Apatite Fission Track Ages Associated with the Altered Igneous Intrusive in Beacon Sandstone near the Base of CRP-3,
Victoria Land Basin, Antarctica
P.G. FITZGERALD
Department of Earth Sciences. Syracusc University
204 Heroy Geology Laboratory. Syracusc. New York 13244-1070 - U.S.A. (pgfii~@.syr.cclu) Received 20 February 2001; accepted in revised form 9 July 200 1
Abstract
-
T h e C a p e Roberts drillhole 3 ( C R P - 3 ) o n t h e western e d g e of the Victoria Land Basin. Antarctica cored through a highly altered igneous intrusion intruded into Paleozoic sedimentary basement. T h i s intrusion was regarded as highly enigmatic as its origin could represent volcanism associated with early rifling of the Victoria Land Basin, later renewed rifting within the Terror rift, or Jurassic tholeiitic magmatism. Direct methods to date the intrusion by U-Pb dating of zircon o r fission track analysis failed due to insufficient quantities of these m i n e r a l s . A p a t i t e f i s s i o n track ( A F T ) a n a l y s i s o n t h e a d j a c e n t s e d i m e n t a r y basement, the Devonian Beacon Supergroup sandstone, yielded an age of 101 56 Maand a mean track length of 12.3 pm with a 1.9 prn standard deviation. The fission tracks were not annealed in the C e n o z o i c a n d thus the intrusion must b e o l d e r t h a n this. This observation. DILIS , L trace element chemistry of the intrusion suggest it is most likely the s a m e age and original composition as the middle Jurassic sills and dykes of the Transantarctic Mountains. The AFT age is similar to the onshore regional AFT stratigraphy and reflects complete thermal overprinting in the Jurassic, residence in an apatite partial annealing zone, followed by exhumation in the early Cenozoic and down-faulting at least 3 km to its present position. However, the sample of Beacon sandstone has an AFT age "too young" and a confined track length distribution "too short" relative to the results a sample f r o m an onland equivalent stratigraphic position should yield in the simplest scenario. This is possibly d u e to the position of the CRP-3 basement on the western edge of the West Antarctic rift system, where it underwent periods of rifting and elevated thermal gradients in the Jurassic, Cretaceous. Eocene and Oligocene causing annealing of the sample. Alternatively, this sample reflects a more complex thermal history involving Cretaceous as well as Cenozoic denudation, prior to being down faulted to its present position.
INTRODUCTION
Cape Roberts drill-hole 3 (CRP-3) was the third of three holes drilled off Cape Roberts on the western e d g e of the Victoria Land Basin (Fig. 1). T h e scientific objectives of the Cape Roberts drilling included: (1) investigation of the history of the East Antarctic Ice Sheet and the climate record associated with ice sheet inception at c. 34 Ma, (2) constraining the early history of the West Antarctic rift system and uplift of the Transantarctic Mountains (TAM). These objectives, the geological setting and the results of CRP-3 are well discussed in previous papers (Barrett et al., 1995) and initial reports from earlier drill-holes (Cape Roberts Science Team, 1998, 1999, 2000). For this paper, it is sufficient to say only that CRP-3 lies on the Roberts R i d g e on the edge of the Victoria Land Basin and is separated from the uplifted TAM, to the west, by the Transantarctic Mountains Front (TMF), a zone of steeply dipping normal faults.
CRP-3 drilled through Early Oligocene to Late
Eocene (c. 34 Ma) glacial sediments that represent a cold polar climate (Cape Roberts Science Team.
2000). T h e transition t o a warmer pre-ice s h e e t climate was not found. Near the base of CRP-3 the sediments are cut by a (steeply dipping?) shear zone.
Just below this shear zone, glacial sediments s i t unconformably on well-cemented quartz sandstone belonging to the Beacon Supergroup (Fig. 2). T h e q u a r t z o s e composition and rounded g r a i n s a r e characteristic of several formations in the lower (Devonian) part of the Beacon Supergroup, and of these the Arena Sandstone was considered most likely correlative (Cape Roberts S c i e n c e Team, 2000).
However further sampling has shown a higher proportion of feldspar than is typical of the Arena S a n d s t o n e , and present judgement is that t h e sandstone is from lower in the section and is best described stratigraphically as "lower Taylor Group".
F u r t h e r work is being carried o u t f o r a better judgement on the stratigraphic equivalence of this sandstone with onshore stratigraphy (P.J. Barrett, personal communication).
Fig. I - Generalized geological map of southern Victoria Land. Modified from (Fitzgerald. 2001). Geology and location o f fanlls ancl lineaments is after Gunn and Warren (1962). Warren (1969). McKelvey and Webb (1962). Findlay et al. (1984). Gleadow and Fil/pcralil (1987). Fitzgerald (1992) and Wilson (1995, 1999). Offshore faults are from McGinnis et al. (1985). Damaske et al. (1994). Barren cl al.
(1995). Approximate Cenozoic çroc uplift>> contours (marked by black dashed lines with rock uplift amount in km from 4.5 to 2 . 5 ) l'or (In- Dry Valleys block are modified from Fitzgerald (1992). KES = Kukri erosion surface. RL = Radian lineament. EL = Blue lineament.
Intruded into the lower Taylor Group sandstone
from 9 0 0 t o 9 2 0 mbsf (m below sea-floor) is a (mbsf)
.
,clay sand' '
-16 m thick body of highly altered magmatic rock.
Constraining the age of this intrusive body of rock is the focus of this paper as its presence in the base of the hole was regarded as highly enigmatic by the on- site scientific team (Cape Roberts Science Team, 2000, p. 7). Does this intrusion represent volcanism associated with early sifting of the Victoria L a n d Basin (part of the volcanic rocks of unit V6), or is it a part of later renewed rifting within the Terror rift?
Alternatively, is this intrusion simply a dyke of Ferrar Dolerite intruded into Devonian sandstone in the mid- Jurassic ( H e i m a n n e t a l . , 1994)? Under this l a s t scenario, the intrusion and surrounding basement were subsequently down-faulted into its present location in the Cenozoic following the initiation of the main phase of u p l i f t a n d d e n u d a t i o n of t h e T A M (Fitzgerald, 1992).
T h e texture of t h e i g n e o u s intrusive a n d i t s geochemistry (Cape Roberts Science Team, 2000, p.
130-13 1 ) suggest the intrusive may simply be highly altered Ferrar Dolerite, especially as the trace element geochemistry indicate a tholeiitic affinity. However, lack of graphic intergrowths, severe alteration i n contrast to dolerite seen onland (Hamilton e t al.:
196.51, and its importance if it was Cenozoic in age given its position near the base of CRP-3, meant constraining the age of this intrusion was important.
KEY
Beacon sandstone Sa17dstone lower Taylor Group fluvial sediments
E
Mudstone- M Diamictite conglomerate
Fig. 2 - Stratisraphic column for the basal part of CRP-3 (modified from Cape Roberts Science Team. 2000).
Apatite l-'ission Track Agcs Associated with tlic Altn'ctI Igneous Intrusive
' I i r l ~ , I - Samples collcclcd for analysis
To do this, direct means either by U-Pb ion-probe analysis o n single crystals of zircon, or fission track analysis of zircon or apatite were considered. If this direct approach failed, a n indirect approach using apatite fission track (AFT) thermochronology in the adjacent sandstone to look for an overprinting thermal effect was to be used.
Three samples were collected (Tab. l ) , two of the highly altered i g n e o u s intrusion a n d o n e of t h e adjacent lower Taylor Group sandstone (Fig. 2). The direct approach to date the intrusion failed, as neither of the two samples of altered igneous rock yielded sufficient amounts or large enough zircon crystals, or a n y a p a t i t e . H o w e v e r , s a m p l e C R P 3 - 1 f r o m t h e B e a c o n s a n d s t o n e , yielded relatively a b u n d a n t amounts of both apatite and zircon. In order to look f o r thermal overprinting within the sandstone, AFT thermochronology was used because of its relatively low closure tenlperature (- 100°C and relatively low temperature bounds of the apatite partial annealing z o n e (PAZ) (-70- 11 O°C) F i s s i o n t r a c k t h e r n ~ o - chronology on zircon from the sandstone would be less likely to register a thermal overprint because of i t s higher c l o s u r e temperature (-250°C) A basic intrusion of this sort would be expected to have an i n t r u s i o n t e m p e r a t u r e in e x c e s s of 1 0 0 0  ° ( e . g . Sparks, 1992) a n d thus completely anneal fission t r a c k s in a p a t i t e . M e t a m o r p h i c r e a c t i o n s within Beacon sandstone adjacent to Ferrar Dolerite sills in t h e Taylor Valley indicate temperatures exceeded 500° following intrusion of the sills (Haskell, 1964).
Also in southern Victoria Land, Fitzgerald (1982) examined the thermal effects of dolerite intrusion on a 1 8 0 m thick s e q u e n c e of alluvial plain sediments within t h e upper part of the B e a c o n S u p e r g r o u p . These sediments are bounded by an upper sill -30 m t h i c k and a lower sill -60 m t h i c k . M e t a m o r p h i c reactions requiring temperatures of 3 10-3 15OC were estimated for the centre of the sedimentary package, w i t h temperatures c l o s e to 5 5 0  ° a d j a c e n t to t h e chilled margin decreasing to 400-450° 0.5 in away f r o m t h e sill c o n t a c t . S u c h t e m p e r a t u r e s w o u l d completely anneal fission tracks in apatite. Indeed.
throughout the TAM wherever Jurassic dolerite sills a r e present, t h e a p a t i t e f i s s i o n t r a c k t h e r m o - chronometer has been reset. As with the Jurassic sills, if the intrusion cored in CRP3 was mid-Cenozoic in age, then the apatite fission track therinochronometer would also be reset at the time of intrusion.
T o p o f sampk i11d~rvdl (111) I h ~ t l o i ~ f of sarnfle interval ( m )
CR1'-3 BASAL STRATIGRAPHY AND SAMPLE MATERIAL
A n unconformity (at 8 2 3 . 1 1 mbsf) s e p a r a t e s C e n o z o i c strata ( i n c l u d i n g -33 m of b a s a l conglomerate) from Devonian Beacon (lower Taylor Group) sandstone (Fig. 2 ) . Apart from the igneous intrusion ( f r o m c. 9 0 2 to c. 9 2 0 mbsf) B e a c o n s a n d s t o n e extends t o t h e bottom of t h e h o l e ( 9 3 9 . 4 6 nibsf). T h e b a s a l c o n g l o m e r a t e in t h e Cenozoic section is extensively sheared. Faulting in the Beacon sandstone is more localized, but contains considerable brittle deformation, with breccia within the sandstone forming up to 36% of the core.
T h e s a n d s t o n e i s a l i g h t redlbrown m e d i u m - srained quartzose sandstone, generally well stratified, mostly with parallel lamination but with some cross- stratification (Cape Roberts Science Team, 2000.
p . 6 7 ) . Strata i m m e d i a t e l y a b o v e and below t h e igneous intrusion comprises a breccia containing c l a s t s of s a n d s t o n e a n d d o l e r i t e . Away f r o m t h e intrusion sandstone decreases in hardness and colour c h a n g e s f r o m p u r p l e t o light r e d l b r o w n . T h e s e features are indicative of thermal and mechanical alteration along the boundaries of the intrusion.
T h e intrusion itself i s h i g h l y altered a n d t h e original mineralogy is totally destroyed, although in s a m p l e s f r o m t h e u p p e r p a r t ( e . g . C R P 3 - 2 f r o m 907.45 mbsf) the original texture can be seen. T h e rock generally consists of a replacement mineralogy of an anastomosing w e b of carbonates, smectites, serpentine(?), chlorite and haematite.
RESULTS AND DISCUSSION
Sample CRP3-1 of quartzose sandstone gave an AFT age of 101 26 M a (1 o) with a mean confined length of 12.3 pm and standard deviation of 1.9 11m (Tab. 2). The track length distribution indicates that this sample has not had a simple thermal history, but that the fission track population had been subjected to protracted annealing over geologic time. The sample fails the Chi-square test indicating that greater than one population of single grain ages may be present.
This wide distribution of single grain ages can be seen in the radial plot in figure 3. Such a wide spread of single grain ages is likely to be caused by the grains having slightly different con~positions, common in a s e d i m e n t a r y r o c k , a s different a p a t i t e c o m p o s i t i o n s have s l i g h t l y different a n n e a l i n g
Stratigraphic column: southern Victoria Land
.% ,. (not to scale)
A 2. ,.
(-180 Ma)
~s.:J.~:~l
Ktrkpatrick Basalt (Ferrar Supergroup:.,
.<Mawson Formation (Ferrar Supergrou[
Feather Conglomerate (215 m)
Welter Coat Measures (250 m) (-190- 160 Ma) Ferrar Supergroup (Sill)
-190- l 6 0 Mal l I I I I Ferrar Supergroup (sill) Beacon Beacon Hei hts Orthoquartzite (330 m) Su er roup Arena Sandstone (385 m)
~ a y i o r Group Altar Mountain Formation (235 m) New Mountain Sandstone 250 m) (-415-350 Ma) Terra Windy Cotta Gully Slttstone Sandstone (82 (80 m) m)
(-190. 160 M ~ )
1 1 1 1 1
Ferrar Supergroup (Peneplain Sill, 300 (-540- 470 Ma)(-190- 160 Ma)
(-540- 470 Ma)
Granite Harbour Instrusive suite Koettlitz Group
Ferrar Supergroup (Basement Sill, 300
Granite Harbour Instrusive suite Koettlitz Group
Generalized apatite FT age profile for the upper c r u s t )f the northern Dry Valleys block, southern Victoria Lan
Radial plot
Confined track length distribution
Central age: 101 Â 6 Ma Relative error: 22 % Pk2) = 0.004%
ft of xtals: 29
0
0 T t d Lenath lmivmsl
AFT age of sample CRP3-1 101 Â 6 Ma
-Pale0 -1 10' isotherm
Break in slope" marks the onset of significant exhumation within the TAM accompanying uplift of the range,
Apatite fission track age (Ma)
fig. 3 - Generalized AFT age profile for the upper crustal stratigraphy of the northern portion of the Dry Valleys block. Stratigraphy modified from Fig. 4.4 of the CRP-3 initial report (Cape Roberts Science Team. 2000) and (Barrett. 1991). Thicknesses given for each unit of the Beacon Supergroup are niaxiniunis. Note that the stratigraphic column is not to scale. Fission track results generalized Sroin (Gleadow et al.. 1984: Gleadow and Fitzgerald. 1987: Fitzgerald. 1992). The apatite age profile represents a pattern formed following complete overprinting by thermal effects accompanying Jurassic tholeiitic magmatism. followed by residence in the upper crust until exhumation accompanying uplift of the TAM began in the early Cenozoic.
characteristics ( e . g . Green et al., 1986). Thus, long term residence within an apatite PAZ would result in differential annealing on grains with slightly differing compositions, magnifying the statistical spread i n s i n g l e grain a g e s that would b e e x p e c t e d f r o m monocompositional apatites.
The AFT result precludes a Cenozoic age for the intrusion. Given the proximity of t h e s a n d s t o n e sample to the igneous intrusion, we would expect that thermal resetting of the sample would have been complete. However, the AFT data appears to reflect the "regional" pattern of AFT ages that is revealed in onshore data for the Dry Valleys block (Gleadow et al., 1984; Gleadow and Fitzgerald, 1987; Fitzgerald, 1992) (Fig. 3). T h u s , the intrusion i s most likely Jurassic in a g e , as also s u g g e s t e d by t h e geochemistry.
The amount of offset between the TAM and the
basement (the sandstone) of CRP-3 is at least 3000 m. The sandstone in CRP-3 is 1140 m below present mean sea-level (Cape Roberts Science Team. 2000, Fig. 7.8) and lower Taylor Group sandstone crops out at elevations of -2000 m in the TAM inland from Cape Roberts. Given the AFT age and confined track length distribution of the sandstone sample (CRP3-l), the simplest interpretation is that this sample was completely reset by the thermal effects of Jurassic magmatisin, resided within an apatite partial annealing zone before being exhumed in the early Cenozoic, and down-faulted to its present position off the coast of Cape Roberts. This simple scenario is consistent with regional geology, regional A F T stratigraphy, evidence for faulting within the CRP-3 core, plus the documented faulting across the onland portion of the T M F (Gleadow and Fitzgerald, 1 9 8 7 ; Fitzgerald,
1992).
Apatite Fission Track Ages Associated with the Altered Igneous Intrusive 589
Tab. 2 - Apalitc fission track analytical iesulis: ('RP-3 drill hole.
ant arc tic;^.
-. . .. . CI'R3- 1
No. of Cii'clins
P a r e n t h e s e s e n c l o s e n u m b e r of tracks c o u n t e d ( d e n s i t y ) o r measured (track lengths). Standard and induced track densities were measured on inica external detectors (geometry factor = 0.5). and fossil track densities were measured on internal mineral surfaces.
Apatites were mounted in epoxy resin on glass slides. ground and polished to reveal an internal surface. and then etched for 20 s at room temperature in 5N HNO3 to reveal spontaneous fission tracks.
Apatite ages were determined using the external detector method and an automated stage. Samples were irradiated at the Oregon State University Nuclear reactor in the slow soaker position B-3 (Thermal column number 5 ) that has a Cc1 for ALI ratio of 13.6 at the column face. The mounts were counted at a magnification of 1250x under a dry IOOx objective. Ases were calculated using the zeta calibration method (zeta = 361 Â 10 for dosirneter glass CN5) following the procedures of Hurford and Green (1983) and Green (1985). Analytical errors were calculated using the conventional method (Green. 1981). The chi-square test performed on single- grain data (Galbraith. 1981) determines the probability that the counted grains belong to a single age population (within Poissonian variation). If the chi-square value is less than 5%. it is likely that the grains counted represent a mixed-age population with real age d i f f e r e n c e s b e t w e e n s i n g l e g r a i n s . T h e r e l a t i v e e r r o r o r a g e dispersion (spread of the individual grain data) is given by the relative standard deviation of the central age. Where the dispersion is low (<IS) the data are consistent with a single population. and the meantpooled ages and the central age converge. Track l e n ~ t h s were measured using confined fossil fission tracks using only those that were horizontal (Laslett et al., 1984). Tracks were measured under a l00x dry objective using a projection tube and a digitizing tablet attached to a microcomputer.
Standard Track Density Fossil Track . Density Induced Track Density Chi-Squarc Probability Relative 1-rror Central Agc  ± o Mean Track Length Standard Deviation
While the above scenario is certainly the simplest, there are possible complications. Under that scenario, the -100 Ma AFT age of the sandstone sample is -50 m y younger t h a n t h a t would typically b e f o u n d onshore for a similar stratigraphic position (Fig. 3, path a). Onshore, AFT ages of -100 Ma are usually found near the level of the Ferrar Dolerite basement sill (Gleadow and Fitzgerald, 1987; Fitzgerald, 1992), that is considerably lower in the stratigraphic column than the lower T a y l o r G r o u p (Fig. 3 , path b). In addition, onshore basement samples with AFT ages of -100 Ma have confined track length distributions with means typically of 12.7-13.3 pm, even up to 13.8 pm along the Mt Doorly spur, and standard deviations of
near 2 urn. Tlms (lie CRP-3 sandstone sample has an AFT agc "too young" a n d a confined track length distribution "too short" l'or its str;itigraphic position.
This indicates that under this simple scenario. this s a m p l e . compared to samples onshore having a similar stratigraphic position, has undergone a greater amount of annealing.
Annealing is c a u s e d b y residence at higher temperatures. I n this case. h c t o r s contributing to
"eater annealing in CRP-3 include 1 1
1
CRP-3 lies on the edge of a rift zone acl.jacent to a rift-flank uplift, and rift zones are areas of elevated heat flow, [2] \the basement of CRP-3 has been down-faulted to deeper crustal levels (within a rift /.one), and hence hotter temperatures.Rifti~~g and exhumation events': The age of rifting f o r this part of the Victoria Lancl Basin may b e represented by the a g e of the Cenozoic s t r a t a (c. 34 Ma) just above the unconformity with the at the base of CRP-3 (Cape Roberts Science Team, 2000 - c h a p t e r 7). It is likely that this Late E o c e n e - Oligocene phase of rifting elevated the geotherm.
Other phases of rifting, that may also have elevated the geotherni, occurred within the West Antarctic rift system in the Cretaceous (e.g. Lawver and Gahagan, 1994; Fitzgerald and Baldwin, 1997), and probably also in the early Eocene, coeval with the onset of exhum.ation as recorded by AFT profiles along the TAM (e.g. Davey and Brancolini, 1995; Fitzgerald, 2001). It should be noted that aside from the main phase of exhumation within the TAM begun in the early Cenozoic, other phases of exhumation also occur in the Early and Late Cretaceous, including a period of Cretaceous exhumation documented in the K u k r i H i l l s of s o u t h e r n Victoria L a n d ( F i g . 1 ) (Fitzgerald, 1995). Theoretically, sample CRP3-1 could lie just above a -100 Ma break in slope similar to that seen in the Kukri Hills. As of yet there is no definitive evidence for Cretaceous exhumation in the n o r t h e r n part of t h e Dry Valleys b l o c k , but t h a t possibility should not b e discounted. On the other hand. Cretaceous thermal alteration of samples cannot be ruled out either, as a mid-Cretaceous thermal event h a s been d o c u m e n t e d f r o m 4 0 A ~ l ~ ~ A r a g e s o n hydrothermally altered Ferrar Dolerite (e.g. Molzahn et al., 1999) in the TAM of northern Victoria Land. In the Dry Valleys block, there is not yet direct evidence for a mid-Cretaceous thermal event, although three samples from the Mt Jason profile gave anomalously y o u n g A F T a g e s of -100 M a (Gleadow a n d Fitzgerald, 1987). These three samples should also have had AFT ages of 140- 150 Ma. In addition to the regional elevated heat flow brought about by episodes of rifting, local thermal perturbations d u e to late Cenozoic alkaline volcanism (McMurdo volcanics) in southern Victoria Land (from c. 25 Ma, LeMasurier and Thornson, 1990, and articles within) are likely to have effected the local geotherm.
X IO'crn X1O6cm-'
X 10'cll12 7r 7c Ma p n
urn
1.60 (49 19) 1.861(1262) 4.963 (3366)
<O. l 22 101 ±
12.3 k 0 . 2 (1 19) 1.9
Fdniniif. T h e timing of down-l'aulting constrains h o w l o n g t h e b a s e o f C R P - 3 has been cit clcpth. A s the lowermost Cenozoic glacial sediments (c. 3 4 M a ) a r c c u t Ely f a u l t s a n d a l a r g e s h e a r z o n e , f a u l t i n g within the sedimentary package was ongoing after 3 4 M a . However. 81s the clepositional level o f sediments cored in C R P - 3 s h o w n progressive d e e p e n i n g f r o m fluvial to near-shore snarine 1 0 deltaic ( C a p e Roberts S c i e n c e T e a m , 2 0 0 0 , p . 1 9 6 - 1 9 7 ) , f a u l t i n g t o n e a r present l e v e l s w a s p r o b a b l y c o m p l e t e i n t h e O l i g o c c n e . A s t h e m a j o r p a n o f t h e B e t i c o n Supergroup section and a c c o t i i p a n ) , i ~ i ~ ~ e r r a r Dolerite sills must have been eroded off prior to deposition of c. 3 4 M a o l d s e c i i ~ i ~ e ~ ~ t s i t is a l s o clear tluu faulting a c r o s s t h e 'TMF m u s t h a v e siarted earlier than that.
Complicaiing this simple i n t e r p r c t a i i o ~ ~ is the f a d that t h e r a k e of fault s t r i a e o n fault surfcices within t h e C R P - 3 c o r e i n d i c a t e a c h a n g e f r o m 1 0 0 % d i p - s l i p faulting in O l i g o c c n e sediments above 7 8 9 . 5 mbsf to inclucie a s i g t i i f i c a n t c o m p o n c ~ i t o f o b l i q u e - s l i p faulti~i",below t h a t ( C a p e R o b e r t s S c i e n c e T e a m . 2000. p. 22).
CONCLUSIONS
T h e sample of early Devonian lower Taylor G r o u p s ~ i n d s t o n e f r o m n e a r the base of the C R P - 3 drillhole yielded an A P T age eof l 0 1 ± Ma ( I n ) with a mean confined length of 12.3 p111 and standard deviation of 1 . 9 p m . T h i s a g e f r o m i\ s a m p l e a d j a c e n t t o ~ I I I
igneous intrusion does not reflect thermal overprinting in t h e i ~ i i ( i - C e ~ i o x o i c a n d s o t h e i n t r u s i o n i s m o r e likely a h e a v i l y altered d y k e related to intrusion of the Ferrar Dolerite in the mid-Jurassic. However, t h e sample of sandstone has an A F T age "too young" and a c o n f i n e d t r a c k l e n g t h d i s i r i b ~ t t i o n " l o o s h o r t "
relative to the onland equivalent straligraphic position.
This indicates this sandstone scimpie has undergone a g r e a t e r a m o u n t o f annealing c o m p a r e d t o its onlatid stratigraphic equivalent. Annealing may b e related to t h e p o s i t i o n o f t h e C R P - 3 b a s e m e n t , l y i n g o n the w e s t e r n e d g e o f t h e W e s t A m a r c t i c rift s y s t e m . a system t h a t h a s e x p e r i e n c e d p e r i o d s o f s i f t i n g a n d elevated thermal gradients in the Jurassic. Cretaceous, Eocene and Oligoccnc. Alternatively, this sample may h a v e e x p e r i e n c e d exhumation in the C r e t a c e o u s o r a p a r t i a l t h e r m a l o v e r p r i n t in t h c C r e t a c e o u s b e f o r e being down faulted to its present level.
ACKNOWLEDGEMENTS - Supporl is [ r o m the National Science 1:oundation Office of Polar Programs OPP-002824 a n d OPP-003957. Thanks to Peter Barren lor a lliou~lilt'ul review on an earlier di'iifl of this mai~iscripl. Reviews by
l),,, ana :, Laura 13alestrieri and Guiho B i g a r ~ i , and comme~tts
by Ken Vcrosiib helped clarify and improve this paper.
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