Diagenesis of English Lower Jurassic limestones as inferred from oxygen and carbon isotope analysis

Earth and Planetary Soence Letters, 45 (1979) 23-31 © Flsevler Soennfic Pubhshmg Company, Amsterdam - Printed m The Netherlands

23

[51

DIAGENESIS OF ENGLISH LOWER JURASSIC LIMESTONES AS INFERRED FROM OXYGEN AND CARBON ISOTOPE ANALYSIS H S CAMPOS * and A HALLAM

Department of Geologteal Sciences, University of Bzrmmgham, Btrmmgham {Great Britain)

Recewed [ ebruary 10, 1979 Revised version recewed June 20, 1979

Fltty-seven samples of mostly Llasslchmestones and calcite spar were subjected to mass spectrometer analysis to determine 813 C and 6180 The results are used to compare and Interpret the changing isotopic composmon through time of calciteprecipitating pore flmds and to assess the influence of rate and type of sedimentation, dlagenetlc reactions and meteoric water reflux In general mlcrosparlte beds and concretions formed sooner after deposmon m Dorset than in Yorkshire, where sedimentation rates were usually higher Calcite horizons m the Bndport Sands Forniatlon of Dorset are entirely diagenetic In origin Carbon Isotope data from concretions m the Jet Rock Formation of Yorkshire support a model of changing orgamc reactions with increasing depth of burial The influence of meteoric waters is substantially restricted to the formation of post-Llasslc veins

1 Introduction Within the past few years isotopic analysis has come to be an increasingly important tool m the study of carbonate dlagenesxs, supplementing the more tradmonal approach based on structural and textural relationships and trace element analysis Many interesting dmgenetxc problems present themselves m the Enghsh LlaSSlC (Lower Jurassic) but no isotopic mvestlgatmn has been undertaken prevmusly Rock samples were collected from the two classic sections of the Dorset and Yorkshxre coasts A small addltmnal number of samples was collected for comparison from deposits m Dorset and Devon ranging in age from Rhaetlc to Upper Cretaceous, and from post-Llasslc veins m Somerset, South Wales and northwest Scotland The only carbonate phase present m the samples ts calcite Application of the stainmg techmque of Dlckson [1] shows that the early

* Present address Instituto de Flslca da UFBA, Campus da Federaqao, Salvador, Brazil

dlagenetlc concretions are composed of non-ferroan and the later-stage sparry material of ferroan calcite

2 Techmque Samples were crushed and the powder reacted with 100% phosphoric acid at 25°C using the method described by McCrea [2] Isotopic measurements of the CO2 so obtained use made in a Mlcromass 602-D mass spectrometer m the Department of Geological Sciences, University of Birmingham The final values for 813C and 8180 use corrected in the manner described by Craig [3] and the data expressed with reference to the universal PDB standard The NBS-20 was used as work standard The reproduclblhty of measurements was tested by subjecting one sample to analysis on four different dates as indicated in Table 1 From the results obtained the standard devlatxon xs estxmated to be -+0 50%0 for carbon and -+0 07%0 for oxygen The high precision of the results presented m Table 2 is demonstrated by the closely similar data for sam-

24 TABLE 1 Estimate of data reprodumblhty Date

Sample

613C(%)

6180(%)

31 05 78 04 07 78 07 09 78 20 09 78

BS 5 BS 5 BS 5 BS 5

-042 - 0 37 - 0 29 041

-4 -4 -4 -4

71 81 66 61

pies from adjacent strata or different parts of concretlons, such as the Belemnlte Marls, Wtute Llas and Chalk in Dorset and the "pseudovertebrae" and "cannon-ball" concretions m Yorkshire

3 Results The complete data are presented m Table 2 and portrayed graphically m Figs 1 and 2 In Table 2 the results are grouped into several categories

The majority of rocks analysed are calcite mlcrosparltes occurring either as more or less umform beds or as concretions As regards Dorset all the 6180 results are negative, with the lowest values being recorded from the Birch1 nodule bed, a small number of the 6 ~3C results are slightly posmve No slgmficant &fferences are apparent between beds and concrenonary nodules, between the exterior and interior of mdlwdual beds or nodules nor between the Blue Llas, Belemmte Marls and Lower Chalk The Yorkshire results show a much wider scatter for both carbon and oxygen A t t e n n o n was concentrated on the laminated bituminous shale formation known as the Jet Rock, which contains a number of interesting horizons of concretions Thus bed 35 of Howarth [5] contains early-stage concrenons of irregular shape known as "pseudovertebrae" intruded locally by pale brown mlcnte and enveloped by younger large elllpsoldal concretions known as "whalestones" [6] The early stage mlcrospante and mlcrlte are similar m having strongly negative 313C,

TABLE 2 Isotopic analyses of carbonates, maanly from the Llas of Dorset (D) and Yorkshire (Y) * Sample

Honzon (zone and stage)

Locahty

13C (%o)

18 O (%0)

Calctte mtcrospartte beds or nodules

BLD 10 BLD 6 BLD 7 BLD 8 BLD 9 BLD 4 BLD 3 BLD 1 BLD 2 BND 1 BMD 1 BMD 2 BMD 3 BMD4 JBD 1 WLD 1 WLD 2 WLD 3 ) IOD 1 LCD 1 LCD 2 LCD 3

Planorbls (Hettanglan)(bed H1) Angulata (Hettanglan)(base of bed 13) "] Centre of bed 13 t Edge of nodule, bed 15 Centre of nodule,bed 15 Bucklandl (Slnemunan)(bed 29) "] Bed 35 Bed 49 Senucostatum zone (bed 50) Turnen zone (Slnemunan)(Blrchl nodular bed)

Pmhay Bay, Devon

Jamesom zone (Pllensbachlan)

Black Ven, Dorset

Blfrons zone (Toarclan) White Llas (Rhaetlc)

Watton Chff, Dorset Plnhay Bay, Devon

White Llas (Rhaetlc)

Plnhay Bay, Devon

Laevluscula (BaJoclan)

Burton Cliff, Dorset

Cenomaman

Ballard Point, Dorset

Seven Rock Point, Dorset

Lyme Regas, Dorset Charmouth, Dorset

] l

"] )

+2 40 +0 26 +0 49 +0 45 +0 19 +0 05 - 0 44 4 25 - 0 81 - 2 98 -1 34

-2 -2 -1 -2 -2 -2 -3 -3 -5 -6 -3

95 07 77 49 23 9l 29 04 38 43 75

-1 79 0 94 -1 15 +2 86 +2 93 +2 59 +3 82

-3 -4 -3 -3 -2 -4 -3

98 60 84 75 64 19 61

+2 18 +2 65 +2 59 +2 45

-2 -3 -4 -3

23 77 17 94

25 TABLE 2 (continued) Sample

GSY 1 JRY JRY JRY JRY JRY JRY

5 6 10 1 23

JRY JRY JRY JRY JRY JRY JRY

4 7 8 9 11 12 13

] ~

Horizon (zone and stage)

Locahty

13 C (%c)

Tenmcostatum (Toarclan)

Port Mulgrave, Yorkshire

- 1 3 79

- 5 90

12 -12 -13 -10 - 66

5 40 - 5 28 4 69 - 3 01 66 33 65

Falclferum zone (pseudovertebrae concretions of bed 35)

"1 t

7 ~

Brown mlcnte in pseudovertebrae concretions

"Beef" seams SB l a SB 2 "r

'1 ~

Sparry calctte BLD 5 (Ammomte) BVMD 1 "] BVMD 2 (Sep- t tartan vem)

Port Mulgrave, Yorkshire

Whalestone concretions (bed 35)

Port Mulgrave, Yorkshire

Mdlstones concrenons (bed 40) Centre of cannon-ball concretion (bed 33) Edge of cannon-ball concretion

Port Mulgrave, Yorkshire Port Mulgrave, Yorkshire Port Mulgrave, Yorkshire

Calctte cement o f quartz sand BSD 1 Levesquel (Toarclan) BSD 2 ) BSD 3 ) Levesquel (Toarclan) BSD 4

DD 1 DD 2 DD 3

Port Mulgrave, Yorkshire

Burton Chff, Dorset Burton Chff, Dorset

BBS (fault vein) G 1 S2

19 75 00 78 30 31

- 1 1 05 +7 27 +8 26 +7 04 +4 30 12 47 - 1 2 21 +0 -0 -0 0

34 41 16 71

-3 -5 -4 5 -6 -12 -12

47 46 75 35 47 61 48

-6 5 5 -3

31 96 94 38

Turner1 (Smemunan)

Charmouth, Dorset

+017 +1 17

- 7 31 - 6 76

Berrlaslan

Durlstone Bay, Dorset

+1 04 - 0 48 +0 25

- 6 28 - 5 45 - 5 45

Bucklandl (Smemurlan)

Lyme Regas, Dorset

- 4 49

- 4 59

- 1 4 17 - 1 6 65

- 2 34 - 1 81

- 1 5 72

- 2 35

Obtusum (Smemurlan)

Black Ven, Dorset

Obtusum (Smemurlan)

Black Ven, Dorset

Jason (Callovlan)

Chlckerell, Dorset

BVMD 3 BVMD 4~, BVMD 5 ~ LOC 1 JY 1 JY 2 )

18 0 (%~)

-4 -13 -1 -11 -11

23 38 31 03 57

-3 -3 -7 -14 -14

97 45 89 17 87

Jamesom (Phensbachlan)

Robin Hoods Bay, Yorkshire

Senucostatum (Slnemurlan)

Broadford Bay, Skye

+1 06

- 2 2 68

Bucklandl Angulata (Hettanglan)

St Donat's Glamorgan Watcher, Somerset

+1 14 +0 29

- 1 2 21 - 1 0 75

* Llthostratlgraphlc names mdlcated m sample numbers thus BL = Blue Llas, SB = Shales with Beef, BVM = Black Ven Marls, JB = JuncUon Bed, BS = Bndport Sands, IO = Inferior Oohte, LOC = Lower Oxford Clay, D = Durlston Beds, LC = Lower Chalk, ML = Middle Das, WL = White Llas, GS = Grey Shales, JR = Jet Rock, BB = Broadford Beds, Blue Das bed numbers of Lang [4] and Jet Rock bed numbers of Howarth [5]

m u c h m o r e so t h a n a n y t t u n g m Dorset, a n d 6 1 8 0 values m o r e similar t o D o r s e t b u t n e v e r m o r e t h a n - 3 In striking c o n t r a s t the later-stage w h a l e s t o n e s

a n d m o r p h o l o g i c a l l y sxmflar " m d l s t o n e s " o f b e d 4 0 have s t r o n g l y posltwe/513 C values r e a c h i n g a maxam u m o f 8 2 6 , b y far t h e h i g h e s t values f o u n d m t h e

26

6 0

6 0 10

-9

8

7

6

5

-3

?

~r ~F ~2p lb cP lh

o

1

3 -

22 .3

o

o

2~

2

.

210

118

16

-

12

b

10

6

~.

/

y

q

+2 °d~°

0

*6

1

o

2

3 I

~'

t. 6

*L

m+Z.

3

c

4

*2 O

5

-5 (D

O

0

~0

(D

t

9 -4

10 12

12

6

-1 3 -14

14

-15 16

15 a

16 D

17 I

18 10

-9 Colclte Bmdport

I . ,

g

7

6

mlcrospGrlto Sand

t a l c 'e

I

I

5

L

I

I

-3

-2

x -8

-lC

17 L

'8

-1

0

o

Beef ISinemurlap)

x

Bee~ t Berrlas~ap I •

•

o

D

l~]g 1 Oxygen and stable carbon isotope results for Dorset

14

; --I

-22

Calcite

Other

I

I

L

-20

~

I

-18

[]

I

I

-16

o o I

-IL

[

l

I

~,2

L

I

10

mFcrosparRe

0# Whalestones

Siderite

Septoman

mlcroeparlte

mlcrospa

calorie

I

1

8

of p s e ~ d o v e r t e b r a e

colclte

,2

c

mlcrospoFlte

P o s t - L~asslc

whole study The spheroidal "cannon-ball" concretlons of bed 33 differ from both groups in having strongly negative/513 C and (5 ] SO Secondly, four samples were analysed from the Toarclan part of the Brldport Sands m Dorset, which consist of hard bands of calcareous sandstone alternatmg regularly with friable non-calcareous finegrained sandstone All four samples of carbonate from the calcareous sandstone are very similar, with (513C close to zero and moderately negative (5180 like the Blrchl nodules The third category consist of later-stage sparry calcite occurring as seams, veins or cawty fills It has long been known that the thin, beddlng-paralM seams of fibrous calcite (often with cone-m-cone structure) known as "beef" post-date compaction of the envelopmg shales, whereas the concretions predate compaction [7] The Isotopic composition of the Smemunan "beef" in Dorset is very similar to that of the basal Cretaceous Durlston Beds, with (5~3C close to zero and moderately negative (5180 This LlassIc "beef" has more negative g ~80 than any microsparlte The coarse calcite spar of septarlan cracks,

c °

-1 2

Colcite

S e p l a r l a ~ a n d a m m a n te chamber calcite

L

and

~1

6

I

I

t

_~q

-4

o Mdlstones

El

•

x

ire

o

v e i n s ~ non Y o r k s )

0

rig 2 Oxygen and stable carbon isotope results for Yorkslure, with a few veto samples from other areas

veins and ammonite chambers exhibits quite a wide variation It is important to dlstmgmsh the earlier fills of septanan cracks of concretions and ammomte chambers, often with strongly negative (513C and (5180 quite close to zero, with the later veins of Glamorgan, Somerset and Skye, which are associated with post LlaSSlC faults These latter have strongly negative 6180 Septarlan calcite in the Jamesonl zone of Yorkshire shows strongly negative values of both (5~3C and (5180 The septarlan calcite of a Lower Oxford Clay concretion m Dorset has a composition very close to similar material from the same horizon In the Midlands [8]

4 Interpretation As Hudson has made clear in his review [9], significant departures of (513C and (5180 from zero result

27 from quite different processes Negative oxygen values relate either to increased temperature, the introduction of meteoric water or dlagenetlc reactions, while carbon fluctuations relate to the presence of orgamc matter and the different isotopic types of CO2 produced by various organic reactions The isotopic data presented here throw light on several Important geological questions

Type of carbon dtoxMe productton Curtis [15] and Irwin et al [16] have put forward the following model of depth zonatlon in sedimentary burial sequences of organic-rich clays with reference to the generation of CO2 Maximum value of 813C (%~) Zone 1

Prtmary or secondary orlgm of carbonate-rwh beds With regard to the Blue Lias of Dorset, about which there has been long-standing controversy about the sedimentary or dlagenetlc origin of the hmestoneshale rhythm, earlier work by one of us suggested that there were both primary and secondary components, but with dmgenesls invariably playing an important role both in enhancing llthologacal contrasts between limestone and shale and even in creating some limestone beds and concretion horizons from an original marl [10] It is noteworthy therefore that the hmestone beds and obviously secondary concretions are no more distinguishable lSOtOplcally than they are an gross composition and texture The close approximation to zero 6180 values indicates, however, that dlagenetlc segregation of CaCO3 must have taken place shortly after deposition of sediment before there had been much time to modlfy the ISOtOpic composition of the seawater The Belemnlte Marls higher in the Lower Lias show many resemblances to the Blue Llas but there is less chemical contrast between the more and less calcareous beds and Sellwood [11] argued, mainly on the basis of trace fossil motthng, that the beds were primary in origin The calcite cannot, however, be distinguished lSOtOplcally from that of the Blue Llas Kennedy [12] compared the banding of the Lower Chalk in southern England with that of the Blue Laas, but llthologlcally there is a greater resemblance to the Belemnlte Marls Once again, no significant isotopic differences are discernible Much more decisive references can be made about some other deposits An essentially prtmary origin for the calcareous beds of the Bndport Sands was proposed by Davies [13], an interpretation subsequently challenged [14] All the I80 values obtained from samples of this formation depart appreciably from mean seawater values and point strongly to a dlagenetic origin for the CaCO3

BacterialoxldaUon within the top few centimetres, m free contact with seawater

Zone 2

Bacterialsulphate reduction

Zone 3

Bacterialfermentanon, with methane producnon which commences as soon as SO4 reduction stops and continues until the organic substrate is exhausted or a high bunal temperature IS achieved

Zone 4

Ablotlc reactions

-25 25

+15 10 to -25

It follows that, for continuous precipitation, there should be a dramatic change in 813 C from strongly negative to strongly positive values from zone 2 to 3 This is supported convincingly by data from the Jet Rock concretion horizons 35 and 40 That the whalestone concretions of bed 35 are younger than the pseudovertebrae mlcrosparlte and associated mlcrlte IS clearly indicated by the structural and textural relations Like the spheroidal cannon-ball concretions of bed 33, the pseudovertebrae mlcrosparlte and mlcrlte are rich in finely disseminated particles and aggregates of pyrite Petrographic study indicates, however, that precipitation of this mineral had ceased by the time of formation of the elllpsoldal whalestones and millstones (bed 40), suggesting that the supply of reducible sulphate was by then exhausted, as indeed it should have been according to the model Further inferences can be made by relating nodule shape to CaCO3 content It has been plausibly argued that the CaCO3 content of concretions gives an approximate measure of the sediment porosity at the time they were formed [17,18] The pseudovertebrae nodules have a content of approximately 80% [6], Implying a 10% reduction of porosity between their times of formation If the argument cited above is acceptable, this porosity reduction would presumably have been achieved by progressive compaction, which increasingly inhibited vertical migration of pore waters and hence led to the production of elhp-

28 spoldal nodules with long parallel to the bedding, rather than spheroidal nodules hke the cannon-ball concretions (Fig 3) Nodule formataon ceased when continued porosity reduction eventually passed a critical limit beyond which mterpore migration was effectwely inhibited Data relating depth of burial to poros:ty suggest that all the nodules must have formed within the top few metres of sediment [19] The isotopic data throw no further hght on the factors controlhng the complex shape and textural relations of the pseudovertebrae mlcrosparlte and mlcr:te [6] than to lndacate that they were both formed within zone 2 Raxswell's [20] detailed analysis of the cannon-ball concretions led to the proposal of two distractive stages of growth, an early stage related to the actwlty of an umdentlfled bacterml source of CaCO3 and a later stage assocmted with bacterial sulphate reduction, leading to the formatlon of pyritic rims The lSOtOplCdata Indicate, however, no difference between the nodule lntenor and exterior Influence o f meteoric water and dtagenettc reacttons Shackleton and Kennett [21 ] put forward the followmg shghtly revised palaeotemperature equation and argue for a pre-glaclal ocean water compos~taon of around-1 2 T°C = 16 9 - 4 38 (6 c - 6w) + 0 10 (6 c - 6 w ) 2 where (6 c - 6w) is the measured difference m 61So

between calcite and water Using this equation and the water correction 6w = 1 2, 61 s 0 values of - 3 , - 4 , - 6 and - 1 2 respectively give temperatures of about 2 5 , 2 9 , 38 and 65°C On grounds of faunal tolerance, only the first (and conceivably the second) as reahstlc for Llasslc bottom waters But nearly all the Blue Llas and Jet Rock mlcrosparltes m question have CaCO3 contents between 80 and 90% [6,22], implying formatlon within the top few metres of sediment, where the temperature could not be expected to have been significantly higher than that of the overlying bottom water It follows that the hghtenlng of oxygen must result either from an reflux of meteonc water of from dlagenetlc reactions revolving connate seawater The Yorksh:re samples show more negatwe values than Dorset, and geological data suggest that an reflux of meteoric water was perhaps more hkely for Yorkshire, where the Llas is on the whole more sandy than in Dorset and the Middle Jurassic much more so A source land for sediment probably existed at no great distance to the north and north west of the present outcrops, in the region of the Southern Uplands and perhaps also the north Penmnes [23] Wall's [24] palynologlcal investigation of the Jet Rock points to a slmrlar conclus:on In contrast to Dorset, there was probably emergence above sea level at about the end of Llasslc t:me in Yorkshire [25] There Is a maximum of some 110 m of Toarclan overburden above the Jet Rock Formation, but Jet Rock concretxons formed at this tame should have poros-

EARLY

STAGE

Porosity I'- 90% Zone of sulphate reduction (negGhve 8~3C}

LATE STAGE Porosrty -,'-- 80 % Zone of fermentahon (poslhve 613C)

Fig 3 The formation of early- and late-stage concretions m the Jet Rock Formation of Yorkshire Thin hnes signifybedding, length of arrows proportional to the amount of flow of calcite-precipitating mterpore fluxds

29 ltles ranging between about 20 and 30% [19], which is manifestly not the case Therefore any introduction of meteoric water must have taken place no later than after the deposition of a few metres of clay sediment Investigation of the Florida Shelf [26] indicates that freshwater discharge from land Into marine se&ments can extend for a considerable distance offshore The rocks through which the water travels are, however, comparatively permeable carbonates, and not closely comparable with the argillaceous rocks of the Yorkshire Lower Toarclan The strongly negative 180 values of the cannon ball concretions are particularly difficult to explain by a hypothesis of meteoric water reflux because they occur within bituminous shale, which must have lost most of its permeability soon after deposition It seems therefore more plausible to argue both for Yorkshire and Dorset that the hghtenlng of oxygen has been achieved by diagenetic reactions taking place in the connate pore water, because the major isotopic effect is a depletion of 18O associated with the formation of diagenetlc minerals enriched m that isotope A systematic reduction of 6180 with depth is exhibited by DSDP samples, where the influence of meteoric waters can be completely discounted [16] The more negative values for Yorkshire have still to be accounted for, and will be discussed further m the next section The data for sparry ferroan calcite can be explained along the lines proposed by Hudson [9] For the earher septarlan veins and ammonite chamber fills organic influence In CO2 production was conslderable, whereas the later "beef" seams were formed at a ttrne when organic reactions had apparently ceased Whereas the 613Cvalues for the "beef" are closer to zero, the 6180 tend to be more strongly negative, which is consistent with their having formed later Meteoric influence must be revoked m the case of the younger vein calcite associated with faulting, which is charactensed by extremely negative 6180 That In Somerset IS dated as post-LIassIc [27] and presumably the same is true for comparable structures nearby in Glamorgan, on the opposite side of the Bristol Channel Faulting m Broadford Bay, Skye, was apparently provoked by uplift related to early Tertiary Intrusive activity [28] and this points the way to an explanation of the exceptionally negative

6180 value for a vein associated with one of these faults, approximately 11%o lower than the average for the Somerset and Glamorgan veins Taylor and Forester [29] have referred, from oxygen isotope analyses of igneous rocks, a large hydrothermal convectlon system involving heated low -180 meteoric groundwaters estabhshed at the time of intrusion Tan and Hudson [30] have applied this interpretation of heated groundwaters to calcites in the Middle Jurassic of Skye, which exhibit 6 as o values 4-7%0 lower than the values to be expected from normal dlagenesls

Influence of character and rate of sedtmentatton The strongly bituminous character of the Jet Rock shales in Yorkshire, with several percent of orgamc carbon, may suggest that the strongly negative 613C of the concretions is heavily influenced by this However, the situation is more complicated because similar low values were obtained from horizons In non-bituminous sediments in the Middle Llas and Grey Shales formation Furthermore the bituminous horizon of the Blrchl bed in Dorset has a value not notably different from the many non-bituminous horizons sampled In that county A much stronger influence appears to have been exerted by the rate of sedimentation There are now many Indications to suggest that a low sedtmentation rate in carbonate-bearing sediments IS usually associated with early lithificatlon [31,32] The earlier the carbonate was precipitated the closer the isotopic values should approximate to seawater values [9] Early hthlfication of the White Llas of Devon may confidently be inferred from a variety of sedimentary criteria [33], although evidence of exceptionally low sedimentation rate is in this case lacking Such a low rate must, however, have characterised both the Junction Bed and the Inferior Oolite, m each of which several ammonite zones are contained within at most a few metres of rock All these deposits have 61 s O close to zero and slightly positive 613C The latter seems to tmply one or a combination of the followmg hthlfication before the characteristic zone 2 organic reactions could commence, negligible amount of organic matter was originally deposited, organic matter was mostly destroyed by oxidation on the sea bed before burial Comparing the two most thoroughly sampled for-

30 m a t r o n s , t h e average s e d l m e n t a t m n rate d u r i n g d e p o s i t i o n o f t h e Blue Laas (0 73 c m / 1 0 3 y e a r s ) w a s subs t a n t l a l l y less t h a n t h a t for t h e J e t R o c k (3 2 c m / 1 0 a years), as c a l c u l a t e d o n t h e a s s u m p t i o n t h a t o n e amm o n i t e z o n e a p p r o x a r n a t e l y r e p r e s e n t s 1 × 106 years [15] I n d e e d , m o s t o f t h e Llasslc z o n e s in Y o r k s h i r e are a p p r e c i a b l y t h i c k e r t h a n m D o r s e t Whale average sed~mentatlon rates can take no account of shortt e r m v a r i a t i o n s , ~t seems r e a s o n a b l e t o refer a general t e n d e n c y t o w a r d s h i g h e r rates m Y o r k s h i r e T h u s t h e best available e x p l a n a t i o n for t h e t e n d e n c y t o w a r d s m o r e s t r o n g l y negative 6 1 a O m l c r o s p a r l t e results for Y o r k s h i r e is t h a t t h e n o d u l e s a n d b e d s generally f o r m e d later a f t e r d e p o s m o n t h a n t h o s e m D o r s e t T h e g r e a t e r lapse o f t i m e allowed d l a g e n e t l c r e a c t i o n s t o p r o c e e d f u r t h e r a n d h e n c e deplete c o n n a t e p o r e w a t e r in a 8 0

Acknowledgements O n e o f us (H S ) t h a n k s t h e Brazilian G o v e r n m e n t for t h e a w a r d o f a s c h o l a r s t u p a n d Mr R WIlhams for t e c h m c a l advice We are b o t h g r a t e f u l t o Drs C D Curtis a n d J D H u d s o n for c o n s t r u c t i v e c o m m e n t s and crmclsms

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