(1991) 1.825(472) 1.914(495) 17 11 21 237.2F16.8 11.35F0.25 2.57 105 ing
a
te
,
Manya-6 I.R
GC531-4 20 1.284(1991) 3.095(1188) 3.246(1246) 29 7 2 236.2F11.8 11.43F0.19 1.70 76 .
D
GC531-5 15 1.279(1991) 1.662(449) 1.776(533) 16 <1 36 231.0F15.9218.8F26.9 11.21F0.64 2.92 21 u
d
d
y
Manya-5 /T
GC531-6 20 1.274(1991) 1.803(523) 2.048(594) 18 58 3 216.6F14.5 11.92F0.15 1.57 104 ecto
GC531-7 20 1.268(1991) 1.718(468) 2.195(598) 20 <1 29 192.1F13.1195.6F20.8 12.22F0.28 2.00 52 n
o
GC531-8 20 1.263(1991) 0.683(375) 0.697(383) 6 26 18 238.4F18.7 11.23F0.21 2.12 102 ph
y
s
ic
LMwest-1 s
3
GC531-9 20 1.257(1991) 3.812(806) 2.838(600) 26 60 <1 323.6F19.9 12.12F0.22 2.32 108 49
GC531-10 20 1.252(1991) 2.918(762) 2.933(766) 27 66 3 240.2F14.2 11.98F0.16 1.68 106 (2
0
0
2
)
Giles-1 2
5
GC531-11 20 1.247(1991) 3.384(888) 2.706(710) 25 81 3 299.3F17.4 11.95F0.18 1.79 102 1
–
GC531-12 20 1.241(1991) 1.916(704) 1.820(669) 17 <1 28 251.6F15.5233.5F22.3 12.10F0.18 1.74 90 27
5
GC531-13 20 1.236(1991) 1.320(255) 2.138(413) 20 <1 55 148.2F12.6139.6F22.8 11.16F0.19 1.87 100
RhoS=spontaneoustrackdensity;RhoI=inducedtrackdensity;RhoD=trackdensityinstandardglassexternaldetector.
Alltrackdensitiesquotedinunitsof106trackspersquarecentimeter.Bracketsshownumberoftrackscounted.
RhoDandRhoImeasuredinmicaexternaldetectors:RhoSmeasuredininternalsurfaces.
Italicsindicateacentralage—usedwheresamplecontainsasignificantspreadofsinglegrainages[P(v2)<5%].Errorsquotedat F1sigma.
AgescalculatedusingdosimeterglassCN5,withazetaof392.9F7.4.AgeandtracklengthanalystisM.Moore.
P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275 259
conservative choice of a value at the top end of the will be underestimates rather than overestimates.
measured range suggests that our estimates of the net Furthermore, all samples are from relatively shallow
degree of cooling involved in any thermal episode depths, so that the present-day temperatures of all
Fig.4.Singlegrainagehistogramsandradialplots(Galbraith,1990)forsampleswithP(v2)<5%(seeTable2).
260 P.R.Tingate,I.R.Duddy/Tectonophysics349(2002)251–275
samples are less than 70 jC (Table 1), at which beginning at some time between 350 to 200 Ma,
contemporary fission track annealing is minor (e.g. followed by cooling from 70 to 85 jC, at some time
Green et al., 1989a). between110and10Ma.Theresultsfrombothsamples
wouldalsoallowaninterveningthermalepisodewith
4.3. AFTA results coolingfrom paleotemperatures intermediate between
those in the two defined episodes, as illustrated sche-
All AFTA analytical details are presented in Table matically by the horizontal-hatched area in Fig. 7.
2. The mean fission track ages (pooled and central) Support for such an intervening episode comes from
fromdrilleddepthslessthan500-mrangeinagefrom the solution for the deepest Precambrian sample,
approximately190to330Mawithatendencyforthe GC531-13 (Meramangye Formation, present temper-
ages to decrease generally from the southwest to the ature 51 jC). The AFTA solution indicates cooling
northeast. Single grain age variation in four samples from 100 to 105 jC beginning between 210 and 110
(of13)withP(v2)valueslessthan5%isillustratedin Ma,followedbycoolingfrom85to90jCbetween70
Fig. 4. The length parameters show little variation and10Ma.Note,theAFTAresultsforsampleGC531-
across the region (Fig. 5): mean track lengths vary 13 provide no direct evidence for any part of the
between11.2and12.3Amandstandarddeviationsare thermal history prior to 210 Ma because of the near
all greater than 1.5 Am. totalannealingofafissiontracksinthe210to110Ma
Mostsampleshaveaverynarrowrangeofchlorine episode. However, higher paleotemperatures are
content (0.0 to 0.2 wt.% Cl with occasional grains allowed by the data at any time prior to 210 Ma, as
with up to f0.5 wt.% Cl) typical of quartzose sand- illustrated for a notional episode described by the
stones derived from granitic terrains (e.g., sample crosshatched area in Fig. 7. Thus, integration of the
GC531-4 as shown in Fig. 6). Apatite from samples AFTA results from individual samples in this way
of Permian Boorthanna Formation (GC531-2) and allows a more complete picture of the post-Cambrian
Proterozoic Cadney Park Formation (GC531-3) from thermal history of the region to be built, even where
Manya-2 contain apatites with broad ranges of Cl youngerrocksarenolongerpreserved.
contents,up to2.1wt.% insample GC531-2(Fig.6), Timing constraints at F95% confidence limits
suggesting the presence of a range of volcanogenic obtained from the maximum likelihood solutions in
detritus (Duddy,unpublished results; Mitchell,1998). all AFTA samples in the five Officer Basin wells
studiedareillustratedbythehorizontalbarsinFig.8.
4.4. AFTA thermal history interpretations Assuming that the paleo-thermal episodes identified
inindividualsamplesfromawellrepresentpervasive,
Thermal history constraints from each sample in synchronous events, then the overlap in AFTA-
terms of the magnitude and timing of paleotempera- derived timing from the individual samples can pro-
tures (F95% confidence limits) are summarised in videabetterestimateofthetimingofdiscreteevents,
Table 3. The constraints listed in Table 3 are derived as discussed for each well in the following section.
from the maximum likelihood solutions of the AFTA Also shown on Fig. 8 are the time ranges of major
results in each sample for two thermal episodes. regional unconformities: 560 to 540 Ma (Petermann
Examples of these solutions, plotted in time–temper- Ranges Orogeny); f510–490 Ma (Delamerian
aturespace,arepresentedinFig.7forthethreesamples Orogeny); 370–300 Ma (Alice Springs Orogeny),
fromtheGiles-1well.ThesolutionforsampleGC531- 260–150 Ma; and 95–40 Ma, with which the timing
11fromtheCambrianRelief Sst(presenttemperature constraints from individual samples may be com-
35jC)indicatescoolingfrompaleotemperaturesof95 pared.
to100jCbeginningatsometimebetween400to250
Ma, followed by cooling from 75 to 85 jC, at some 4.4.1. Giles-1
time between 125 and 5 Ma. The solution for sample Thermal history solutions for the Giles-1 samples
GC531-12 from the underlying Precambrian Tanana have beendescribedindetail aboveandareshownin
Formation(presenttemperature36jC)isverysimilar, Fig. 7. In summary, at least two, but possibly more,
withcoolingfrompaleotemperaturesof100to105jC thermalepisodesarerequiredbytheAFTAdatainthe