llownhole Temperature, Radiogenic Heat Production, and Heat Flow from the CRP-3 Drillhole, Victoria Land Basin, Antarctica
C.J. BUCK ER^^', R.D. J A R R A R D ~ & T. WONIK'
GGA. Leibniz Institute for Applied Geosciences. Slillcwrg 2, 30655 Hannover - Germany Dept. of Geology & Geophysics. 717 WBB. Univ. o f LJl:ih. Scill Lake City. UT 841 12-01 1 l - USA
'Present address: RWE-DEA AG. Ucbersccring40. 22297 Hamburg - Germany Received 29 October 2000; accepted in revised form 17 November 200 1
Abstract - Cape Roberts drillhole CRP-3 in the northern part of McMurdo Sound (Ross Sea, Antarctica) targeted the western margin of the Victoria Land basin to investigate Neogetie to Palaeogene climatic and tectonic history by obtaining continuous core and downhole logs (Cape Roberts Science Team, 2000). T h e CRP-3 drillhole extended to 939.42 mbsf (meters below seafloor) at a water depth of 297 m. The first downhole measurements after drilling were the temperature and salinity Jogs. Both were measured at the beginning and at the end of each of the three logging phases. Although an equilibrium temperature state may not have been fully reached after drilling, the temperature and salinity profiles seem to be scarcely
disturbed. The average overall temperature gradient calculated from all temperature measurements is 28.5 K/km; remarkably lower than the temperature gradients found in other boreholes in the western Ross Se;' and the Transantarctic Mountains.
Anomalies in the salinity profiles at the beginning of each logging phase were no longer present at the end of the corresponding logging phase. This pattern indicates that drilling mud invaded the formation during drilling operations and flowed back into the borehole after drilling ceased. Thus, zones of temperature and salinity anomalies identify permeable zones in the formation and may be pathways for fluid flow.
Radiogenic heat production, calculated from the radionuclide contents, is relatively low, with average values between 0.5 and 1.0 pW/m3. T h e highest values (up to 2 pW/m3) were obtained for the lower part of the Beacon Sandstone below 855 mbsf. The heat flow component due to radiogenic heat production integrated over the entire borehole is 0.7 mW/m2. Thermal conductivities range from 1.3 to 3 WImK with an average value of 2.1 W/mK over the Tertiary section. Together with the average temperature gradient of 28.5 K/km this yields an average heat flow value of 60 mW/m2.
INTRODUCTION
The main aims of the Cape Roberts Project are to document past variations in Antarctic ice cover and climate and to reconstruct the tectonic history of the nearby Transantarctic Mountains (TAM). A comprehensive overview of the project and a detailed description of the geological setting have been published by the Cape Roberts Science Team (1998, 1999, 2000). A cumulative stratigraphic thickness of 1 5 0 0 m was drilled by the three C R P boreholes.
CRP-3 cored 821 m of Lower Oligocene and possibly Upper Eocene sedimentary rocks and 116 m of the underlying Devonian sandstone (Cape Roberts Science Team, 2000). Drilling and coring was done in two phases: HQ-size 3" drill rod (6.1 cm core diameter) was used for coring the interval from 3 to 346 mbsf, followed by NQ-size 2" coring (4.5 cm core diameter) of the interval from 3 4 6 to 939 mbsf.
Average core recovery was 95 %. The mid-Tertiary section consists primarily of sandstones and muddy
sandstones, which are intercalated with conglomerate beds and less common sandy mudstones and diamictites (Cape Roberts Science Team, 2000).
It is possible that anomalous thermal conditions are associated with the crustal thinning and subsidence of the Ross Embayment and the Cenozoic uplift of the Transantarctic Mountains of up t o 55 m/Ma (Blackman et al., 1987). Temperature logs exist for only a few drillholes in Antarctica, and all of the older measurements consist of discontinuous point measurements. An overview of existing downhole temperature data for this area of Antarctica is given in table 1, the locations of the drillholes are shown in figure 1. The Dry Valley Drilling Project (DVDP) drillholes have been described by Bucher & Decker (1976), Decker (1974, 1975), Decker et al. (1975), McGinnis et al. (1981), Mudrey et al. (1973), Pmss et al. (1974), and Treves & McKelvey (1974).
Information about the MSSTS (McMurdo Sound Sediment and Tectonic Studies) drillhole (depth 220 mbsf) has been published by Sissons (1980), and
*Corresponding author (christian.bueeker@rwedea.de)
'I'nh. I - Compilation of ilownl~ole tcniperature mcasi~rcii~crit resulls in llic 'I'i;iiisaiiliiri.~tir Moiiiiliiins ;iiul Victoria 1,iind I-lilsin.
T h e most reliable tlelcrmii~iilio~is arc s h o w n in boldface ~ y p c . T h e ~ i ~ ~ ~ i p ~ ~ i ' a l i ~ r e yradieiiis ol' ?.,l ancl 2 8 . 5 Klkm lor Ilic C a l x ' K o l n * i ~ ' ~ boreholes arc lower lliaii ~ l i o s c lor all the oilier boreholes. T h e liiplu'st ~ r i ~ i ~ ~ r r i ~ ~ i i i ' c griidifiils wcrr i l c ~ c r n i i i i ~ d for tlic l)Vl)13 I - i o n - l ~ t ~ l i " ~ S,
10. 1 1 and 12. which arc locatcil in tlic T;iyIor Valley (see location 111;1p. l-'ig. l), The ofl'sliorr (Irillliolcs MSS'I'S- I . CIROS- I. m i d DV1 )l' IS a t New Harbour show overall lcrnpcratiire g r i ~ l i c n l s ol'ahout 3." Klkm. The 111ini coluinn ("Iciiipci.ii/iirc "('") gives tlic lcni1~ei';iliit~' :il l i e d e p t h given in Ilic s e c o n d c o l u m n . L a r g e negative gradieiits ;it Don Ju;in Pond (DVI)!' 1 3 ) and at North l-'ork ( I ) V P I ' 1 - 1 ) ; i n '
11VDP-2. -3. McMiircIo
!)Vl>P-2. -3. McMiirdo DVl)l'-2. McMurdo DVIIP-2. McMurclo l)Vl)P-3. McMurdo DVDP-3. McMunIo DV11P-3. McMurdo 1)VllP-4. Lakc Vaiul;i 1)VIlP-4. Lake Vand;i l)VDP-6. Lake Vkl;~
DVIIP-6. Lakc Vida DVDP-8. New Hashor DVlIP-8. New Harbor DVDP-8. New Harlioi DVDP-S. New Harbor DVIIP-10, New Harbor DVDP- 10. New Harbor DVDP-1 1 . Corn. Glacier DV1)I'- I I. Corn. Glacier DVHP-1 1. Corn. Glacier 1)VI)P-12. Lake Leon DVDP-12. Lake Leon D V l l F 13. Don Juan Pond DVOP-13. Don Juan Pond DVIIP-14. North Fork DVPP-14. North Fork DVDI3-IS. McMi~rdo Sound MSSTS- l . Butter Poiiit C'IROS- 1 . Butter Point CIROS-l. Butter Point CIROS-l. Butter Point CRP-212A. Cape Roberts
l)i'1)lll r ; > l l ~ ' Ill
&-
. 4 0 1 X 0 180-270 20- 135
00. ?.do
10-305 30- l 35 10- 135 30- 135 10- 160 IO-300 60-300 10- 1 80 10-75 10-80 10-60 20-220 150-600 280-450 20-620
volciinics. pyrod;is~ics vok'iinics. pyroclastics volciinics. pyroclastics
\olc;inics. pyroclastics voldiiiics. pyrocl;islics volciiiiics. pyrocI;tstics volciinics. pyroclastics
basement hi~scmcnt h;iserneiit hiisciiient
@:.icial XL 111afinc s e d i ~ i ~ e n ~ s glitciiil & marine sediments glacicil & marine sedinients gl~ticial & marine seiiliments glacial & marine sediments glacial & marine scdimcnts
clastic sediments clastic sediments (.l' astic secliments . "
clastic sediments clastic sediiiicnts bi~sement basement basement basement glacigcnic sediments glacigenic sediments gliicigenic sediments glacigenic sediments glacigenic sediments glacigenic sediments
D ~ k e r . l074 IX-ckcr. 197.1 Decker 1074 Decker, I074 decker. 197.4: Priis-. cl :!l,, l') I 1 Pecker 1974: I'niss cl :\l.. D1.l
Decker cl ill.. lO'15 Decker. 1974: Pruss cl ill.. IO1.I Decker. 1974: Priiss cl i l l . . 1'1 1.1 Decker 1974: I'ILISS el itl.. I0i.I Decker 1974: P ~ U S S cl ill.. IO1.I Decker. 1974: Pri~ssel ill.. I01.I Decker. 1974: Pruss cl :d.. D1.l Decker. 1974: Pruss et cil.. 1') /-l
Decker et al.. 1975 Decker et al.. 1975 Decker et al.. 1971 Decker et iil.. 107.5 Decker et al.. 1975 Decker et al.. 1971 Decker et al.. 1975 Decker et al.. 1975 Decker et al.. 1975 Decker et al.. 19'75 Decker et al.. 1075 Deckcr et al.. 197:i Buclier & Decker 1970
Sissons. 1980 White. 1989 White. 1989 White. 1989 Bucker et al.. 2000
CRP-3. Cape Roberts 900 24 28.5 20-920 glacigcnic sediments this paper --
investigations on drillhole CIROS-l, which reached a depth o f 700 m b s f , have been described by Barrett (1987) and White (1989).
All temperature data besides the measurements in the Cape Roberts drillholes indicate temperature gradients greater than 30 K l k m . In particular, the downhole measurements in the drillholes o f the Dry Valley Drilling Project (DVDP) onshore and offshore o f the Transantarctic Mountains show temperature gradients up 80 Klkm for some depth intervals. Since the Transantarctic Mountains are relatively young, elevated temperature gradients and heat flow values would be expected. Blackman et al. ( 1 9 8 7 ) found
"minimum possible" heat flow values in the western Ross Sea o f 66 to 73 mW/m2, which are significantly higher than the continental m e a n o f 57 m W / m 2 (Sclater et al., 1980). Other values in the area are similar (i.e. Della Vedova et al. 1992). Bucher and Decker (1975) give onshore heat flow values o f 67 and 7 9 m W / m 2 at Ross Island (near McMurdo Station) and Lake Vanda (Transantarctic Mountains),
respectively. These data are in agreement with oilier studies suggesting regionally high heat flow in the Western Ross Embayment because o f crustal thinning during Cenozoic sifting (Cooper & Davey, 1985).
Our new geothermal studies in the Cape Robcrts drillholes presented here will be used to examine the role o f thermal conditions in determining the tectonic history o f the Ross Embayment.
MEASUREMENTS AND RESULTS DOWNHOLE TEMPERATURE AND MUD CONDUCTIVITY MEASUREMENTS
A detailed description o f the downhole logging tools used in CRP-3 is given in the Initial Reports (Cape Roberts Science T e a m , 1999, 2 0 0 0 ) . Temperature measurements were made w i t h a combined salinityltemperature tool at a sampling rate o f 0.1 m . T h e accuracy o f t h e temperature
/Â¥7,i; 1 - I.ociitio11 map of bedrock tlrillholcs in McMurcIo Sou~i(I ancl tlic :~(l,jacciit ' T r ' ~ i i s x i ~ c ~ ' c t i c M o i ~ i i l i i i ~ i s CI'AM). T h e m a j o r I':iiilt between the Vicloria Land li;isi~i (VI.15) a n d T A M is iiulicaicti: U = upthrown s i d e . 1 ) -2 d o w i i t l ~ ~ ~ o w n s i d e . ODP ilrillholc 1 165 f r o m Leg 188 i n 'ryil/ liay i \ the deepesl cli'illliole i n bedrock in Aiitarctica (999 m iu'low sea 1'loor O i ~ b s f ) ) : CRP-3 rciiclicd ;I depth of 939.42 mbsf i n November 1999.
All Dry Valley Drilling Project ( D \ / P P ) clrillliolcs a r e in the TAM e x c e p t D V D P 15 i ~ i
McMiinIo S o u n d iincl DVDP l -
1. which arc on Ross Island. The C R P drillholes arc ;it the northern p of R o b e r t s Riclge. F i g u r e
~ i i o ~ l i l ' i c d from 13tin'ctf et i l l .
( 1 986).
measurements is about 0.1 'C (Cape Roberts Science Team, 1999). The same tool also records the electrical conductivity (salinity) of the mud with two adjacent electrodes.
Downhole logs in CRP-3 were recorded in three phases of CRP-3 drilling. Due to time and weather constraints, it was not possible to wait a reasonable t i m e for t e m p e r a t u r e equilibrium after drilling operations, and measurements had to be made only hours after drilling was concluded. Because of the cold conditions and to prevent freezing, the drilling mud was heated to 20 'C in the circulation system before it was pumped down-hole (Bucker e t a l . , 2000). Downgoing temperature measurements were m a d e at t h e beginning and a t the end of e a c h downhole-logging p h a s e , with about two d a y s between each pair of temperature logs. These sets of temperature measurements provide a unique chance to observe transient effects in mud conductivity and downhole temperatures.
Although the measured temperatures may not reflect undisturbed formation temperatures due to the constraints mentioned above, temperature and salinity changes measured shortly after drilling completion may indicate fluid flow and thus- permeable zones (Rider, 2000; Serra, 1986). Since the circulating mud presumably cooled the d r i l l h o l e , the calculated temperature gradients and heat flow values must be regarded as "possible minimum values".
Ail example of a continuous downhole temperature log taken at the end of the third logging phase after two days of logging operations is shown in figure 2.
T h e temperature ranges from -1.95 'C at the sea floor (freezing point of sea water) to 23.7 'C at the bottom of the hole. The average temperature gradient o v e r the e n t i r e measured section from 2 5 0 to 9 1 0 mbsf a s calculated by linear regression is 28.5 Klkm; deviations from this average are very small as indicated by the 95 5% confidence interval shown in the figure. However, these small deviations from the linear temperature gradient are enlarged in the reduced temperature curve ( r i g h t column in F i g . 2), produced by subtracting t h e linear temperature gradient from the measured temperature curve. T h e anomalies in this reduced temperature curve (marked with Grey shading) occur at the same depths as anomalies in previous logging runs, but the amplitudes are smaller than in those logs. These negative anomalies indicate an influx of cold fluids into the borehole. They can be detected mainly in the s a n d - d o m i n a t e d section of the b o r e h o l e below 500 mbsf and occur in conjunction with lonestones and/or conglomerates. The Tertiary section of the borehole from 500 to 780 mbsf seems to be cooled, the reduced temperature curve shows mainly negative values, in contrast to the section above 500 mbsf. The d o l e r i t e thrust zone at about 8 0 0 mbsf and the underlying Beacon sandstone show mainly positive
/Â¥'i',q 2 - Example of a downhole tciiipcriii~irc logclown to 920 mbsf from the end of (lie last logging phase on 21 November 1999, two days iifler drilling was concluded and after all o~lier ilow~ihole measurements. The iiveriige ovcriill cinperature gradient is 28.5 Klkni: (lie 95 '/<
c o n f i d e n t i a l interval is very s m a l l . T h e iinomalies (inarkecl by Grey shading) i n the rcduced temperature curve (right part of the l i g u r e ) a r e at the s a m e d e p t h s as in the previous logs. but with lower ainplit~icles. A n i~klitional anomaly can be seen at 845 mhsf in the Beacon Sandstone. The lower amplitudes in the temperature anomalies m a y be due io a rehouiid effect of the formation. 1:or lithology (light column) see Barrett (this volume).
Temperature
('C)
'>.,(21.11.1999) icd Tempeiature ( Â ¡ C .g m
0 5 10 15 20 25
_ 0 2 o;, 0values f o r reduced temperature with one distinct negative anomaly at 840 mbsf.
All d o w n h o l e m e a s u r e m e n t s taken in CRP-3, together with the temperature log taken in CRP-2, are shown in figure 3 in the middle column. Temperature variations over the entire borehole are only minor and occur in a narrow band along the linear temperature gradient. Only the temperature log from CRP-2 shows a s y s t e m a t i c offset of a b o u t 1 K to h i g h e r temperatures, possibly d u e to different b o r e h o l e conditions and a different time between the end of drilling and the starting of logging operations. In CRP-2, the temperature was logged first in the last logging p h a s e a f t e r d r i l l i n g was c o n c l u d e d i n December 1998 and is shown for comparison with the CRP-3 l o g s . T h e d e v i a t i o n s of t h e d o w n h o l e temperatures from the linear temperature gradient are enlarged in the reduced temperature curves (Fig. 3), produced b y subtracting the average temperature gradient of 28.5 K l k m . A l l reduced t e m p e r a t u r e curves show negative anomalies, which indicate cold fluid inflow into the borehole. Enlarged peaks can be seen at depths of 265 mbsf, 535 mbsf, 605 m b s f , 750 m b s f , a n d at 840 m b s f . In CRP-2, a l a r g e negative a n o m a l y c a n b e s e e n b e t w e e n 520 a n d
600 mbsf. This anomaly has already been described a ies ;it
by Bucker et al. (2000). The temperature anon) 1' 605 mbsf and 750 mbsf in CRP-3 are accompanied by anomalies in the mud conductivity. But these mud conductivity anomalies (which can b e inverted to salinity anomalies) from the first measurements at the beginning of the logging phases are no longer present at t h e e n d of the c o r r e s p o n d i n g l o g g i n g phases.
Unfortunately, there was no time to c o n d u c t any active f l u i d flow m e a s u r e m e n t s o r t o t a k e fluid samples at depth.
A closer look at the temperature profiles and their variation with time shows that for each logging phase.
the first temperature log has lower temperatures than the second temperature log, which was run about two days later. T h e second temperature run is always warmer than the first one, confirming the argument t h a t observed gradients a r e m i n i m a and negative thermal spikes a r e f r o m mud i n f l u x . T h i s occurs despite the fact that equilibrium temperatures for most of t h e b o r e h o l e a r e < 20°C i n d i c a t i n g t h a t mud passage through the -2OC water column cools it to n e a r f r e e z i n g (after having b e e n h e a t e d at t h e surface). The difference between the temperature runs proves that equilibrium was not reached by the first
Downhole Temperature, Radiogenic Heat Production, and Heat Flow
Fig. 3 - Downhole temperatures and salinities. From left to right: (i) lithology, (ii) gamma ray (0 - 150 API, see Bucker et al., this vol.), (iii) temperatures measured on the indicated dates (-5 to +25 'C), (iv) reduced temperatures (-1 to +l 'C) calculated from temperature curves with an average temperature gradient- of 28.5 K/km (the reduced temperature curve of borehole CRP-2/2A was calculated using the thermal gradient of 24 K/km), and (v) mud conductivities (salinities) (50 - 150 mSEm).
All reduced temperature curves show anomalies that indicate inflow of cold fluid into the borehole. Distinct peaks can be seen at 265 mbsf, 530 mbsf, 610 mbsf, 750 mbsf, and at 840 mbsf. The temperature anomalies at 605 mbsf and at 750 mbsf are accompanied by anomalies in the mud conductivity. But these mud conductivity (salinity) anomalies from the first measurements at the beginning of the logging phases are not present at the end of the corresponding logging phases.
run. but it may have been reached by the second r u n . Obviously temperature equilibrium is achieved many times faster in small-diameter holes such as the C'apc Roberts boreholes than in large-cliameter holes. The n i i i x i m ~ ~ m difference between two corresponding temperature measurements is about 1 K. However.
s i n c e the approach to temperature equilibrium is nonlinear, with decreasing temperature clif~ferences w i t h time. it cannot b e e x p e c t e d that the t r u e formation temperature will be much higher than the logged temperatures. As a result, the temperature curves and temperature gradients presented here inust be considered as minimum possible values. By taking into account the highest temperatures in CRP-3 as rccorded on 13 Nov. 1999 (red curve in Fig. 3). a temperature gradient of 30 K/kin can be estimated, RADIOGENIC HEAT PRODUCTION
Radiogenic heat production is controlled by the decay of the radionuclides of potassium, thorium, and u r a n i u m . It can be d e t e r m i n e d by t w o different methods. The first method uses the spectral gamma ray m e a s u r e m e n t s of t h e c o n t e n t s of p o t a s s i u m , thorium, and uranium (Bucker et al., this volume).
Radiogenic heat production is then calculated using the following formula (Rybach, 1986):
Where A is heat production in pW/m2. p is rock d e n s i t y in k g / m \ a n d c,,, c
,,,,,
a n d c K a r e t h e radioactive element concentrations in ppm for thorium and uranium, and in percent for potassium.The second method simply uses the correlation between the measured gamma ray values and the heat production of Biicker & Rybach (1996):
w h e r e GR i s t h e g a m m a ray i n A P I u n i t s . Although this formula was originally set up for hard rocks, it is also valid f o r t h e sediments drilled in CRP-3 with considerable reliability.
Since radiogenic heat production shows a close correlation to gamma ray, its depth pattern can b e compared to the gamma ray curve shown in figure 4.
Overall, a bimodal distribution of heat production values was observed with peaks at 0.5 and l.0p/Wm3.
T h e s e values can be attributed to sandstones a n d m u d s t o n e s , respectively. D i a m i c t i t e s a n d c o n - glomerates show intermediate heat production values.
The highest heat production was calculated for the lower part of the Beacon Sandstone below 855 mbsf with values greater than 2 p/Wm2, d u e to elevated thorium values (Bucker et al., this volume).
The depth integrated heat production rate gives the
iii110~111t of lictit produced by radioactive c l e n u ~ n i s i n
the drilled s e d i m e n t s and is shown i n f i g n i v , l , Integrating the radiogenic heiit pi~ocliiction over i l ~ ~ p ~ l i yields a value of 0.7 mW/m3. which is about I 'L of (lie measured surface heat flow (Biicher & D r r k ~ ~ i .
0 7 5 ; McGinnis et al.. 198 1 ) . Thus. the rad ioac-live heat production in the drilled s e q u e n c e s I1ii.s n o significant influence o n heat flow.
' 1 IERMAL CONDUCTIVITY AND I IEAT F1 .O\V T h e r m a l conductivity was mcasured t i n c o r r samples from the Tertiary section of drillhole CKI' i with a high precision, noncoinact method iisiiiy, i i i i
optical scanning device (Popov, 1997). The tIicoi'i.-tic*iil model f o r this kind of measurement is bciscd oil scanning a sample surface with a focused, mobile iiinl continuously operated heat source in c o m h i i r I I ' 1 0 1 1
with infrared t e m p e r a t u r e s e n s o r s . F o r thcsi.~
measurements no polishing or sawing is nccesszn'y, and flat and cylindrical surfaces of the c o r e s a r e acceptable. The measurements were calibrated against standards; replicate measurements were made o n i l l 1
samples. The relative error in thermal cond~ictivity is less than 15%.
Thermal conductivities range from 1 .3 to 3 W/niI<
with an average value of 2.1 W/mK over the Tertiary section of CRP-3 (Fig. 4). I n the upper p a n of [lie borehole, where the lithology consists of sandstones, m u d s t o n e s , d i a m i c t i t e s . a n d c o n g l o m e r a t e s . a reasonable correlation between quartz content a n d t h e r m a l c o n d u c t i v i t y can be s e e n (Fig. 4, S i O , measurements by Sproveri et al., this volume). In the lower part of the borehole below 500 mbsf-, where clean sandstones d o m i n a t e t h e lithology, thermal conductivities show an excellent correlation with q u a r t z c o n t e n t . S i n c e q u a r t z h a s a high thermal conductivity of 7 W / m K , it is evident that quart/, controls the thermal conductivity in this depth range.
The few non-correlating data points, with very high t h e r m a l c o n d u c t i v i t i e s up to 3 W I m K , can be attributed t o high-density conglomerate sections.
Despite these few high values, thermal conductivity is g e n e r a l l y l o w below a b o u t 6 3 0 mbsf. T h i s is remarkable because the sandstones below and above 6 3 0 m b s f s h o w d i s t i n c t l y d i f f e r e n t s e d i m e n t provenance (Bucker et al., this volume; Sniellie, this volume).
Heat flow was then calculated by multiplying the a v e r a g e t h e r m a l g r a d i e n t of 2 8 . 5 K / k m w i t h t h e results of thermal conductivity measurements (Fig. 4).
Because of the linear relationship, heat flow shows the same trend as thermal conductivity. The average heat flow is 6 0 mW/m2, insignificantly higher than t h e a v e r a g e c o n t i n e n t a l h e a t f l o w of 5 7 m W / m 2 (Sclater et al., 1980). However, two sections with elevated heat flow values can be observed. The first is
l
i^lithflL!_SQL_È.cad.iheat Sin? 1QQ heat flow mean 1 therm. cond. 330 heat flow 9(
Fig. 4 - Thermal properties of the rocks in the CRP-3 drillholc.
From left to right: (i) lithology. (ii) gamma ray log (0 - 150 API).
(iii) log of integrated radiogenic heat production (0 - 1 m W / n ~ ~ ) . (iv) thermal conductivity measured on core samples (dots) (scale from 1 to 3 W/mK. mean thermal conductivity is shown as vertical grey line at 2 . 1 WImK) and S i O ? content taken at 1 m c o r e intervals (scale 60 - 100 ¡'c data from Sproveri et al.. this volunie).
( v ) heat flow ( d o t s ) (scale 30 - 60 mW/ni2) calculated from thermal conductivity and a constant thermal gradient of 28.5 Klkm.
mean heat flow is shown as vertical Grey line at 60 WImK. The integrated radiogenic heat production was calculated from the uranium. t h o r i u n ~ . and potassium c o n t e n t s . T h e highest heat production values were calculated for the lower part of the Beacon sandstone below 855 mbsf. T h e integrated r a d i o g e n i c h e a t production is 0.7 mW/m2. which is about 1 % of the measured surface heat flow (Bucher & Decker 1975).
T h e mean heat flow for the C R P - 3 drillhole of 60 m W / m 2 is indicated as a vertical grey line.
at about 130-250 mbsf and the second is below about 500 mbsf, with peak values up to 9 0 mW/m2. These peak values may be attributed to conglomerates and maybe also affected by local convection effects.
DISCUSSION AND CONC'LUSIONS Downhole temper;iii.ire logs were run during three logging phases i n drillhole CRP-3 i n order to obtain reliable i l i i t i i ahout ihe tnic I'ormation iemperatiire and ienipcriiliiri.- griidient. Altogcthcr. six dow~iliolc logs in CRP-3 iiiul oiuL downhole log in CRP-2 provide the basis I'or the i~iii~rp~~etation of the tcniperature Held at h e C;ipe Roberts drill s i t e s . However. d u e to Antarctic eo~lstrai~lts i t was not possible to wait for temp(~i~ifliire eqnilihriiim after cirilling ceased. Usually the first log was r u n shortly al'tcr the drill bit was taken out of the drillhole. The second tempcraturc log of o n e loggiiig phase was taken at the end of the phase. about l \ W clays later. The ai-lvantage of having several downhole teniperature logs taken at different t i m e s after drilling a l l o w s a differentiated interpretation of the logs and an estimate of transient effects.
An example of a downhole temperature log on 21 November 1999 is given in figure 2. Although the log was taken only a few hours after drilling was stopped, i t shows a very linear trend with depth. The 95 % confidential interval of the linear regression is very small, indicating only small temperature fluctuations.
Anomalies from the linear regression can be seen in the reduced temperature curve in figure 2. The zones of negative values for reduced temperature marked in figure 2 indicate permeable zones from which cold mud flowed back f r o m t h e f o r m a t i o n into t h e borehole. All other clownhole temperature logs show behavior similar to this one. Even the temperature a n o m a l i e s , which a r e e n l a r g e d in t h e r e d u c e d temperature curves, occur at the same depth in each t e m p e r a t u r e l o g . T h e s e n e g a t i v e t e m p e r a t u r e a n o m a l i e s i n d i c a t e i n f l o w of c o l d f l u i d i n t o t h e borehole and mark the locations of permeable zones.
However, on the basis of an individual temperature curve it cannot be decided whether these anomalies indicate an aquifer or if mud is simply flowing back from the formation into the borehole.
In general, the temperature curve run first during a logging phase shows about I K lower temperatures than t h e temperature c u r v e taken at t h e e n d of a logging phase. This can be attributed to a temperature rebound effect. T h e temperature field, which was disturbed by the drilling activities, is rebounding to t h e true undisturbed f o r m a t i o n temperature. T h i s temperature rebound is not a linear process. Most temperature changes occur during a short time after t h e d i s t u r b a n c e of t h e t e m p e r a t u r e f i e l d c e a s e s . Presumably there will be only minor temperature c h a n g e s after t h e s e c o n d l o g r u n of o n e l o g g i n g phase.
The effects of the small diameter of the borehole (3" in t h e lower p a r t , s e e C a p e R o b e r t s S c i e n c e Team, 2000) are rapid formation rebound and rapid cooling of mud while traversing the water column.
However, due to the temperature rebound effects, the
158 C.J. Biickcr et ill,
t e t ~ ~ p e r a t u r e c u r v e s m u s t be taken a s minimum possible values. T h e same is true for the calculated tcniperau~re gradients, A careful look at the reclucccl temperat~ire curve in figure 3 shows that the grralie~nt also changes slightly with time. T h e tempcritture c u r v e logged o n 1 3 N o v e m b e r 1999 s h o w s the highest temperature measured in CRP-3 ( F i g 3) at that time. The temperature gradient calculated o n the basis of this tet~ipcrau~rc curve is 30 Klkm. which can be e s t i ~ i ~ i u e d as an upper vahte for the temperature griuiicnt at the C a p e R o b e r t s drill s i t e CRI'.?.
However, at the Cape Roberts drillsites CRP-2/2A, a t e m p e r a t u r e g r a d i e n t of 24 Klkm was estimated Blicker et al.. 2000). This lower tcniperdt~~re gradient ntay be result of f l u i d flow. which niiglit give ci
bigger effect in the more i~nconsolidatcci sediments with higher porosities in the upper sections of CRP- 212A.
T h e mud c o ~ i c l u c t i v i t i e s ( F i g 31, which were measured with the s a m e probe a s the clownhole temperati~res, are directly related to the mud salinity.
KC1 was a d d e d to tlie mud to prevent i t from freexirig, the borehole from collapsing. and the clays from swelling, resulting i n a mud conductivity ot 130 mS1cm. whereas the seawater salinity was about 30 mS1ct11. The temperature anomalies at 610 mbsf and 750 mbsf are accompanied by negative anomalies in the mud conductivity indicating lowered salinity.
But these mud conductivity (salinity) anomalies from the first m e a s ~ ~ r e ~ ~ ~ c m s at the beginning of a logging phase a r e no l o n g e r present at the e n d of the con'espon0ing logging phase. This observation clearly indicates that the observed temperature anomalies tire not related to ;in a q u i f e r IILH to backflow of miid invaded the f o r m a t i o n d u r i n g d r i l l i n g . However, temperati~re and salinity anomalies indicate perincable z o n e s in t h e drilled f o n n a t i o t i . 'The temperc~titre anomalies at 260 and 530 inbsf also correltue will'!
f r a c t u r e z o n e s d e t e r m i n e d by Wilson et a l . ( t h i s volume),
A heat flow profile was calculated using thermal conductivity measLirernents made on core samples and a n average thermal gradient of 2 8 . 5 Klkm (Fig. 4).
The resulting average heat flow of about 6 0 mW/m3 is only slightly higher than the average continental heat flow. If the cstimateci higher thermal gradient of 3 0 Klkm is used, a h i g h e r a v e r a g e heat flow of M mWlm3 is ohittined. These heett flow values tire considerably lower than all the oilier published values m e n t i o n e d a b o v e . T h e r i f l i n g event of the Transaritarctic Mountains about 4 0 ti1.y. ago would have caused increased heat flow and thermal gradient at the C R P boreholc sites over the entire borehole compared to normal continental crust. Obviously this is not the case, which may result in the imeyretation that the tectonic activity ceased a long time ago.
As the Cape Roberts drillholes are at the tnorthcm end of Roberts R i d g e , w h i c h is ~ r i . ~ n c i ~ ~ e d by t h e M a c k a y S e a Valley, t h e m e a s u r e d relatively low
temperature gradient and thus low heat flow values at the C a p c Roberts sites may be a result of ice that flowed through the Mckay Sea Valley c l u r i n ~ ~ e r i o d s o f more extensive glaciation, D u r i n ~ t h e last glacial maximum, the entire thermal gradient would have been o f f s e t to much lower t e m p e r a t u r e s , and sitbsec~~ent warming a d s like ;I l~ect spike propagclting downward from the seafloor and t a k i n ~ l i o i ~ s a ~ ~ d s of years to reach 500-1000 111 dcptl'!. The observed linear thermal gradient indicates that a new equilibrium temperature profile has been established and thermal memory of the cooling event has been lost.
T h e topographic location of the C a p e Roberts clriliholes and the lithologic contrast of crystalline r o c k s i n t h e Tre~nsttntarctic M o i t m a i n s and sedimentary rocks in the Victoria Land Basin may also have an effect on the measured lietit flow values.
Calculations by llelisle (pers. comm.) indicate that this effect is small: only 3 mW/iii2. which would have to be added to the heat flow value given above.
In summary, the heat flow estimates given here for the C a p c R o b e r t s drillsites. based on r e p e a t e d d o w n h o l e t e ~ n p e r a l u r c and thermal c o n d u c t i v i t y measurements o n samples, arc lower than expected.
13111 they confirm the estimates by Smeilie (this vol.) regarding timing of the uplift of the Tre~~isantarctic Mountains cincl subsidence of the Victorici L m d basin.
Blackmail et al. (1987) has already pointed out that the tl~ermal conditions play an importa~it role in the tectonic history of the region. However. we still need more information on the temperature field and i t tiiusl be recognized that we are still tit the beginning o f Antarctic geothennal exploration.
ACKNOWLEDGEMENTS - This work was supported by tlic A~ryd-l4~cgt'nei'-In.'i!i!n! f i i r Polcir- imd Mwrcsforvchiiiig (AWI), the Hm~dc'i~it.s~~il!,fiir Cn'owissensc~fiei'i mu!
Roh.s!off'e (BGR), and the German Science Founclatiot~ DFG ( W 0 6 7 2 ) . Special ihiinks go to Peter Barrctt and Franz Tessensol-in for their constiiiit encouragement ancl entli~isitisi~~. The paper benefited from cxcellcni coinments by Ladislaus Rybach and an anonymous reviewer, many tl1211iks to tlietti! We tvould like to i11a11k 2 \ 1 1 lliose who helped u s carry out the liorehole metisurei~~cnis, for which the logistics in these "icy" environments was quite difficillt.
'Shanks to Petcr Schuix, Fcrdinand Hoischer. Pal Cooper, Alex Pyne, J o h n Alexander. Jitn Cowic. and all the others
;tt Cape Bob Camp, Scoll Base and Ci-;try Lab McMiirdo not named here. The qiu~rtx diila from M. Sprovcri were very helpfiil for ii~lcrprctins the thermal condnct.i\;it),
~iicasi~rcmenls, The manuscripl benefited from Sr~titl'ul discussions with I-', Tesscnsohn a n d G. Delisle ( B G R Hannover). llilke Detjcn and Phillip Wolf condi.ictet.l tlie then'nal concluctivity nieastirements.
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