Annales Societalis Geologorum Poloniae (1997), vol. 67: 271-286.
DIAGENETIC REGIMES AND THE FORAMINIFERAL RECORD IN THE BEAUFORT-MACKENZIE BASIN AND ADJACENT
CRATONIC AREAS
David H. McNEIL
G e o lo g ic a l S u rvey o f C anada, 3 3 0 3 3 3 r d St. NW ., C algary, A lb erta , T2L 2A 7, C a n a d a
McNeil, D. H., 1997. Diagenetic regimes and the foraminiferal record in the B eaufort-M ackenzie Basin and adjacent cratonic areas. Ann. Soc. G eol Polon., 67: 271-286.
A b stra c t: Terrigenous clastic, deltaic-dominated, sedimentary basins, such as the B eaufort-M ackenzie B asin o f A rctic Canada, are thick, areally extensive bodies o f sediment that undergo a wide variety o f diagenetic processes in a wide variety o f physical and chemical environments. Diagenetic processes and effects are unique to various parts o f the basin, but four diagenetic regimes can be isolated to encompass many o f the diagenetic processes affecting sedimentary basins. These are the early, burial, overpressured, and meteoric diagenetic regimes. Forami
niferal preservation may be affected by chemical, physical, and biological activities in the diagenetic regimes.
Early diagenetic conditions are generally favourable for the preservation o f foraminifers, but bacterial production o f C O i could lead to the dissolution o f calcareous foraminifers and early diagenetic pyrite often infills foraminiferal tests.
The burial diagenetic regime is dominated by increasing temperature, pressure, and compaction. Increased tem perature is reflected by thermal, colour alteration o f organic cement in the test o f agglutinated foraminifers.
Thermally controlled mineralogical changes are also evident in the burial diagenetic regime and include silicifi
cation o f the agglutinated test, chloritization o f calcareous foraminifers, and precipitation o f secondary clay minerals such as kaolinite, smectite, illite, and chlorite w ithin the foraminiferal test. Thermal m aturity can be assessed by application o f the Foraminiferal Colouration Index (FCI). M ineralogical changes in foram inifers allow for the establishm ent o f four broad burial diagenetic zones (A to D).
The overpressured regime may be responsible for a retardation in thermal alteration o f agglutinated foram i
nifers and silicification o f agglutinated foraminifers may be a precursor as well as an effect o f overpressured fluids in sedimentary basins.
The meteoric regime is significant during periods o f extensive erosion. Interactions betw een organic and inorganic detritus and meteoric waters may lead to dissolution o f calcareous foraminifers and precipitation o f secondary minerals such as kaolinite and minor amounts o f quartz.
A b stra k t: K lastyczne, przeważnie deltowe, osady o dużej miąższości, deponowane w basenie sedym entacyjnym B eauforta-M ackenzie (arktyczna część Kanady) podlegały różnego rodzaju procesom diagenetycznym w w arun
kach szerokiej zmienności param etrów fizycznych i chemicznych. Procesy te i ich efekty są inne w każdej części basenu, jakkolw iek można w yróżnić cztery środowiska, które charakteryzują diagenezę w basenie sedym entacyj
nym. N ależą do nich procesy związane z: (i) w czesną diagenezą, (ii) pogrzebaniem osadów, (iii) nadciśnieniem płynów porow ych oraz (iv) działalnością wód meteorycznych. Stan zachow ania otwornic może zależeć od oddziaływania czynników chemicznych, fizycznych i biologicznych w czasie diagenezy osadów.
W warunkach wczesnej diagenezy, stan zachow ania otwornic je st dobry, jakkolw iek biologiczna produkcja CO2 może spow odow ać rozpuszczanie ich węglanowych skorupek. Skorupki są wtedy w ypełniane wczesnodia- genetycznym pirytem.
W trakcie pogrzebania osadów w zrasta temperatura, ciśnienie i kompakcja. W zrost tem peratury pow oduje zmiany składu mineralnego skorupek otwornic. N ależą do nich sylifikacja skorupek otw ornic aglutynujących, chlorytyzacja skorupek otwomic węglanowych i wytrącanie wtórnych m inerałów ilastych (kaolinit, illit i chloryt) wewnątrz skorupek. Dojrzałość term iczną można określić stosując wskaźnik zmiany barwy otw ornic (Foram ini
feral Colouration Index). Zm iany mineralogiczne skorupek otwomic pozwoliły na ustalenie czterech stref przeo
brażeń diagenetycznych (A -D ).
Procesy związane z nadciścieniem wód porowych m ogą być odpowiedzialne za spow olnienie procesów przemian term alnych w skorupkach otwornic aglutynujących. Sylifikacja skorupek może być zarówno prekur
sorem ja k i pierwszym z efektów tych procesów.
W pływ wód m eteoiycznych jest znaczący w czasie procesów silnej erozji osadów. W zajemne oddziaływanie detrytusu pochodzenia organicznego i nieorganicznego z wodami meteorycznym i może prow adzić do rozpusz
czania węglanowych skorupek otw om ic oraz krystalizacji wtórnych minerałów (kaolinitu i podrzędne ilości kwarcu).
272
D. H. McNEILKey w o rd s: diagenesis, benthic foraminifers, sedimentary basins, thermal colour alteration, silicification, clay minerals, geopressure, meteoric water.
Manuscript received 10 July 1996, accepted 30 A pril 1997
INTRODUCTION
D ia g e n e sis re fe rs to th e p h y sic a l a n d c h e m ic a l c h an g es u n d e rg o n e b y a se d im e n t, in c lu d in g its b io lo g ic a l c o m p o n e n ts , a fte r in itial d e p o s itio n a n d c o n tin u in g th ro u g h lith ifi- ca tio n , e x c lu s iv e o f w e a th e rin g a n d m e ta m o rp h ism . D ia g e n e tic e ffe c ts h a v e b e e n stu d ie d e x te n s iv e ly b y se d im e n to lo - gists a n d g e o c h e m ists in o rd e r to u n d e rs ta n d th e m o d ific a tio n s th a t se d im e n ts u n d e rg o o n th e ir p a th to b e c o m in g ro c k s , in p a rtic u la r as re s e rv o irs fo r h y d ro c a rb o n s. P a le o n to lo g is ts h a v e stu d ie d e a rly d ia g e n e sis m o s tly in a ta- p h o n o m ic c o n te x t, b e in g c o n c e rn e d w ith th e e a rly d ia g e n e tic h is to ry o f a n o rg a n is m a n d e ffe c ts o n p re se rv a tio n . In c o n tra st, th e e ffe c ts o f la te d ia g e n e sis (b u ria l d ia g e n e sis)
Fig. 1. Location o f B eaufort-M ackenzie area
h a v e re c e iv e d v e ry little a tte n tio n fro m m ic ro p a le o n to lo g ists. B u ria l d ia g e n e tic e ffe c ts, h o w e v e r, are c o n sp ic u o u s a n d a c c u m u la tiv e in fo s sil fo ra m in ife rs an d p ro v id e v a lu -
UPLANDS
ab le in fo rm a tio n o n th e th e rm a l h is to ry o f se d im e n ta ry b a sins (M c N e il e t a l., 1996).
In th is p a p e r, d ia g e n e sis is re v ie w e d fro m n u m e ro u s p e rs p e c tiv e s b a se d la rg e ly o n in fo rm a tio n c u lle d fro m th e d ia g e n e tic lite ra tu re a n d o n o b s e rv a tio n s a n d re s e a rc h into th e d is trib u tio n s o f b e n th ic fo ra m in ife rs in th e B e a u f o r t- M a c k e n z ie B a sin o f A rc tic C a n a d a (F ig. 1). T h is b a sin is o f in te re st fo r se v e ra l re a s o n s, a n d g e n e ra liz a tio n s can b e in fe rre d to o th e r s im ila r b a sin s. It h a s b e e n th e s ite o f th ic k a c c u m u la tio n s o f d e lta ic in flu e n c e d te rrig e n o u s c la s tic s e d i
m e n ts (1 0 to 15 k m p re s e rv e d ) th ro u g h m o s t o f th e C e n o z o ic a n d is c u rre n tly re c e iv in g s e d im e n t a n d a c tiv e ly su b sid in g . It is a p e tro le u m e x p lo ra tio n b a s in th a t h a s re c e iv e d a s ig n ific a n t a m o u n t o f e x p lo ra tio n , b u t still h o ld s m u c h p o te n tial fo r fu tu re e x p lo ra tio n . Its b io s tra tig ra p h y re lie s h e a v ily u p o n b e n th ic fo ra m in ife ra l a s s e m b la g e s w h ic h h a v e b een re c o v e re d th ro u g h a w id e sp e c tru m o f te rrig e n o u s c la stic se d im e n ta ry e n v iro n m e n ts.
It w as o b se rv e d a t a n e a rly sta g e in th e e x p lo ra tio n o f th is se d im e n ta ry b a sin th a t tw o a sp e c ts o f th e fo ra m in ife ra l re c o rd w e re a b u n d a n tly c le a r. T h e first c e n tre d a ro u n d th e c o n sp ic u o u s re c ip ro c a l re la tio n s h ip b e tw e e n th e d is trib u tio n s o f a g g lu tin a te d v ersu s c a lc a re o u s b e n th ic fo ra m in ife rs in v ario u s p a rts o f th e b a sin a n d a t v a rio u s tim e s th ro u g h the h isto ry o f th e b asin (S c h rö d e r-A d a m s & M c N e il, 1994).
T h e se c o n d s te m m e d fro m th e o b s e rv a tio n th a t th e m in e ra lo g ical, te x tu ra l, a n d o v e ra ll v is u a l a p p e a ra n c e o f th e fo ra m in ife ra l a ss e m b la g e s c h a n g e d p ro g re s s iv e ly (th ro u g h b u ria l d ia g e n e tic p ro c e s se s) as d e p th o f b u ria l a n d te m p e ra tu re in c re a se d (M c N e il e t a l., 1996).
E m p iric a l d a ta fro m th e B e a u fo rt-M a c k e n z ie B a s in in d ic a te d th a t d ia g e n e tic p ro c e s se s h a d a ffe c te d fo ra m in ife ra l asse m b la g e s in d iffe re n t p a rts o f th e b a sin in a v a rie ty o f w a y s. T h e se d iffe rin g d ia g e n e tic a ffe c ts c a n be d e s c rib e d in th e c o n te x t o f d ia g e n e tic re g im e s (F ig . 2 ) p re v io u s ly o u t
Fig. 2. Generalized distribution o f diagenetic regimes in a continental margin, terrigenous clastic sedim entary basin such as the B eaufort-M ackenzie Basin. Coarsely stippled areas represent coarse clastic sediments; darker patterns represent finer grained mudstones and shales
DIAGENETIC REGIMES AND THE FORAM INIFERAL RECORD
273
lin ed in th e d ia g e n e tic lite ra tu re , fo r e x a m p le , B j0rly k k e (1 9 8 3 ) a n d G a llo w a y (1 9 8 4 ). T h e d ia g e n e tic reg im es e x c lu d e p re -d ia g e n e tic p ro c e s s e s su c h as d is so lu tio n by u n d e r
sa tu ra te d s e a w ater, a b ra s io n , p re d a tio n , b o rin g s, etc. T h e first re g im e , o r early d ia g e n e tic reg im e, c o n tro ls th e in itial fo s s iliz a tio n o f b e n th ic fo ra m in ife rs in th e u p p e rm o st s e v era l m e tre s o f s e d im e n t im m e d ia te ly b e lo w th e se d im e n t- w a te r in te rfa c e . T h e s e c o n d re g im e is th a t o f b u ria l d ia g e n e sis w h ic h is c o n tro lle d by in c re a sin g te m p e ra tu re a t d ep th . In th is re g im e , o rg a n ic m a tte r is p ro g re ssiv e ly m a tu re d , d i
a g e n e tic flu id s are m o b iliz e d , an d s e c o n d a ry m in e ra liz a tio n a n d re c ry sta lliz a tio n o c c u rs . T h e th ird re g im e is th e o v e r
p re s s u re d re g im e in w h ic h flu id s, tra p p e d in ra p id ly d e p o s ited se d im e n ts, a ttain a b n o rm a lly h ig h p o re p re ssu re . T h e o v e rp re s s u re d re g im e is im p o rta n t b e c a u se o f p o te n tia l a f fe c ts on th e m a tu ra tio n o f o rg a n ic m a tte r a n d affe c ts on o th e r c h e m ic a l re a c tio n s in an a b n o rm a lly p re s su re d flu id e n v iro n m e n t. A fo u rth d ia g e n e tic re g im e is re fe rre d to as th e m e te o ric re g im e d e fin e d b y th e flo w o f m e te o ric w a te rs into th e s u b s u rfa c e p o te n tia lly to d e p th s o f 2 km c o n tro lle d b y h y d ra u lic p re s su re a n d p e rm e a b ility p a th w a y s. T h is re g im e m a y h a v e a lim ited a ffe c t o n th e p re s e rv a tio n o f ca lc a re o u s fo ra m in ife rs a n d also a m in o r in flu e n c e o n th e m o b iliz a tio n a n d p re c ip ita tio n o f silic a an d c la y m in erals.
D ia g e n e tic re g im e s in se d im e n ta ry b asin s h a v e b een re v ie w e d by n u m e ro u s a u th o rs. B ro ad o v e rv ie w s o f th e to p ic h a v e b e e n p re s e n te d by G a llo w a y (1 9 8 4 ), C h ilin g a ria n a n d W o lf (1 9 8 8 ), B j0 rly k k e (1 9 8 9 ), a n d H a rriso n an d T e m p e l (1 9 9 3 ). T h e c h e m ic a l-b io lo g ic a l e n v iro n m e n t o f th e early d ia g e n e tic z o n e has b e e n re v ie w e d b y H e sse (1 9 9 0 ). N u m e ro u s a c c o u n ts on ta p h o n o m y a n d th e fossil re c o rd are c o m p ile d in th e p u b lic a tio n s o f A lliso n a n d B rig g s (1 9 9 1 ) a n d D o n o v a n (1 9 9 1 ). C o m p ila tio n s on a w id e v a rie ty o f d i
a g e n e tic to p ic s can a lso b e fo u n d in p u b lic a tio n s b y M a r
sh a ll (1 9 8 7 ), a n d M c llre a th a n d M o rro w (1 9 9 0 ). S p ecific stu d ie s on th e g e o c h e m istry a n d p e tro g ra p h y o f the B e a u fo rt-M a c k e n z ie B a sin c a n b e fo u n d in B lo c h an d Issler (1 9 9 6 ), K o (1 9 9 2 ), D ix o n (1 9 9 6 ), a n d S c h m id t (19 8 7 ).
THE EARLY DIAGENETIC REGIME
In n o rm a l m a rin e e n v iro n m e n ts, th e e a rly d ia g e n e tic re g im e b e g in s in su b a q u e o u s se d im e n ts a n d c o m p rise s th e u p p e rm o s t 10 m e tre s o r so o f s e d im e n t c o n sistin g o f a b ro w n is h -y e llo w is h o x id iz e d lay er, u su a lly less th a n 10 cm th ic k , u n d e rla in by a m u c h th ic k e r g re y ish -b la c k is h re d u c ing la y e r (F ig . 3). In a n o x ic b o tto m w a te r e n v iro n m e n ts, the o x id a tio n z o n e is a b s e n t a n d e a rly d ia g e n e sis o c c u rs en tire ly in th e re d u c in g zo n e. T h e m a jo rity o f b e n th ic fo ra m in ife rs liv e a t th e s e d im e n t/w a te r in te rfa c e an d w ith in th e o x id iz in g z o n e , b u t so m e sp ecies c a n to le ra te a n a e ro b ic liv in g c o n d i
tio n s a n d b u rro w a s h o rt d is ta n c e into th e re d u c in g la y e r (M o o d le y & H ess, 1992; S e n G u p ta & M a c h a in -C a stillo , 1993; K ita z a to , 1994).
E a rly d ia g e n e sis is d o m in a te d b y ch e m ic a l a n d b io lo g i
cal in te ra c tio n s d e p e n d e n t on th e n a tu re o f th e p o re w ater, th e a m o u n t an d ty p e o f o rg a n ic m a tte r, th e a b u n d a n c e o f r e a c tiv e iron, th e s e d im e n ta tio n rate, th e lev el o f b io tu rb a tio n , an d m o s t im p o rta n tly th e a c tiv ity ^ o f e ith e r a e ro b ic o r a n
a e ro b ic b a c te ria (H esse, 1990). F o ra m in ife rs are in v o lv e d d ire c tly in early d ia g e n e tic p ro c e s s e s as c o n trib u to rs o f c a l
ca re o u s a n d silic e o u s tests, as in fa u n a l in h a b ita n ts a n d bio- tu rb a to rs o f th e o x id iz e d zo n e, a n d as su p p lie rs o f o rg a n ic m a tte r a n d tra c e s o f re a c tiv e iron.
In n o rm a l m a rin e se d im e n ts, p o re w a te rs a t sh a llo w b u r
ial d e p th in th e o x id iz in g z o n e a re ty p ic a lly o x y g e n a te d , c a l
c iu m c a rb o n a te sa tu ra te d , an d p H n e u tra l. B e r n e r ’s (1 9 8 1 ) m e a s u re m e n ts o f h u n d re d s o f e s tu a rin e an d n o rm a l m arin e se d im e n ts in d ic a te d th a t 9 0 % w e re b e tw e e n p H 6 .5 a n d 7.5.
U n d e r th e se c o n d itio n s c a lc a re o u s te s ts w ill g e n e ra lly be stab le, p a rtic u la rly b e c a u se o f th e a ffe c ts o f n a tu ra l b u ffe rs (c la y s, d is so lv e d g ases, o rg a n ic a c id s , etc.). T h e re are, h o w ever, n u m e ro u s p ro c e s se s c a p a b le o f p ro d u c in g a c id s, c a r
b o n ic a c id in p a rtic u la r, th a t a re p o te n tia lly d e s tru c tiv e fo r fo ra m in ife rs. In th e a e ro b ic zo n e, b a c te ria l d e c a y o f o rg an ic m a tte r p ro d u c e s C O2 th ro u g h th e sim p lifie d e q u a tio n C H2O + 0 2 = C O2 + H2O . H y d ra tio n o f C O2 le a d s to th e h ig h ly d is so c ia te d c a rb o n ic a c id (H2C O3) in th e re a c tio n C O2 + H2O = H + + H C0 3- (G o lu b ic & S c h n e id e r, 1979). I f p H is n o t b u ffe re d (an u n lik e ly situ a tio n ), a b u ild -u p o f c a rb o n ic a c id w o u ld lead to the d is so lu tio n o f c a lc a re o u s fo ra m in i
fers. F lu id e x c h a n g e w ith o v e rly in g m a rin e w a te r a lso n e g ates th e p o te n tia l fo r c a rb o n ic a c id b u ild u p (K id w e ll &
B o se n c e , 1991).
A e ro b ic b a c te ria l a c tiv ity a c tin g d ire c tly o n fo ra m in i
fers can p la y a s ig n ific a n t ro le in d ia g e n e tic a lly a lte rin g th e fo ra m in ife ra l reco rd . F re iw a ld (1 9 9 5 ) d o c u m e n te d th a t b a c te ria l d e g ra d a tio n o f o rg a n ic m a te ria l w ith in C ib ic id e s lo- b atu lu s c re a te d m in u te ly lo c a liz e d b u ild u p s o f C O2. M icro - sc o p ic -sc a le d is so lu tio n o c c u rre d , ev e n th o u g h s u rro u n d in g p o re w a te rs w e re s a tu ra te d w ith re s p e c t to c a lc iu m c a rb o n ate, b e c a u se C O2 w as tra p p e d u n d e r a b io film se c re te d by th e b acte ria.
B a c te ria l d e c o m p o s itio n o f o rg a n ic m a te ria l in th e o x i
d atio n zo n e is g e n e ra lly so a c tiv e th a t it d e p le te s a v a ila b le o x y g e n b e lo w th e u p p e r few c e n tim e tre s o f se d im e n t as sed im e n ts b e c o m e b u rie d a n d p a ss into th e a n a e ro b ic re d u c ing zone. D e c o m p o sitio n o f o rg a n ic m a tte r in th is z o n e o c cu rs m a in ly th ro u g h a n a e ro b ic , b a c te ria lly m e d ia te d , su l
p h a te re d u c tio n alth o u g h n itra te , iron, a n d m a n g a n e se r e d u c tio n m a y b e im p o rta n t lo c a lly . A sim p lifie d e q u a tio n fo r th is p ro c e s s is 2C H2O + S O42 - = H2S + 2H C O3- (B e rn e r, 1985; H esse, 1990). A c o n se q u e n c e o f th is g e n e ra liz e d re a c tio n is an o v e ra ll in c re a se in C O2 (c a rb o n ic a c id a n d its d is so c ia te d sp e c ie s b ic a rb o n a te H C O3- a n d c a rb o n a te C O32 - ; H esse, 1990, p. 2 8 3 ) an d p o te n tia l d is so lu tio n o f c a lc a re o u s fo ssils in so m e facies su ch as b la c k sh a le s (C u rtis, 1980). In m o s t s h e lf se d im e n ts, h o w e v e r, th e su lp h a te re d u c tio n p ro c ess lead s also to th e re d u c tio n o f iro n w h ic h ra ise s p H (C u r
tis, 1980; B o u d re a u & C a n fie ld , 1988; A b e rc ro m b ie , p e r s o n a l com m u n ication ). Iro n is g e n e ra lly a b u n d a n t in te r
rig e n o u s c la stic s h e lf se d im e n ts in th e fo rm o f o x id iz e d co a tin g s on iro n -b e a rin g d e trita l g rain s.
A n im p o rta n t a s p e c t fo r b o th c a lc a re o u s a n d a g g lu ti
n a te d fo ra m in ife rs in the re d u c in g z o n e is th e fo rm a tio n o f d ia g e n e tic p y rite. P y ritiz a tio n o c c u rs in th e su lp h a te re d u c ing zo n e u n d e r a n a e ro b ic c o n d itio n s . O rg a n ic m a tte r is m e ta b o liz e d b y a n a e ro b ic su lp h a te re d u c in g b a c te ria . H y d ro gen su lp h id e p ro d u c e d in th e re d u c tio n o f su lp h a te e ith e r
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D. H. McNEILEARLY D IA G E N E TIC REGIME
STAGNANT MARINE NORMAL MARINE DELTAIC
BOTTOM WATER
- w a te r stratification - limited circulation - dysaerobic to aero b ic - limited b en th o s - m arine organic rich - sulphate rich
- active circulation - oxygenated
- diverse, a b u n d an t b en th o s - m arin e/terrestrial organics - sulphate rich
- rapid,turbid, sed im en tatio n - oxygenated
- a b u n d a n t b e n th o s, low diversity
- te rre stria l organics - iron rich, sulphate p o o r
OXIDIZING LAYER
m m - c m
- oxidizing layer a b sen t - if b o tto m w a te r is dysaerobic, oxidizing layer is thin, as oxygen is quickly d e p le te d by aerobic b acteria
- aero b ic b acte ria feed on m arine organic m aterial, depleting oxygen
- pH variable w ithin limited range ( ~ 7 . 0-7.5)
- sed im en ts a re well oxygenated
- a e ro b ic b acte ria m ay d e p le te organic m a tte r
REDUCING LAYER
c m - 10 m
- an aero b ic b acte ria re d u c e and d e p le te sulphate - principal p ro d u cts are bicarb o n ates, hydrogen, and bisulphide ion
- pH may b e acidic causing dissolution o f calcareous foram inifers
- pH could be buffered by silicate hydrolysis
- an aero b ic b a c te ria re d u c e sulphate and ferric iron - p y rite is fo rm e d from re d u c e d iron and dissolved HS
- ferric iron in agglutinated foram inifers is red u ced , possibly leading to disaggregation of agglutinated te s t
- an aero b ic activity m ay be limited by lack of m arine organic m a tte r
- agglutinated foram inifers a re w ell p re s e rv e d and calcareous sp ecies can be dissolved by acidic m e te o ric w a te r
FOSSIL
RECORD If e
Fig. 3. Summary o fflu id s, sediments, chemistry, bacterial activity, and foraminiferal preservation in the oxidizing and reducing layers o f the early diagenetic regime, a. Trochammina, GSC 109450, aberrant growth in low oxygen conditions; b. Stainforthia, GSC 89548, adapted for survival in low oxygen environment; c. Turrilina, GSC 89524, normal m arine habitat; d. Reticulophragmium. GSC 109537, specimen replaced partly by pyrite (light areas); e. Portatrochammina, GSC 89532, excellent preservation in deltaic habitat;
f. Asterigerina, GSC 109538, partial dissolution, iron oxide encrusted
m ig ra te s u p w a rd to be re -o x id iz e d to su lp h a te o r re a c ts w ith d e trita l iro n m in e ra ls to fo rm a series o f su lp h id e s w ith th e e n d p ro d u c t b e in g p y rite (B e m e r, 1985). In a n o x ic m a rin e e n v iro n m e n ts, th e o x id a tio n z o n e is a b s e n t a n d H2S is n a tu ra lly a b u n d a n t in b o tto m w a te rs. P y rite fo rm a tio n m a y be lim ite d b y th e su p p ly o f re a c tiv e iron a n d /o r su lp h a te o r o r
g a n ic m a te ria l (B e m e r, 1985).
In n o rm a l m a rin e s h e lf settin g s, in te n se b io tu rb a tio n m a y d is ru p t th e a b o v e c ite d p ro c e s se s an d le a d to th e d is s o lu tio n o f c a lc a re o u s fo s sils (L e w y , 1975; A ller, 1982) in th e fo llo w in g m a n n e r. B io tu rb a tio n in h ib its an in c re a se in p H , d e sp ite h ig h rates o f s u lp h a te re d u c tio n an d b ic a rb o n a te ion p ro d u c tio n , b y th e a d d itio n o f c a rb o n d io x id e th ro u g h re s p i
ratio n . B io tu rb a tio n su s ta in s a su p p ly o f su lfa te -ric h b o tto m w a te r, a n d in th e p re s e n c e o f re a c tiv e d etrital iron (a b u n d a n t in te rrig e n o u s c la s tic s e d im e n ts), p y rite fo rm s lo c a lly by su lp h a te re d u c tio n . F u rth e r b io tu rb a tio n e x p o se s th e p y rite
to o x id a tio n a n d p ro d u c e s s u lfu ric a c id m ic ro e n v iro n m e n ts cap a b le o f d is so lv in g c a lc a re o u s te sts. In B a ltic S e a se d i
m en ts, L e w y (1 9 7 5 ) n o te d th a t b io tu rb a tio n in th e u p p e r 20 c m o f se d im e n t c o u ld re c y c le c a lc a re o u s fo ra m in ife rs to th e su rfa c e w h e re th e y w e re d is so lv e d by s e a w a te r. B e lo w th e level o f b io tu rb a tio n , a n a e ro b ic d e c o m p o s itio n o f o rg a n ic m atter, su lp h a te re d u c tio n , a n d re d u c tio n o f iro n su p p o rts g o o d p re s e rv a tio n o f c a lc a re o u s fo ssils. L e w y (1 9 7 5 ) an d A lle r (1 9 8 2 ) h a v e b o th d o c u m e n te d th e p re s e rv a tio n o f deep b u rro w in g c a lc a re o u s sh e lly o rg a n ism s.
A n o th e r p o te n tia lly d e s tru c tiv e e a rly d ia g e n e tic m e c h a n ism a ffe c tin g a g g lu tin a te d fo ra m in ife rs w a s p ro p o s e d by S c h rö d e r-A d a m s a n d M c N e il (1 9 9 4 ). T h e y s p e c u la te d th a t th e in te n se a c tiv ity o f su lp h a te re d u c in g b a c te ria in fin e g ra in e d m a rin e m u d s w ith a b u n d a n t o rg a n ic m a tte r c o u ld lead to th e d isa g g re g a tio n o f a g g lu tin a te d fo ra m in ife rs. D is a g g re g a tio n o c c u rs b e c a u se th e te s t-s ta b iliz in g re a c tiv e fer-
DIAGENETIC REGIM ES AND THE FORAM INIFERAL RECORD
275
ric iro n in th e o rg a n ic c e m e n t o f a g g lu tin a te d fo ra m in ife rs is re d u c e d to fe rro u s iro n a n d u tiliz e d in th e fo rm a tio n o f p y rite. E v id e n c e to s u p p o rt th is h y p o th e s is cam e fro m s e d i
m e n ts c o n ta in in g a b u n d a n t p y rite a ss o c ia te d w ith rare p y ritic s te in k e rn s o f a g g lu tin a te d fo ra m in ife rs in th e m id st o f w ell p re s e rv e d a ss e m b la g e s o f c a lc a re o u s fo ra m in ife rs (F ig. 3 c,d ). T h e p y ritic s te in k e rn s w e re in te rp re te d as sp arse re p re s e n ta tiv e s o f a d is a g g re g a te d a ss e m b la g e o f a g g lu ti
n a te d fo ra m in ife rs.
S e d im e n ta tio n ra te is an im p o rta n t facto r in e a rly d i
a g e n e sis (H e sse , 1990, p. 2 8 3 ). In h ig h d e p o sitio n a l rate e n v iro n m e n ts, su c h as th e B e a u fo rt-M a c k e n z ie B a sin an d m a n y c o n tin e n ta l s h e lf a re a s, m o re o rg a n ic m a tte r su rv iv es th e o x id a tio n zo n e an d is p re s e rv e d in a re a c tiv e state in th e re d u c in g zo n e. T h e in c re a se d s e d im e n t th ic k n e ss also tra p s th e p ro d u c ts o f early d ia g e n e sis b e c a u se d iffu sio n an d ad- v e c tio n is d e c re a s e d . T h e c o m b in a tio n o f th e se fa c to rs leads to m o re in te n se d ia g e n e tic re a c tio n s su c h as su lp h a te re d u c tion in ra p id ly b u rie d se d im e n ts. S u lp h a te re d u c tio n is th e in itial ste p in th e fo rm a tio n o f p y rite an d th is p ro c e ss im p a c ts on fo ra m in ife rs in b o th c o n stru c tiv e an d d e stru c tiv e w ay s.
F o rm a tio n o f e a rly d ia g e n e tic p y rite c a n be c o n stru c tiv e in the p re s e rv a tio n o f fo ra m in ife rs, p a rtic u la rly fo r a g g lu ti
n a te d sp e c ie s, b y re in fo rc in g th e te s t in te rio r a g a in s t c o l
lapse fro m b u rial p re s su re . P y ritiz a tio n in fo ra m in ife rs c o u ld a lso b e d e stru c tiv e i f p y rite is b ro k e n d o w n later by o x id a tio n fro m w e a th e rin g p ro c e s se s. T h is p ro c e ss o f o x id a tio n c a n b e sim u la te d in la b o ra to ry p ro c e s s in g i f c a lc a re o u s fo ra m in ife rs w ith p y ritic m o u ld s are p ro c e s se d w ith h y d ro g en p e ro x id e . H o d g k in so n (1 9 9 1 ) has c a u tio n e d m ic ro p a le o n to lo g ists a g a in s t u sin g h y d ro g e n p e ro x id e to p ro cess p y ritiz e d m ic ro fo s sils, b u t e x p e rie n c e h as sh o w n th a t it c a n b e u s e d i f d ilu te d (1 0 % ) a n d p ro c e s s in g tim e is m in im ized .
THE BURIAL DIAGENETIC REGIME
T h e b u ria l d ia g e n e tic re g im e o c c u rs a t d ep th in s e d i
m e n ta ry b a s in s as te m p e ra tu re s a n d p re s su re s in c re a se an d th e re are n o d ire c t links to th e m e te o ric o r th e m a rin e h y d ro sp h e re (F ig . 2). H e sse (1 9 9 0 ) n o te d th a t b u rial d ia g e n e tic re a c tio n s b e g in a t a b o u t 7 5 °C as b a c te ria lly m e d ia te d d e c o m p o s itio n re a c tio n s in o rg a n ic m a tte r g iv e w a y to th e rm a lly c o n tro lle d (k in e tic ) re a c tio n s , su c h as th e th e rm a l m a tu ra tio n o f k e ro g e n . D e p e n d in g o n th e g e o th e rm a l g ra d ie n t and b u ria l ra te , k e ro g e n m a tu ra tio n b e g in s ta k in g p la c e as risin g te m p e ra tu re s g e n e ra te th e a c tiv a tio n e n erg y n e c e ss a ry fo r b re a k in g o rg a n ic m o le c u le s. In a n o rm a lly c o m p a c tin g c o n tin e n ta l s h e l f b asin , th e se th e rm o c h e m ic a l d ia g e n e tic p ro c e sse s b e g in o p e ra tin g a t d e p th s o f 2 - 3 km . T h e b u rial d ia g e n e tic re g im e a p p lie s th ro u g h a te m p e ra tu re ran g e o f a b o u t 7 5 °C to a b o u t 2 0 0 -2 5 0 ° C a n d b u ria l d e p th s u p to a b o u t 8 km w h e re m in e ra l tra n s fo rm a tio n s o f th e e a rly m e ta m o rp h ic re g im e b eg in .
F o ssil fo ra m in ife rs e x h ib it m a n y o f th e cla ssic a l th e r
m a l e ffe c ts th a t o c c u r p ro g re s s iv e ly in o rg a n ic m a tte r an d v a rio u s m in e ra l p h a se s th ro u g h th e b u rial d ia g e n e tic reg im e.
F o r e x a m p le , th e F o ra m in ife ra l C o lo u ra tio n In d ex (F C I) re c e n tly d e v e lo p e d fro m B e a u fo rt-M a c k e n z ie B asin data by
Table 1
F o ra m in ife ra l C o lo u ra tio n In d e x (F C I) o f M c N e il e t al.
(1 9 9 6 ) re la tiv e to sta n d a rd c o lo u rs o f th e M u n s e ll Soil C o lo u r C h a rt
FORAMINIFERAL COLOURATION INDEX (FCI)
FCI MUNSELL COLOUR STANDARD
0 7.5YR6/6 reddish yellow 1 1OYR0/1, 10YR8/2 white 2 10YR7/1, 10YR7/2 light grey
3 10YR6/1, 10YR6/2 light brownish grey to grey 4 10YR5/1, 10YR5/2 grey to greyish brown 5 10YR4/1, 10YR4/2 dark grey to dark greyish brown 6 10YR3/1, 10YR3/2 very dark grey to very dark greyish brown 7 10YR2/1, 10YR2/2 very dark brown to brownish black 8 10YR2/1, N2/0 very brownish black
9 N2/0 black (partially translucent)
10 N2/0 black (opaque)
M cN eil e t al. (1 9 9 6 ) d o c u m e n ts th e rm a lly c o n tro lle d c o lo u r alte ra tio n o f th e o rg a n ic c e m e n t in a g g lu tin a te d fo ra m in ife rs (T a b le 1). F C I d a ta an d c a lc u la te d b o re h o le te m p e ra tu re s in th e B e a u fo rt-M a c k e n z ie B asin in d ic a te th a t th e rm a l m a tu ratio n o f fo ra m in ife rs b e g in s a t te m p e ra tu re s o f 6 0 -7 0 ° C a n d th a t a fa irly ra p id c o lo u r c h a n g e to b ro w n is h b la c k o c curs as te m p e ra tu re s rise to 1 0 0 -1 4 0 ° C , m a k in g fo ra m in i
fers se n sitiv e th e rm a l in d ic a to rs in th e e a rly s ta g e s o f p e tr o leu m g en eratio n .
F o ra m in ife rs a re also s e n s itiv e in d ic a to rs o f th e rm a lly c o n tro lle d d ia g e n e tic m in e ra liz a tio n th a t o c c u rs in th e b u ria l d ia g e n e tic re g im e in v o lv in g silic a , c a rb o n a te s, a n d cla y m in e ra ls (M c N e il e t a l., 1996). F ro m a m ic ro p a le o n to lo g i- cal p e rs p e c tiv e , th e m o s t w id e s p re a d a n d re a d ily re c o g n iz a b le o f th e se m in e ra lo g ic a l c h a n g e s is th e s ilic ific a tio n o f a g g lu tin a te d fo ra m in ife rs (F ig. 4 ). S ilic ific a tio n o c c u rs by p re c ip ita tio n o f s e c o n d a ry q u a rtz , as o v e rg ro w th s on q u a rtz g ra in s in th e fo ra m in ife ra l te s t (M c N e il e t al., 1996). M in eral a ss e m b la g e s a n d te m p e ra tu re s c o n tro l th e a m o u n t o f p o re -flu id silic a a v a ila b le fo r p re c ip ita tio n . In itia lly , d u rin g early d iag en e sis, p o re flu id s are o v e rs a tu ra te d w ith re s p e c t to q u artz, b u t p re c ip ita tio n d oes n o t g e n e ra lly o c c u r u n til k i
n e tic lim ita tio n s on q u a rtz p re c ip ita tio n are o v e rc o m e in th e b u rial d ia g e n e tic re a lm (A b e rc ro m b ie e t a l., 1994). In th e bu rial d ia g e n e tic re a lm , th e silic ific a tio n o f a g g lu tin a te d fo ra m in ife rs is p ro g re ssiv e w ith in c re a s in g te m p e ra tu re an d b u ria l (M c N e il e t al., 1996). M a p p in g o f th e h o riz o n at w h ic h silic ific a tio n in itia lly o c c u rs p ro v id e s a re a d ily re c o g n iz a b le d atu m fo r th e u p p e r lim it o f b u ria l d ia g e n e tic p ro c esses a ffe c tin g fo ra m in ife rs. T h is is a p o te n tia lly im p o rta n t d atu m in h y d ro c a rb o n b a sin s sin c e it m a rk s th e p o in t at w h ic h p o ro s ity a n d c e m e n ta tio n in re s e rv o irs are a lso lik e ly to be a ffected . B j0 rly k k e an d E g e b e rg (1 9 9 3 ) h a v e n o te d th a t in n o rm a lly s u b s id in g b a sin s, m o s t q u a rtz c e m e n t fo rm s at te m p e ra tu re s a b o v e 9 0 - 1 0 0 °C .
T h e rm a l m a tu ra tio n o f a g g lu tin a te d fo ra m in ife rs o c c u rs as th e o rg an ic c e m e n t (g ly c o s a m in o g ly c a n ) th a t c o a ts all g ra in s w ith in an d a ro u n d th e a g g lu tin a te d te st w a ll is h e a te d an d v o la tile c o m p o n e n ts are d riv e n o ff. G ly c o s a m in o g ly - can s, p re v io u sly re fe rre d to as m u c o p o ly s a c c h a rid e s , are u n b ra n c h e d p o ly s a c c h a rid e c h a in s o f p ro te o g ly c a n s c o m p o s e d
276
D. H. M cN E ILC
Fig. 4. Progressive silicification in the wall o f agglutinated foraminifers. a. Recent, unaltered specimen o f Recurvoides. GSC 112234; b. well developed quartz overgrowths in asilicified speci
men o f Reticulophragmium. GSC 112235, burial depth 4115 m; c.
solid overgrowth cem entation in silicified specimen o f Haplo- phragm oidesl. GSC 112236, estimated burial depth 6 -8 km
o f re p e a tin g d is a c c h a rid e s e q u e n c e s o f am in o su g ars (L an g er, 1992). L ittle is k n o w n a t p re s e n t a b o u t th e d e ta ile d c h e m ic a l re a c tio n s in v o lv e d in th e m a tu ra tio n o f o rg a n ic c e m en t, b u t h e a tin g e x p e rim e n ts u sin g a p y ro ly s is a p p a ra tu s h av e s im u la te d g e o lo g ic a lly o b se rv e d c o lo u r c h a n g e s in both R e c e n t a n d fo ssil fo ra m in ife rs (M c N e il e t a l., 1996).
T h e p re s e rv a tio n o f o rg a n ic c e m e n t in fo ssil fo ra m in i
fers is a fu n d a m e n ta l p re re q u is ite fo r th e ir u se as th erm al m a tu ra tio n in d ices. C o lo u r c h a n g e s th ro u g h p y ro ly s is e x p e rim e n ts p ro v id e v isu a l e v id e n c e o f th e rm a l m a tu ra tio n . S E M e x a m in a tio n o f th e m ic ro s tru c tu re o f a g g lu tin a te d fo ra m in ife rs p ro v id e s d ire c t e v id e n c e o f th e o c c u rre n c e and d is trib u tio n o f fossil o rg a n ic c e m e n t. In o rd e r to se p a ra te o r
g a n ic c e m e n t fro m o rig in al d e trita l g rain s, u s u a lly q u a rtz , in th e te s t w all, fo ssil sp e c im e n s w e re e tc h e d b rie fly in h y d ro flu o ric acid. F ig u re 5 illu s tra te s fo ssil o rg a n ic c e m e n t s e p a rated fro m an a g g lu tin a te d g ra in o f q u artz.
Fig. 5. Q uartz grains (QZ) w ith remnants o f organic ccment (OC) in the wall o f Bathysiphon, GSC 109545. after etching in dilute hydrofluoric acid
T h e a p p lic a tio n o f F C I in a ss e ssin g th e rm a l m a tu rity in se d im e n ta ry b a sin s has b e e n d e a lt w ith th o ro u g h ly b y M c N e il e t al. (1 9 9 6 ). D a ta fro m o n e w ell in th e B e a u fo rt S e a (A m a u lig a k J-4 4 ) are p re s e n te d h ere to illu s tra te th e a p p lic a tio n o f F C I (Fig. 6; T a b le 2). Q u a n tific a tio n a n d a c c u rate d e te rm in a tio n o f fo ssil c o lo u r is in itia lly a c h ie v e d by v isu al c o m p a riso n o f in d iv id u a l sp e c im e n s a g a in s t the sta n d a rd M u n se ll C o lo u r C h a rt. T h e s ta tistic a l m e a n a n d sta n d a rd d e v ia tio n o f m e a s u re m e n ts fo r e a c h s a m p le th en p ro v id e s a re lia b le a ss e s s m e n t o f th e th e rm a l m a tu rity fo r th e e n tire sa m p le . A n o m a lie s in th e d is trib u tio n o f F C I v a l
u es w ith in an y o n e sa m p le a re u su a lly e x p la in e d e ith e r by re w o rk e d m ic ro fo s sils o r b y c o n ta m in a tio n th ro u g h ca v e d w ell c u ttin g s, as the d a ta fro m sa m p le 3 7 9 7 m in A m a u lig a k J-4 4 illu stra te s (F ig. 6; T a b le 2). F C I th e re fo re p ro v id e s a q u a n tita tiv e m e a s u re o f w ell s a m p le q u a lity .
In a d d itio n to c o n sp ic u o u s c o lo u r c h a n g e s, b u ria l d i
ag e n e sis is re s p o n sib le fo r te x tu ra l an d m in e ra lo g ic a l c h a n g e s in fo ra m in ife rs (M c N e il e t a l., 1996). A s a lre a d y n o ted , silic ific a tio n o f a g g lu tin a te d fo ra m in ife rs (F ig . 4 ) is
DIAGENETIC REGIMES AND THE FORAM INIFERAL RECORD
277
Table 2
F C I d a ta fro m w ell c u ttin g s sa m p le s o f th e A m a u lig a k J-4 4 w ell in th e B e a u fo rt S ea (fro m M c N e il e t a l., 1996)
Depth (m below
KB)
Average TV D (m below SL)
FCI data
Average
1 2 3 4 5 6 7 8 9 10 FCI
2 20 0-22 1 0 2150 1 2
2 39 5*2405 2 34 5 8 130 34 1 2 .1 6 ± 0.49
2 59 7-26 0 3 2 54 7 23 42 62 2 2 .3 3 ± 0 .7 8
2 78 0 -2 0 0 4 2 73 0 34 56 90 6 2 .3 7 ± 0.82
2 99 9 -3 0 0 5 2 94 9 8 3 22 23 3 .0 7 ± 1.02
319 7-32 0 3 3 1 4 7 5 2 17 7 1 2 3 .09 ± 1.24
3 39 8*3404 3 34 6 3 25 133 101 42 11 3.5 9 ± 0.97
3 59 9-36 0 5 3549 9 6 4 16 31 14 4 .2 0 ± 1.56
3 79 7-38 0 3 3 74 7 14 19 22 23 14 8 3 .28 ± 1 .3 4
3 99 8*4002 3 94 8 5 9 5 18 52 44 15 5 .9 9 ± 1.44
o n e o f th e m o s t o b v io u s o f th e s e d ia g e n e tic c h an g es, b u t o th e r d ia g n o s tic m in e ra ls c o m m o n ly fo u n d in fossil fo ra m in ife rs in c lu d e k a o lin ite , sm e c tite , illite, ch lo rite, an d r e c ry s ta lliz e d calc ite. A z o n a tio n o f m in e ra lo g ic a l a n d te x tu ra l c h a n g e s o b se rv e d in fo ssil fo ra m in ife rs has b e e n e s ta b lish ed b y M c N e il e t al. (1 9 9 6 ). F ig u re 7 su m m a riz e s b rie fly th e m a in fe a tu re s o f th is z o n a tio n . M in e ra lo g ic a l a n d te x tu ra l fe a tu re s o f th e d ia g e n e tic s e q u e n c e are e a sily o b serv ed th ro u g h S E M te c h n iq u e s.
E
Oo CO
CDw
2
-§ 3 -
<D -Q
Q_
CD
Q 4 -
5 -
1 2 3 4 5 6 FCI
I I I I I I
7 8 9 10
I I I I
Amauligak J-44
Fig. 6. Plot o f FCI data versus depth in the A m auligak J-44 well, see Table 2 for raw data, (from McNeil et a l 1996) Note:
sample at 3797 m with anomalously low FCI average value o f 3.28 represents caved well cuttings
B U R IA L D E P T H A N D T E M P E R A T U R E
IN C R E A S IN G
Z O N E A
Z O N E B
Z O N E C
Z O N E D
Burial deplh: 0 -2 4 0 0 m T e m p e ra tu re : 0 -7 5 °C W a ll texture u n a lte re d , gra ins loos ely a g g re g a te d , so m e com paction at d epth. N o sec o n d a ry m ineralization.
F C I: 0 - 2 . 5
Burial depth: 2 4 0 0 - 3 5 0 0 m T e m p e ra tu re : 7 5 U-1 1 0 ‘>C W a ll te x lure g ra n u lar, porosity re d u ce d by qu a rtz m ineralization. O rg an ic c e m e n t p re se n t, slight carbonization an d d arker.
F C I: 2 . 5 - 3 . 5
Burial deplh: 3 5 0 0 - 6 0 0 0 m T e m p e r a lu r e ^ iO M S O ^ W a ll te x tu re crystalline, porosity re d u ce d by
m ineralization of q ua rtz, kaolin an d s m ec tite precipitated.
O rg an ic c e m e n t c a rb o n ize d to d a rk brow n colour.
FC I: - 3 . 5 ■ 7 .5
Burial d e p lh : - 6 - 8 km T e m p e ra tu re : 1 50 °-2 5 0 °C W a ll texture crystalline, porosity negligible, silicification c o m plete, calcite re cry stalized . illile a n d chlorite precipitated. O rg a n ic c e m e n t c arb o n ize d (black).
FC I: - 7 . 5 - 1 0
Fig. 7. Zonation o f burial diagenetic trends in foraminifers related to approximate temperatures, burial depths, and Foram iniferal C olouration Index (FCI) (m odified from McNeil et al., 1996)
C la y m in e ra ls are a d is tin c tiv e s e c o n d a ry m in e ra liz a tio n fe a tu re in bo th a g g lu tin a te d a n d c a lc a re o u s fo ram in ifers (F ig s. 8, 9). T h e ir d is trib u tio n fo llo w s p re d ic ta b le tren d s in cla y m in e ra lo g y th ro u g h in c re a sin g te m p e ra tu re reg im es. A t lo w le v e ls o f th e rm a l m a tu ra tio n , k a o lin ite a n d sm e c tite are the sta b le c la y m in e ra ls an d th e ir m o re o r less sim u lta n e o u s p re c ip ita tio n can be o b se rv e d w ith in th e w all o f silicified , a g g lu tin a te d fo ra m in ife rs (F ig . 8). A t h ig h e r lev els o f th e r
m a l m a tu ra tio n , illite an d c h lo rite are th e sta b le cla y m in e r
als (F ig. 9). T h e o c c u rre n c e o f sm e c tite a n d illite in h y d ro c a rb o n b a sin s h as b een th e fo c u s o f m u ch re s e a rc h b ased on th e in c re a se in th e illite /s m e c tite ra tio a t b u rial d ep th s g re a te r th a n a p p ro x im a te ly 2 km a n d th e re la tio n sh ip o f th e s m e c tite /illite re a c tio n to th e g e n e ra tio n an d m ig ratio n o f p e tro le u m (B u rs t, 1969). G e n e ra lly , th is tra n sitio n is related
to te m p e ra tu re , b u ria l rate, g e o th e rm a l g ra d ie n t a n d p o re fluid co m p o sitio n . A b e rc ro m b ie e t al. (1 9 9 4 ) h a v e sh o w n th a t th e re a c tio n m a y b e lin k e d to a q u e o u s silic a a c tiv ity and o c c u r b etw e e n te m p e ra tu re s o f 5 0 °C to 150°C d e p e n d in g on re la tiv e b u rial ra te s, i.e. tim e -te m p e ra tu re re la tio n sh ip s . M c N e il e t al. (1 9 9 6 ) re c o g n iz e d illite as a d ia g n o s tic m in eral in fo ra m in ife ra l b u rial d ia g e n e tic z o n e s C a n d D (Fig.
7).
T h e d e v e lo p m e n t o f sig n ific a n t a m o u n ts o f c h lo rite (F ig . 9 ) w as o b se rv e d in re c ry s ta lliz e d c a lc a re o u s b e n th ic fo ra m in ife rs. C h lo ritiz a tio n o f c a lc a re o u s fo ra m in ife rs is n o t a lo c a liz e d p h e n o m e n o n as it h a s b e e n o b se rv e d in b o th th e B e a u fo rt-M a c k e n z ie B asin an d th e W e ste rn C a n a d ia n S e d i
m e n ta ry B asin . C h lo ritiz a tio n is p e rv a s iv e an d can a lte r th e e n tire c a lc a re o u s fo ra m in ife ra l a ss e m b la g e . In th e fo ra m in i-
278
D. M. McNEILFig. 8. Diagenetic quartz and clay minerals in the tests o f agglutinated foraminifers. a. Diagenetic quartz crystals (QZ). kaolinite (KO), and sm ectite (SM) in the interior o f the agglutinated wall o f Labrospira, GSC 1 12290, from Amauligak .1-44. 3599 m; b. Diagenetic quartz (QZ) and kaolinite (KO) filling chamber lumen in Reticulophragmium, GSC 109544. from A mauligak J-44, 3197 m
Fig. 9. Clay m ineralization in agglutinated and calcareous benthic foraminifers. a, b. Illite (IL) filling the interior o f Bathvsiphon, GSC 112293, estimated burial depth approximately 5 km; c, d. Chlorite (CL) and recrystalized calcite ( r e f in the wall o f Margimilma, GSC 112294, estimated burial depth o f 6 -8 km
D IAGENETIC REGIMES AND THE FORAM INIFERAL RECORD
279
feral b u ria l d ia g e n e tic z o n a tio n o f M c N e il e t al. (1 9 9 6 ), c h lo rite is d ia g n o s tic o f z o n e D s ig n ify in g b u rial te m p e ra tu re s in th e ra n g e o f 1 5 0 -2 5 0 ° C an d b u rial d e p th s o f 6 to 8 k m . C h lo rite is a d d itio n a lly s ig n ific a n t as a g e o th e rm o m e ter, sin c e it c ry s ta lliz e s in a series o f te m p e ra tu re c o n tro lle d p o ly m o rp h s (H a y e s, 1970).
A n o th e r te m p e ra tu re c o n tro lle d b u ria l d ia g e n e tic e ffe c t in fo ra m in ife rs w as d e s c rib e d b y R e ise r (1 9 8 8 ) w h o re c o g n iz e d th a t th e a ra g o n itic c a lc ite o f th e fa m ily R o b e rtin a c e a w a s sta b le a t low te m p e ra tu re a n d b u rial d ep th , b u t c o n v e rte d to c a lc ite by m e a n s o f th e rm a l a lte ra tio n . In L o w e r C re ta c e o u s sh a le s o f n o rth w e s t G e rm a n y , th is tra n sfo rm a tio n o c c u rre d a t d ep th s o f a b o u t 3 k m a n d p a le o te m p e ra tu re s o f 100°C an d th u s c o u ld se rv e as a u sefu l to o l fo r p a le o te m - p e ra tu re re c o n s tru c tio n .
THE OVERPRESSURED REGIME
T h e o v e rp re ssu re d re g im e o c c u rs in th e su b s u rfa c e o f se d im e n ta ry b asin s w h e re flu id p re s su re s c a n sig n ific a n tly e x c e e d n o rm a l h y d ro s ta tic p re s s u re (F ig. 2). O v e rp re ss u re d flu id s h a v e lo n g b een o f in te re s t to th e p e tro le u m in d u stry fo r a v a rie ty o f reaso n s in c lu d in g th e ir re la tio n sh ip to the g e n e ra tio n a n d m ig ra tio n o f h y d ro c a rb o n s, effe c ts on th e p o ro s ity a n d p e rm e a b ility o f re s e rv o irs, a n d sa fe ty facto rs d u r
ing d rillin g .
T h e o v e rp re ssu re d c o n d itio n d e v e lo p s as flu id s b ec o m e tra p p e d w ith in o r b e n e a th lo w p e rm e a b ility se d im e n ts u n d e rg o in g c o m p a c tio n a n d b u ria l. S e v e ra l facto rs are u su a lly c ite d in th e d e v e lo p m e n t o f o v e rp re ssu re d flu id s. T h e se in c lu d e ra p id d e p o sitio n o f fin e g ra in e d se d im e n t, te c to n ic stress, e x p a n sio n o f h e a te d flu id s, m in eral tra n sfo rm a tio n s, a n d h y d ro c a rb o n g e n e ra tio n (S h a rp e t a l., 1988; S n o w d o n ,
1995). O f th e s e p ro c e sse s, th e m o s t sig n ific a n t is th o u g h t to b e ra p id d e p o sitio n o f fin e -g ra in e d se d im e n ts le a d in g to c o m p a c tio n d is e q u ilib riu m a n d th e e n tra p m e n t o f in c re a s
in g ly p re s s u riz e d p o re flu id s.
In th e B e a u fo rt-M a c k e n z ie B a sin , th e m ain o v e rp re s su re d zo n e o c c u rs at d e p th s b e lo w 2 km , b u t o c c u rs p ro g re s
siv e ly d e e p e r (3 o r 4 k m ) in o ffsh o re a reas w h ere th e P lio c e n e —P le isto c e n e Ip erk S e q u e n c e a tta in s th ic k n e sse s o f 2 to 4 k m . H itc h o n e t al. (1 9 9 0 ) c o n sid e re d th a t th e m ain o v e r
p re s s u re d z o n e in th e B e a u fo rt-M a c k e n z ie B asin d ev elo p e d d u rin g th e m id -C e n o z o ic a n d w a s d e p re s se d later b y re la tiv e ly ra p id d e p o sitio n o f th e Ip e rk S e q u e n c e in the late C e- n o z o ic . O v e rp re s s u re d flu id s, tra p p e d sin ce the m id -C e n o - z o ic , are n o w b u rie d m u ch d e e p e r in an o ffsh o re d ire c tio n as a re s u lt o f su b s id e n c e u n d e r th e th ic k P lio c e n e -P le is to c e n e Ip e rk S e q u e n c e . T h e g e o th e rm a l g ra d ie n t, o n th e o th e r hand, has a d a p te d to th is d o w n w a rp in g as th e 100°C iso th erm (F ig. 2 ) is fa irly u n ifo rm a t a d ep th o f a p p ro x im a te ly 3.8 km (H itc h o n e t a l., 1990). T h e o v e rp re ssu re d zo n e th u s o ccu rs th ro u g h a ra n g e o f g e o th e rm a l en v iro n m e n ts.
H itc h o n e t al. (1 9 9 0 ) re p o rte d th a t th e salin ities o f fo r
m a tio n w a te rs a v e ra g e 10%o in g e o p re s su re d z o n es a n d 21%o in n o rm a l h y d ro s ta tic a lly p re s s u re d z o n es. T h e y c o n c lu d e d th a t th e lo w sa lin itie s o f g e o p re s su re d w a te rs is a shale m e m b ra n e e ffe c t filte rin g flu id s th a t m o v e d th ro u g h m u d d y se d im e n ts, b u t th is in te rp re ta tio n h as b e e n re je c te d g e n erally
as a v iab le m e c h a n ism fo r p ro d u c in g low sa lin ity su b s u rfa c e w a te rs in c o m p a c tio n a l re g im e s (H a n o r, 1994). It is m o re lik ely th a t lo w sa lin ity w a te rs o f m e te o ric o rig in w ere tra p p e d in p o ro u s u n its a n d th e n b e c a m e o v e rp re ssu re d . In co n trast, a s im ila r o c c u rre n c e o f lo w sa lin ity w a te r c o n c e n tra te d in th e u p p e r p a rt o f th e g e o p re s s u re zo n e in th e G u lf o f M e x ic o , h as b een a ttrib u te d to w a te r re le a s e d fro m sm e c - tite /illite c o n v e rs io n (M o rto n & L a n d , 1987).
A c o m p le te u n d e rs ta n d in g o f d ia g e n e tic p h e n o m e n o n w ith in o v e rp re ssu re d z o n e s is d iffic u lt a t best, an d is c e r
ta in ly b e y o n d th e sc o p e o f th is p a le o n to lo g ic a lly o rie n te d o v e rv ie w . In th e c o n te x t o f a g g lu tin a te d fo ra m in ife rs, h o w e v er, tw o a sp e c ts o f d ia g e n e sis in th e o v e rp re s s u re d re g im e a re o f p a rtic u la r in te re s t - silic ific a tio n an d th e rm a l c o lo u r a lteratio n .
S ilic ific a tio n o f a g g lu tin a te d fo ra m in ife rs is a w id e s p re a d d ia g e n e tic p h e n o m e n o n . In th e p a ra g e n e tic z o n a tio n o f M c N e il e t al. (1 9 9 6 ), silic ific a tio n o c c u rs p ro g re ssiv e ly th ro u g h z o n es B to D re p re s e n tin g b u ria l d e p th s o f 2 .4 to a b o u t 8 km a n d te m p e ra tu re s o f 75 to 2 0 0 °C o r m o re. A p re lim in a ry e x a m in a tio n o f a n u m b e r o f w e lls in th e B e a u f o r t- M a c k e n z ie B a sin in d ic a te s th a t silic ific a tio n b e g in s to o c c u r a b o v e o r n e a rly c o in c id e n t w ith th e m a in o v e rp re ssu re zone.
F ig u re 10 illu stra te s a ty p ic a l e x a m p le . T h is e m p iric a l e v i
d e n c e is im p o rta n t b e c a u se it im p lie s th a t th e silic ific a tio n o f fo ra m in ife rs m ay b e u se d a p re d ic tiv e to o l fo r re c o g n iz in g o v e rp re ssu re d z o n es in e x p lo ra tio n d rillin g . In o ffsh o re areas, w h ere th e ra p id ly d e p o site d Ip e rk S e q u e n c e h as d e p re sse d th e m ain o v e rp re s s u re d zo n e , th e o n s e t o f silic ific a tio n also o c c u rs d eep er. T h is is a k in e tic e ffe c t (tim e /te m p e ra tu re ) d o c u m e n te d b y FC I tre n d s (M c N e il e t a l., 1996) an d is c o n siste n t w ith A b e rc ro m b ie e t al. (1 9 9 4 ) w h o sh o w e d th a t o n se t o f q u a rtz p re c ip ita tio n m a y b e d e la y e d in
— - FC I
0 1 2 3 4 5 6 7 8 9
Do CECD
O
LU Q
X * P O R O S IT Y %
Fig. 10. D istribution o f FCI and shale porosity trends in the Reindeer D-27 well o f the M ackenzie D elta (from McNeil et a i, 1996). Silicification o f agglutinated foraminifers (top o f burial diagenetic zone B) begins at 1624 in; overpressured zone begins at approximately 2000 m as indicated by reversal in porosity trend
280
D. H. M cNEILb asin s u n d e rg o in g ra p id b u rial a n d hig h h e a tin g rates.
T h e in te rio r o f th e a g g lu tin a te d te s t a p p e a rs to be a r e m a rk a b ly re c e p tiv e e n v iro n m e n t fo r th e fo rm a tio n o f q u a rtz o v e rg ro w th s (Fig. 4 ). Q u a rtz o v e rg ro w th s a re co n fin e d to th e in te rio r o f th e te s t w a ll a n d do n o t ex tru d e fro m th e o u ter w all o f th e fo ra m in ife ra l test, a p p a re n tly in h ib ite d b y th e th ic k o u te r o rg a n ic la y e r. M c N e il e t al. (1 9 9 6 ) c o n c lu d e d th a t silic ific a tio n in fo ra m in ife rs re s u lte d fro m q u a rtz o v e r
g ro w th s p re c ip ita te d fro m q u a rtz -sa tu ra te d p o re flu id s c o n tro lle d by te m p e ra tu re a n d m in e ra l asse m b la g e s. B loch and F lu tch eo n (1 9 9 2 ) h a v e d e sc rib e d p ro c e s se s in v o lv in g q u a rtz m in e ra liz a tio n in sh a le m ic ro e n v iro n m e n ts. T h e m o s t lik ely m e c h a n ism fo r th e re d is trib u tio n o f q u a rtz is th e d is s o lu tio n /p re s s u re so lu tio n o f d e trita l g rain s, p a rtic u la rly a m o r
p h o u s b io g e n ic silic a . T h e re la tiv e im p o rta n c e o f p o te n tia l so u rc e s fo r q u a rtz c e m e n ta tio n in se d im e n ta ry ro c k s is c o n tro v e rsia l (M c B rid e , 1989); n o ta b ly th is in c lu d e s silic a r e le a se d th ro u g h th e sm e c tite /illite reactio n .
In th e N o rth S ea B a sin , flu id in clu sio n d a ta fro m q u a rtz c e m e n ts in d icate th a t m o s t o f th e q u a rtz c e m e n ta tio n tak es p la c e a t te m p e ra tu re s a b o v e 9 0 - 1 0 0 °C (B j0 rly k k e & E g e- b erg , 1993) a n d a t d e p th s b e lo w 2.5 to 3 .0 km . T h is c o m p a re s b ro a d ly w ith d e p th s g re a te r th a n 2 .4 km an d te m p e ra tu re e stim a te s o f 7 5 °C o r m o re fo r q u a rtz o v e rg ro w th s in a g g lu tin a te d fo ra m in ife rs in th e B e a u fo rt-M a c k e n z ie B asin (M c N e il e t al., 1996; z o n e s B - D ) . A t an d a b o v e th e se te m p e ra tu re s , q u a rtz p re c ip ita tio n resu lts w h e n flu id s m ig ra te u p w a rd an d cool (M c B rid e , 1989, an d B j0 rly k k e & E ge- b erg , 1993). M ig ra tio n o f flu id s in sh ales a n d m u d sto n e s h o w e v e r is g e n e ra lly th o u g h t to b e lim ited a n d in sig n ific a n t as a m e c h a n ism fo r tr a n s fe r o f silic a fo r d ista n c e s g re a te r th an a few m e tre s (B j0rly k k e & E g eb erg , 1993; B lo c h &
H u tc h e o n , 1992), so th a t silic ific a tio n in fo ra m in ife rs p ro b a b ly re p re se n ts a re m o b iliz a tio n o f silic a fro m local so u rces such as p re s su re s o lu tio n o r re a c tio n s b e tw e e n silic a te m in erals.
A n in trig u in g p o te n tia l so u rc e o f silic a fo r the silic ific a tion o f fo ra m in ife rs, an d o n e th a t h as a p o ss ib le d ire c t lin k to th e d e v e lo p m e n t o f o v e rp re ssu re , is th e m u c h re se a rc h e d a n d still c o n tro v e rsia l s m e c tite /illite re a c tio n . T h is re a c tio n is a w ell d o c u m e n te d p h e n o m e n o n in se d im e n ta ry b asin s an d o c c u rs w ith in c re a s in g b u rial te m p e ra tu re s a n d d ep th (P o w e rs, 1967; H o w e r e t a i , 1976; F o sc o lo s e t al., 1976; E l
lio t e t a l., 1991). T h e re a c tio n o f sm e c tite to illite c o n su m e s p o ta s s iu m an d a lu m in iu m an d p ro d u c e s silic a an d w ater, a m o n g s t o th e r ions. F o s c o lo s (1 9 9 0 ) n o ted th a t q u a rtz in c re a s e s in a b u n d a n c e w ith b u rial d ep th a n d c o n sid e re d th a t th e silic a g e n e ra te d b y cla y re a c tio n s w as p re c ip ita te d as q u a rtz o v e rg ro w th s. T h is is a p o te n tia l m e c h a n ism fo r th e silic ific a tio n o f fo ra m in ife rs sin c e it g e n e ra te s b o th th e silica an d the w a te r n e c e s s a ry to tra n s p o rt th a t silic a into th e fo ra m in ife ra l test. F u rth e rm o re , th e sm e c tite /illite re a c tio n m ay its e lf b e a p o ss ib le fa c to r in cau sin g o v e rp re ssu re d c o n d itio n s in shales. F re e d an d P e a c o r (1 9 8 9 ) p ro p o s e d th a t th e sm e c tite /illite re a c tio n p ro d u c e d c o a le sc e d illite p a c k e ts th a t d e c re a s e d lo c a l p e rm e a b ility le a d in g to a m o re e ffic ie n t g e o p re s s u re seal an d a c o rre sp o n d in g in c re a se in p o re flu id p re s su re . T h e fa c t th a t c la y m in e ra l re a c tio n s p ro d u c e a s ig n ific a n t a m o u n t o f e x tra w a te r c o n trib u te s as w ell to th e geo - p re s su re d c o n d itio n . T h e a p p e a lin g a sp e c t o f th e s m e c tite /il
lite re a c tio n is th a t it a p p e a rs to b e c o in c id e n t w ith silic ific a tio n tre n d s a n d th e d e v e lo p m e n t o f th e o v e rp re s s u re d re gim e. A b e rc ro m b ie (p e r so n a l co m m u n ica tio n ), h o w e v e r, b e lie v e s th a t sm e c tite is p re s e n t o n ly in sm a ll q u a n titie s in th e B e a u fo rt-M a c k e n z ie B a sin a n d th a t th e re fo re its ro le in th e g e n e ra tio n an d m a in te n a n c e o f o v e rp re s s u rin g is q u e s
tio n ab le.
T h e o th e r a sp e c t o f th e o v e rp re s s u re d c o n d itio n th a t m ay be o f re le v a n c e to fo ra m in ife ra l d ia g e n e sis is its p o te n tial e ffe c t o n th e c o lo u ra tio n o f th e rm a lly a lte re d fo ra m in i
fers. In th e su b s u rfa c e o f th e M a c k e n z ie D e lta , M c N e il e t al.
(1 9 9 6 ) o b se rv e d a c o rre la tio n b e tw e e n o v e rp re s s u re d zo n es an d a re ta rd a tio n in th e rm a l a lte ra tio n c o lo u r as m e a s u re d by F C I in a g g lu tin a te d fo ra m in ife rs in th e R e in d e e r D -2 7 w ell (Fig. 10). F u rth e rm o re , n e a r th e b a se o f th e w ell, F C I a c tu ally d e c re a s e d c o in c id e n t w ith a m a rk e d in c re a se in p o ro s ity c a u se d b y a p e a k in o v e rp re ssu re . T h e se o b s e rv a tio n s w ere p re lim in a ry , b u t w ill be fo llo w e d up b y fu rth e r a n a ly s is an d in v e stig a tio n s in o th e r w e lls. It is o f in te re st, h o w e v e r, th a t re c e n t in v e s tig a tio n s in th e T e rtia ry b a s in s o f th e S o u th C h in a S ea by F a n g e t al. (1 9 9 5 ) h a v e re a c h e d sim ila r c o n c lu s io n s re g a rd in g th e re ta rd a tio n o f o rg a n ic -m a tte r m a tu ra tio n in o v e rp re ssu re d e n v iro n m e n ts. F a n g e t al. (19 9 5 ) d o c u m e n te d th a t th e th e rm a l m a tu rity o f n o rm a lly p re s su re d se d im e n ts w as sig n ific a n tly h ig h e r th a n th e th e rm a l m a tu rity o f o v e rp re ssu re d se d im e n ts as m e a s u re d b y v itrin ite re f le c ta n c e (% R 0). T h is o rg an ic m a tu rity a n o m a ly c o u ld n o t be e x p la in e d by v a ria tio n s in a c tiv a tio n e n e rg ie s, c o n d u c tiv ity co n trasts, o r h y d ro lo g ic a l e ffe c ts. S in ce th e d iffe re n c e b e tw e e n th e p re d ic te d a n d m e a s u re d v itrin ite re fle c ta n c e level in c re a se d e x p o n e n tia lly w ith in c re a s in g p o re flu id p re ssu re , F an g et al. (1 9 9 5 ) c o n c lu d e d th a t in c re a se d p o re flu id p re s
su re in c re a se d th e a c tiv a tio n e n e rg ie s o f o rg a n ic m a tte r m a tu ra tio n reactio n s.
S im ila r c o n c lu s io n s w e re re a c h e d fro m e x p e rim e n ta l w o rk b y P ric e a n d W e n g e r (1 9 9 2 ) u sin g a q u e o u s p y ro ly sis te c h n iq u e s. T h e se e x p e rim e n ts a tte m p te d to a p p ro x im a te n atu ral sy s te m s by g e n e ra tin g th e rm a l re a c tio n s u n d e r p re s su re in c lo se d , w a te r-w e t sy s te m s . T h e re a c tio n p ro d u c ts (h y d ro c a rb o n s) w e re m e a s u re d b y gas c h ro m a to g ra p h y , an d P rice an d W e n g e r (1 9 9 2 ) c o n c lu d e d th a t in c re a sin g static flu id p re s su re stro n g ly re ta rd e d th e d e c o m p o s itio n o f k e ro g en , th u s c a u sin g a d e c re a se in th e a m o u n t a n d n a tu re o f h y d ro c a rb o n p ro d u c ts g e n e ra te d . F u rth e rm o re , th e y c o n c lu d e d th a t u n d e r n a tu ra l g e o lo g ic a l c o n d itio n s th e e ffe c ts o f o v e r
p re s su re w o u ld be ev en g re a te r b e c a u se o f th e sig n ific a n t p a rtia l p re s su re c o n trib u tio n s fro m h y d ro c a rb o n g a se s as o p p o sed to h e liu m w h ich w a s u se d in th e p y ro ly s is e x p e ri
m ents.
In an a tte m p t to a n a ly s e re s e rv o ir d ia g e n e sis a n d h y d ro carb o n m ig ratio n , S w a rb ric k (1 9 9 4 ) n o te d th a t th e in flu e n c e o f high p re s su re o n re a c tio n k in e tic s is n o t w e ll k n o w n , b u t th a t e x p e rim e n ta l stu d ies (E n g u e h a rd e t a l., 1990) sh o w e d th a t in c re a sin g p re s su re d e c re a s e s th e ra te o f th e rm a l c ra c k ing o f la rg e r to sm a lle r h y d ro c a rb o n m o le c u le s . H ig h p re s
su re also re ta rd s th e c a rb o n iz a tio n o f o rg a n ic m a tte r in a sealed a q u e o u s sy stem , as illu s tra te d b y e x p e rim e n ta l stu d y on c o n o d o n ts (E p ste in e t a l., 1977).
T h e o v e rp re ssu re d re g im e is o b v io u s ly a sig n ific a n t co m p o n e n t in an y b asin a n a ly s is stu d y . P ric e an d W e n g e r
