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Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
Journal ofAfricanEarthScience$, Vol. 13, No. 3/4, pp. 343-358, 1991.
0899-5362/91 $3.00+0.00 © 1991 PergamonPmu pie
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Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt ANTAZA. ABDEL-WAHABand *P. TUaN~ Departmentof GeologicalSciences.Universityof Texas at Austin.Austin TX 78713 *Schoolof Earth Sciences,BirminghamUniversity,Birmingham,England (Acceptedfor publication 14 May, 1990) Abstract - Four lithofacies are recognized in Late Cretaceous sandstones that crop out on both sides of the basement complex in the central EamtemDesert.Bar,ed on lithologicalclmr~teristi~ and paleocurrentanalysis. these facies can be described as follows, starting with the oldest: a) paleosols, b) Irough cross-beddedpebbly sandstone,c) planar-tabularcross-beddedsandstone,and d) goethite-ccmealcdtabularcross-beddedsandstone. These sandstoneshave be4msubjectedto a variety of diageneficalterationswhich stronglyaffected the original compositionofthesediments. Fourdiageneticstagesme recorded: 1) eodiagenesis,2)immature mesodiagenesis (mechanicalcompaction),3) maturemesodiagenesis,and4) telodiagenesis.Themaindiageneficfeaturesidentified are: 1) mechanical infdtration of clay, 2) dissolution of feldspars and unstable ferromagnesiansilicates, and replacementby clays, 3) neomorphismof kaolinitefrom feldsparand mica, and 4) formationof an authigenicsuite ofminerals,whichincludesquartz,K-feldspar,clayminerals,carbonate,hematite,andgoethite. Secondaryporosity is dominant and is closely related to the decarbonizationprocess of the me.sodiagenesisstage.
INTRODUCTION The s e q u e n c e of t h i c k s a n d s t o n e b e d s exposed o n b o t h s i d e s of t h e Nile Valley in s o u t h e r n Egypt, n o r t h e r n S u d a n , a n d c e n t r a l Sinai h a s a t t r a c t e d t h e a t t e n t i o n of a g r e a t n u m b e r of w o r k e r s since t h e last c e n t u r y . The t e r m Nubia F o r m a t i o n w a s given to t h e b r o w n i s h , highly dissected, a l m o s t horizontal, e x t e n s i v e l y c r o s s - b e d d e d a n d s p a r s e l y fossiliferous s a n d s t o n e b e d s t h a t a r e widely d i s t r i b u t e d over t h e s o u t h e r n p a r t s of Egypt. It r e s t s u n c o n formably on the peneplalned basement complex and underlies marine Upper Cretaceous sedimenta r y rocks (Quseir Variegated Shale). According to Y o u s s e f (1957), N u b i a s a n d s t o n e s s h o u l d have b e e n d e p o s i t e d u n d e r similar conditions a n d t h e i r age s h o u l d be r e s t r i c t e d to t h e C a m p a n i a n .
Study a r e a T h r e e localities of t h e Nubia F o r m a t i o n were c h o s e n for detailed studies. Two sections were s a m p l e d f r o m t h e e a s t e r n f l a n k of t h e P r e c a m b r l a n belt a r o u n d t h e Q u s e i r a r e a at EI-Beida, EIAnbagi, a n d EI-Wekala E I - H a m r a . T h e o t h e r s e c t i o n w a s s a m p l e d f r o m t h e w e s t e r n f l a n k at Wadi H a m m a m a t (Fig. 1). T h e s e t h r e e stratig r a p h i c s e c t i o n s w e r e c h o s e n b e c a u s e t h e Nubia F o r m a t i o n is c o n c o r d a n t o n b o t h sides of t h e Precambrian rocks and the stratigraphic s e q u e n c e s a r e t h e s a m e at e a c h locality.
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]Previotm w o r k A l t h o u g h t h e Nubia F o r m a t i o n h a s b e e n s t u d i e d in g e n e r a l t e r m s b y m a n y workers, few detailed s t u d i e s h a v e b e e n p u b l i s h e d on its sedimentology, petrology, m i n e r a l o g y or g e o c h e m i s t r y . In particular, t h e r e h a v e b e e n no ddtafled d e s c r i p t i o n s of t h e diagenetic h i s t o r y of t h e N u b i a Formation. The Nubia F o r m a t i o n h a s b e e n s t u d i e d in different p a r t s of Egypt b y a n u m b e r of workers, i n c l u d i n g S h u k r i (1945), Youssef ( 1957), Issawi ( 1968, 1972), W h i t e m a n {1971), E I - H i n n a w i et al. (1973), B i s h a r a a n d Nasr (1973), O m a r a et al. (1974), W a r d a n d McDonald (1979), V a n H o u t e n a n d B h a t t a c h a r y y a (1979), Klitzseh et al. (1979), a n d V a n H o u t e n e t a/. (1984).
S c o p e a n d l l m of t h e p r e s e n t work There is c u r r e n t l y no satisfactory c o m p r e h e n s i v e s t u d y of t h e Nubia F o r m a t i o n t h a t c a n be r e g a r d e d as definitive. O u r p r e s e n t knowledge of t h e s e s a n d s t o n e s is still in its infancy. The age of t h e s e q u e n c e is poorly u n d e r s t o o d a n d is d e b a t a b l e in m o s t areas, a n d t h e deposiUonal e n v i r o n m e n t s of t h e s e s a n d s t o n e s h a v e n o t b e e n fully interpreted. The a i m of t h i s p a p e r is to describe the lithofacies and diagenetic history of the Nubia F o r m a t i o n in t h e c e n t r a l E a s t e r n D e s e r t of Egypt.
METHODS Field w o r k i n c l u d e d g r a p h i c logging of t h e sections, s a m p l e collection a n d m e a s u r e m e n t s of
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Fig. 1. Location map and measured straUgraphic secUons at the present studied localities (The geological map is simplified and modified after the EgypUan Geological Survey Map, 198 I). The first three facies are identified at all localities u n d e r c o n s i d e r a t i o n , b u t t h e last facies is recognized only in the u p p e r m o s t part of the Wadi H a m m a m a t section (Fig. 1). The lowest unit (soft horizon), or kaolinitized zone, is clearly identified Just above Precambrian rocks displaying the typical characteristics of laterite. This facies w a s d e s c r i b e d by m a n y workers in the early and middle parts of this century. Moreover, it h a s been recently identified and d e s c r i b e d b y W a r d and McDonald (1979) and Van Houten et aL (1984). This unit is 3 to 6 m thick. At m o s t localities a trough cross-bedded sandstone lies conformably on the soil horizon. This unit corresponds to the "Lower Nubia" of Attia (1955) in the vicinity of Aswan. This unit is also equivalent to Unit A described by Ward and RESULTS McDonald (1979), a n d equivalent to Facies i identified by Van Houten et al. (1984) in the vicinity of The Iden¢!fied f a c i e s Aswan. It is 6 to 35 m thick. In the p r e s e n t studied sections, four lithofacies The p l a n a r - t a b u l a r c r o s s - b e d d e d s a n d s t o n e are identified. They are: facies is ubiquitous in all the areas u n d e r study. 1. soft horizon {rootlet bed); This lithofacies is the one m o s t commonly identi2. trough cross-bedded pebbly sandstone; fied as the Nubia Facies. This unit corresponds to 3. planar-tabular cross-bedded sandstone offlu- the "Upper Nubia" of Attla (1955) and the Taref viatile origin; a n d Sandstone of Awad a n d Ghobrial (1965). It is Unit 4. goethite-cemented planular-tabular sandstone B of Ward a n d McDonald (1979) and Facies 2 of of coastal to shallow-marine origin. Van Houten et al. (1984). It is 22 to 33 m thick. the orientation of cross-bedding. The a m o u n t and direction of cross-bedding dip were corrected for tectonic dip a n d strike of strata. About 150 thin sections impregnated with a green dye (araldite CY 212 and h a r d e n e r HY 951) were prepared. Carbonate c e m e n t s were stained using a combination of p o t a s s i u m ferricyanide and Alizarin Red-S (Dickson, 1966). A s c a n n i n g electron microscope (SEM) with energy dispersive X-ray analyzer (EDAX) was also used to interpret textures and identify minerals. Luminescence characteristics of sandstone (CL) were s t u d i e d u s i n g a n electron-probe microanalyzer (EPMA) and a conventional cold cathode instrument. Clay minerals was identified using Xray diffraction.
Diagonosis of the Nubia Formation, Central Eastern Desert, Egypt The g o e t h i t e - c e m e n t e d t a b u l a r c r o s s - b e d d e d s a n d s t o n e facies h a s b e e n recognized only at the Wadi H a m m a m a t locality. This facies is quite different from t h e facies below it. It h a s a polymodal p a l e o c u r r e n t direction t h a t is t h e m o s t characteristic feature of this facies. The facies is probably equivalent to Unit D of Ward a n d McDonald (I 979) a n d m a y be equivalent to Facies 3 of Van H o u t e n et al. (1984). It is approximately 100 m thick.
Environmental significance O u r p a l e o c u r r e n t d a t a from cross-bedding is d o m i n a n t l y unlrnodal, especially at the e a s t e r n flank of t h e P r e c a m b r i a n belt. The u n i m o d a l p a t t e r n is compatible (PettiJohnet a/., 1972} with t h e interpretation t h a t the m a j o r part of t h e Nubia Formation is fluviatile. The m e a n direction of crossb e d d i n g is 22 °, 56 °, a n d 44 ° at El-Wekala ElHamra, EI-Anbagi, a n d EI-Beida respectively; this indicates t h a t t h e p a l e o c u r r e n t flow w a s generally toward t h e n o r t h e a s t , a n d t h a t t h e m a i n source of detritus was h i g h l a n d s to t h e s o u t h a n d southeast. In t h e w e s t e r n area (Wadi H a m m a m a t ) , unim o d a l c r o s s - b e d d i n g is s e e n in t h e lower twot h i r d s of t h e succession. A polymodal or bipolar t r e n d h a s b e e n n o t e d in t h e u p p e r m o s t t a b u l a r c r o s s - b e d d e d facies. This facies m a y have suffered s o m e m a r i n e influence associated with T e t h y a n t r a n s g r e s s i o n d u r i n g Late C e n o m a n i a n - T u r o n i a n time. F u r t h e r east, s t r e a m s c o n t i n u e d to t r a n s p o r t s e d i m e n t s from a h i g h region t h a t roughly parallelled t h e p r e s e n t Red Sea graben. Sandstone composition S a n d s t o n e s of t h e Nubia from b o t h flanks of t h e P r e c a m b r i a n belt in t h e Central E a s t e r n Desert are f n e - to m e d i u m - g r a i n e d , a n d m o s t are brown to r e d d i s h b r o w n b e c a u s e of iron oxide pigment. T h i n sections s h o w t h a t today t h e s a n d s t o n e s have the c o m p o s i t i o n of quartzarenites, b u t t h a t considerable a m o u n t s of u n s t a b l e grains have b e e n lost by dissolution. Today t h e average s a n d s t o n e h a s 62 % detrltal quartz. 4 % rock f r a g m e n t s {clay clasts. c h e r t a n d mica schists) a n d 1 % each of feldspar a n d heavy minerals. C e m e n t a n d r e p l a c e m e n t minerals are variable in a b u n d a n c e , b u t are chiefly clays (kaolinite. dickite, chlorite, illite w h i c h total 16 9/0). quartz {6 %). iron oxides (3 %) a n d calcite (3 %). Porosity r a n g e s from 5 to 39 % {avg. = 18 %). a n d m u c h of it is secondary. OUTLINE OF DIAGENETIC PROCESSES General C o m m e n t s The Nubia F o r m a t i o n in t h e central E a s t e r n Desert of Egypt s h o w s various diagenetic features. These include: 1. m e c h a n i c a l infiltration of clay;
345
2. dissolution of u n s t a b l e detrital silicate grains, s u c h as feldspar a n d f e r r o m a g n e s i a n silicate, a n d carbonates; 3. clay replacement of unstable detritalgrains: 4. n e o m o r p h i s m of k a n d i t e s (dickite a n d kaolinite) from K-feldspar a n d mica, a n d neoformation of o t h e r clay minerals; 5. formation of a n authigenic m i n e r a l suite t h a t i n c l u d e s calcite, quartz, K-feldspar, clay minerals, martite, hematite, and younger c a r b o n a t e a n d goethite; 6. compacUonal features s u c h as m e c h a n i c al fracturing, deformation, a n d b e n d i n g of ductile a n d cleavable grains. These diagenetic features are c o m m o n to a large n u m b e r of c o n t i n e n t a l red b e d s (Walker 1967, 1976; Walker e t a L , 1978; Turner, 1980). M e c h a n i c a l Infiltration o f Clay Mechanically infiltrated clay r e p r e s e n t s the earliest diageneUc process in the Nubia Formation, particularly in the lowest part of the sections. It is p r e s e n t locally at o t h e r horizons. Using the SEM, there is no difficulty in identiflng clay of this origin a n d it c a n be easily d i s t i n g u i s h e d from replacem e n t or authlgenic clay. Three textural varieties of mechanically infiltrated clay have b e e n detected. The first texture is m a r k e d by clay material t h a t occurs in pores a n d m a y fill or nearly fill interstitial voids (Fig. 2A) or occurs as seepages w h i c h percolate t h r o u g h the s e d i m e n t with or w i t h o u t a preferred orientation, d e p e n d i n g on t h e textural a n d petrophysical properties. The second texture occurs as grain coatings with different clay-sized platelets t h a t are oriented parallel to t h e grain surface. The third texture h a s oriented coatings or "skins" (Fig. 2B), a texture previously identified by Walker et aL (1978). Mechanical infiltration of clay into p o r o u s a n d permeable s e d i m e n t s as grain coatings is a ubiquit o u s feature in alluvial red beds. Most coarsegrained alluvium is essentially free of interstitial clay w h e n first deposited (Walker eta/., 1978} d u e to sorting by s t r e a m flow. The soil horizon at t h e base of the Nubia F o r m a t i o n m a y have served as a s u p p l y for clay. D i s s o l u t i o n o f F r a m e w o r k Silicates In the Nubia Formation, s e c o n d a r y porosity p r o d u c e d by the dissolution of detrital grains a n d possibly authigenic m i n e r a l s is extensive. The complete a b s e n c e of pyroxene, amphibole, a n d other u n s t a b l e heavy m i n e r a l s is probably t h e result of extensive dissolution, b e c a u s e t h e source rock is s u p p o s e d to be either igneous or m e t a m o r phic. However, t h e relics or complete dissolution of grains t h a t have b e e n observed frequently (Fig. 2C,
346
Am'AR A. AaDm -W,~q,~n and P. A
4
Fig. 2 ~L pore-filling infllterated clay. PPL, EI-Anbagi area. B. Clay skin coating the framework grains and oriented parallel to the grain surface. SEM, EI-Anbagl area. C. Oversized pores attributed to complete dissolution (DG), Wadi Hammamat area. D. Oversized pores due to partial dissolution, PPL, EI-Wekala EI-Hamra area. E. Complete dissolution ofdetrital grains with iron oxides and/or clay minerals indicating the original boundary. PPL, El-Wekala EI-Hamra area. F. Feldspar (F) partially dissolved along the cleavage planes. PPL, EI-Wekala EI-Hamra area. D, a n d E) m a y b e a t t r i b u t e d to a m p h i b o l e a n d p y r o x e n e dissolution. The dissolution t h a t t a k e s place in this m a n n e r m a y leave no clue to t h e n a t u r e of t h e original grains. T h e d i s s o l u t i o n of f e l d s p a r s in t h e N u b i a F o r m a t i o n is a crucial feature. Chemical a n a l y s i s a s well a s m o d a l a n a l y s i s indicates t h a t t h e s e s a n d s t o n e s are p o o r i n alkalies, w h i c h s u g g e s t s t h a t m o s t of t h e f e l d s p a r s were dissolved. Petro-
graphic o b s e r v a t i o n s indicate t h a t m o s t of t h e r e m n a n t s are strongly altered. The d i s s o l u t i o n of silicate g r a i n s t a k e s place b y hydrolysis. Na, Ca, a n d Mg are r e l e a s e d from t h e silicate f r a m e w o r k r a t h e r easily, w h e r e a s Fe, AI, a n d Ti t e n d to r e m a i n b e h i n d (Turner 1980). The a c t u a l n a t u r e of t h e hydrolytic r e a c t i o n d e p e n d s u p o n the c o m p o s i t i o n of the m i n e r a l involved. For p o t a s h f e l d s p a r it m a y b e written:
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
347
>KAIsSLtO2o(OH)4+4K÷ grains, b u t also authigenic K-feldspar. The released K, Al, a n d Si p r o d u c e d during t h e break+ 8H4SiO 4 down of feldspars m a y be responsible for the K-feldspar inite f o r m a t i o n of a u t h i g e n i c p o t a s s i u m m i n e r a l s K-feldspar grains t h a t have b e e n etched by dis- including illite (Fig. 3B). Microcline is the m o s t frequent K-feldspar mineSolution are a very c o m m o n feature (Figs. 2F a n d 3A). In general, t h e dissolution t a k e s place prefer- ral due to its high stability. However, it shows high entially along the cleavage plane of the grains degrees of alteration embracing various degrees of (Fig. 2F). The dissolution involves not only detrital c r o s s h a t c h twining obliteration (Fig. 3C). 5KA1ShOs+ 4H'+ 16H20
Fig. 3 A. Feldspar dissolution having deep surface voids. SEM. B. Authigenic K-feldspar (AKF) post-datlng authigenie fllite (AIL}.SEM,Wadi Hammamat. C. alteration ofmlcrocllne (Mic)displayingvarlous degrees of cross-hatched twinning obliteration. XN, EI-Anbagi area. D. The dissolution of siderite (Sid). SEM, Wadl Hammamat area, E. The dissolution ofauthigenic calcite ACa). SEM, Wadl Hammamat. F. The dissolution of quartz overgrowth and/or the detrltal grain and partial replacement by clay. (CI), Wadi Hammamat area.
348
ANTARA. ABDEL-WAHABand P. TURNER
It is n o t e w o r t h y t h a t m o s t of t h e m e t a s t a b l e a n d u n s t a b l e heavy m i n e r a l s have b e e n dissolved or strongly altered to opaques, a n d t h e heavy mineral suites are r e p r e s e n t e d by t h e ZTR group. This m a y indicate t h e s t r o n g effect of i n t r a s t r a t u m solutions. The dissolution of c a r b o n a t e minerals (siderite a n d calcite) is a very c o m m o n feature (Fig. 3D, E). Its effect is considerable in producing large a m o u n t s of s e c o n d a r y pores. The features previously m e n t i o n e d indicate t h a t t h e dissolution p r o c e s s started early, before t h e formation of authlgenic minerals, a n d c u l m i n a t e d d u r i n g shallow or i n t e r m e d i a t e burial (mesodiagenesis).
Replacement by Clay The clay r e p l a c e m e n t process is closely related to the dissolution process. As a result, different elements released into t h e interstitial solution were deposited as authigenic minerals. The two processes are responsible for c h a n g i n g the extent of textural a n d mineralogical m a t u r i t y as well as the b u l k chemical composition of the sediment. In the Nubia Formation, clay r e p l a c e m e n t is very c o m m o n in individual mineral grains (Fig. 3F) a n d rock fragments. Many features reported herein are very similar to t h o s e described by Walker e t al. (1978). S u c h r e p l a c e m e n t s are considered authigenic (Wilson a n d Pittman, 1977). Very little ferromagnesian minerals (mostly hornblende) are present, b e c a u s e m o s t have been completely dissolved leaving oversized pore spaces. Using t h e EDAX analyzer with SEM, differences in chemical c o m p o s i t i o n were noted along a traverse of a n altered h o r n b l e n d e (Fig. 4). Ca, Fe, a n d Mg progressively decrease toward t h e alteration product, a c c o m p a n i e d by a n increase in b o t h Al a n d Si. A m a r k e d increase in iron oxide n e a r the original h o r n b l e n d e grain w a s also seen. Clay replacement is not confined to detrital grains, b u t is also p r e s e n t in authigenic K-feldspar. The r e p l a c e m e n t of authigenic K-feldspar by authigenic illite is a c o m m o n feature (Fig. 5A).
Neomorphism The t e r m n e o m o r p h i s m is u s e d here in the sense of Kantorowicz (1983) as the in situ s t r u c t u r a l alteration of one p h a s e to a n o t h e r involving t h e loss a n d / o r addition of n o n - s t r u c t u r a l or mobile cations w i t h o u t dissolution a n d reprecipitation of the original p h a s e . This t e r m s h o u l d not be confused with neoformation, in w h i c h a new p h a s e is formed from the interstitial solution. Neomorphism a n d dissolution are closely associated. Kaolinite, w h i c h formed as a result of mica a n d feldspar alteration, is c o m m o n (Fig. 5B) a n d quite different in s h a p e from t h e neoformed type. The
Parent Parent rock--- Altered zone--- rock
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% % SiO2 42.9 44.3 AI203 II.2 19.3 TiOz 1.46 1.02 Fe203 16.3 II.2 MgO I0.4 6.32 CaO 12.1 7.21 NazO 0.97 0.42 KzO 0.76 0.38
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former still preserves t h e p r e c u r s o r grain habit, a n d t h e alteration c o n t a c t c a n be seen. Moreover, t h e perfect crystalline forms were missing.
Authigenic Minerals A u t l ~ e n l c m i n e r a l s identified in t h e p r e s e n t s t u d y Include two categorles: non-clayey mlnerals s u c h as quartz, feldspar, a n d iron oxides; a n d authlgenlc clay minerals. Authlgenic quartz Quartz overgrowth in the Nubia F o r m a t i o n is hlghlyvarlable. We believe t h a t the variation of quartz c e m e n t a t i o n c a n be u n d e r s tood a n d interpreted t h r o u g h p o r e - s p a c e availability within the rock before c e m e n t a t i o n . T h e s e pores were c e m e n t e d before c o m p a c t i o n by carbon a t e at a very early stage of diagenesis. The developed overgrowth m a y be c o n s i d e r e d as a f u n c t i o n of dissolved c a r b o n a t e or unfllled pores (Fig. 5C). Many quartz overgrowth f e a t u r e s m e n t i o n e d
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
349
Fig. 5 A. The replacement ofau thigenic K-feldspar(AKF)by authigenic illlte(AIL}.SEM, El-Wekala EI-Hamm area. B. Neomorphosls of feldspar to vermiform Kaolinite. SEM, Wadi H a m m a m a t area. C. Patches of calcite a s pore-filllng which is still incompletely dissolved. XN, El-WekalA El-Hamra area. D. Quartz overgrowth with euhedral faces grown on detrltal grains coated with clay skin. SEM, Wadi H a m n m m a t area. E. The quartz overgrowths merge forming incomplete crystal faces. SEM, EI-Wekala EI-Hamra area. F. Overgrowth of prismatic projections on monocrystalline grains forming a large prismatic form. SEM, EI-Anbagi area
previously by W a u g h (1970a, b), Pittman (1972), and others were identified in the present samples. Quartz overgrowth with euhedral faces has occurred on detrital grains, and is a c o m m o n feature (Fig. 5D). Due to saturation conditions, two stages of the overgrowth on the grain surface merge forming incomplete crystal faces (Fig. 5E). At first glance, s u c h a feature m a y lead to its interpreta-
tion as dissolution features. An advanced stage in the formation of euhedral, prismatic crystals (Fig. 5F) is represented, from which perfect crystal faces of authigenic quartz cement developed locally, having hexagonal pyramidal form (Fig. 6A). A lot of work has been reported on the origin of secondary silica (Sippel, 1968; Hawkins, 1978;
350
AbrrARA. ABDEL-WAHABand P.
C
AI
Si
EDAX Analysis NA 0~5 K AL 12.2 SI 67.4 ~A 18.9 0.17 BA 0.18
7
Fig. 6 A. A perfect crystal ofauthigenlc quartz having hexagonal pyramidal termination s to prism faces. SEM, El-Wekala El-Hamra area. B. Further stage of K-feldspar (AKF) forming large crystal faces (adularla). SEM, Wadl Hammamat area. C. EDAX spot analysis {x}indicating K-feldspar composition (marked point in B). D. K-feldspar in the form of tabular prismatic crystals showing signs of dissolution. SEM, Wadi Hammamat area. E. Authlgenlc K-feldspar (AIG~developed on a grain surface coated with authigenic fllite (AIL).SEM, El-Wekala EI-Hamra area. F. K-feldspar (AKF)altered to authigenic illlte (AIL).SEM.
and Hutcheon, 1983). The dissolution of feldspar and rock fragments and their replacement by clays, as well as the replacement of quartz, mica, and feldspar, are the most reasonable potential sources of silica.
hedron-like crystals of authigenic monoclinic Kfeldspars (term of Baskin,1956). The second is tabular prismaUc crystals (Fig. 6D). These forms were previously described by Stablein and Dapples (1977). Authlgenic K-feldspars also occur as pore fillings, Authlgenlc Feldspars Two different habits of locally forming from authigenic fllite (they postauthigenic K-feldspar are present in the Nubia. date mite) (Fig. 6E). A later stage of authigenic fllite The first is adularia (Fig. 6B, C), tiny rhombo- formed from authigenlc K-feldspar (Fig. 6F).
Diagenesis of the Nubia Formation, CentralEastern Desert,Egypt
351
The growth of authlgenic K-feldspar in sedimen- kaolinite/dickite as both neoformed and neotary deposits requires t h a t the diagenetic environ- morphosed particles requires substantial acid water m e n t be enriched with K, Al, and Si ions (Waugh flux ( H e l g e s o n et al., 1969; F a n n i n g a n d (1978)). These ions could be released by the disso- Keramides, 1977). Smectite a n d mixed-layer fllite-smectite interlution of detrital alkali feldspar and other silicate framework grains from alkaline intrastrata solu- preted to be ofauthigenic origin are present but not tions. The chemical conditions required for the abundant. These phases display flake-like crystals precipitation ofauthlgenic feldspars are controned which often curl up from the grain surface at high by the pH and relative a m o u n t s of various cations: angles (Fig. 7E). Thls curly appearance m a y result K, Na, Ca, a n d Si. A relatively high Na*/H ÷ or K÷/ from dehydration, perhaps during the SEM coatH ÷ ratio associated with high H4SiO4 activities is ing process. Their presence in small a m o u n t s needed for feldspar authigenesis. The alkali m e t a l / supports the belief that the Nubia Formation h a s hydrogen-ion ratio, with a m i n i m u m a m o u n t of not undergone very deep burial. Chlorite is less a b u n d a n t and is present in the H4SiO4in solution, is indicative of feldspar stability with respect to a solution and t h u s indicative form of pore-fill rosettes growing normal to detrital of its possible precipitation (Garrels and Christ, grains (Fig. 7F). Honeycomb chlorite is another 1965; PettiJohn et aL, 1972). Hemley and J o n e s c o m m o n variety. Authlgenic fllite is present locally, generally as (1964) suggested t h a t slightly elevated temperatures with moderate to deep burial are convenient pore-linlng t h a t coats detrital grains. A hair-llke, conditions for feldspar precipitation. However, delicate form is c o m m o n (Fig. 8A). A flake-like illite Kastner and Siever (1979) stated that temperature that is commonly curled is also present. A boxwork is not the m o s t important consideration for feld- or cellular texture of authigenic fllite is recognized spar authlgenesis and t h a t there is no minimal in a few samples (Fig. 8B). depth of burial t h a t would correspond to m i n i m u m In the present study, two s u b s e q u e n t authlgenic geothermal heating before authlgenesis can occur. illites are recognized. The first was formed at a n The authlgenic feldspars (mainly K-feldspars) in early stage of diagenesis before authigenic K-feldthe Nubia Formation seem to have formed at low spar (Fig. 6E). The other post-dates the authigenic temperatures; stratigraphic data suggest the Nubia K-feldspar and formed from adularia dissolution (Fig. 6F). Formation h a s not been deeply buried.
Clay Mlneralm Authigenic kaolinite (including dickite) is present in nearly all samples of the Nubia Formation in a m o u n t s up to several percent. However, there are local occurrences of expandable clays (smectite and mixed-layer lnite-smectite), several morphologic types of fllite, and traces of chlorite and mixedlayer chlorite-vermiculite. Kaolinite is present as both pore-fill and porelining. P s e u d o h e x a g o n a l plates are a r r a n g e d normal to the grain surface {Fig. 7A). A pseudohexagonal pore-filling crystal with monoclinic habit and twinning typical of dickite is also present. Dickite consists of m a n y t h i n n e r lamellae {Fig. 713). Thick packets lacking parallel lamellae are also present. Bohor and Hughes {1971) could not identify the factor controlling packet thickness. Kaolinite with a delicate crystalline habit in the form of rosettes (Fig. 7C) is also present. It is also pore-fill, and displays typical vermicular form. It consists of stacked, long sequences ofpseudohexagonal plates growing between detrital grains and exhibiting bridges (Fig. 7D). Most authigenic kaolinite herein is neoformed, precipitated from interstitial pore water or neomorphosed, formed from a precursor silicate framework. The identification of dickite is based on SEM coupled with the XRD analysis. The dissolution of feldspars and formation of
Iron Oxide Minerals Iron oxide minerals include hematite, martite and goethite. Hematite is ubiquitous in the Nubia Formation. Botryoidal and cellular textures are found in the form of pore lining and interstitial matrix (Fig. 8C), where they are closely associated with clay minerals. Crystalline hematite ofacicular habit, fringing the pore spaces and coating detrital quartz (Flg. 8D), is very c o m m o n in the uppermost part of the Wadi H a m m a m a t section. Another c o m m o n form is elongate laths which coat quartz overgrowths and are oriented parallel with the grain surface (Fig. 8E). In m a n y samples, more t h a n one generation of hematite is present, which suggests the persistence of reddening for a long time. It is not clear w h e t h e r hematite (alpha Fe~Os) forms directly from magnetite or maghemite (gamma Fe20 s) or a phase intermediate between Fe2Osand Fe30 ~ is involved (Turner, 1980). Martite is c o m m o n in the u p p e r m o s t part of the Wadi H a m m a m a t section, where it is associated with hematite and is clearly a low-temperature authlgenic mineral. It probably formed from the oxidation of magnetite or titanomagnetite. The apparent absence of magnetite in the samples studied suggests t h a t it m a y have been the precursor of hematite (cf. Turner, 1980). Authigenic goethite was identified only in the
352
Am'As A. ABDEL-WAHABand P. TURNER
i
A
Fig. 7/k Pseudo-hexagonal plates of kaolinlte arranged normal to the grain surface.SEM, Wadi Hammamat area. B. A typical dickite(DK)ofbook shape displaying pseudohexagonal, stacked plates with characteristic twinning and parallel lamellae. SEM, EI-Anbagi area. C. Pseudo-hexagonal idiomorphic form with a delicate crystalline habit in the form of rosettes. SEM, Wadi Hammamat area. D. A kaolinite pore-fill displaying typical vermlcular form, consisting of pseudo-hexagonal plates, growing between the detrital grains and exhibiting bridges. SEM, EI-Anbagiarea. E. Flake-like smectlte crystals (Sm)which curl up from the grain surface. SEM, El-Wekala EI-Hamra area. F. Authlgenic chlorite in the form of rosettes developed as pore-llning. SEM, EI-Anbagi area. u p p e r m o s t h o r i z o n of W a d i H a m m a m a t . It w a s observed mainly along the cross-bedding planes, h i g h l y c e m e n t e d a n d w i t h a d a r k b r o w n metallic a p p e a r a n c e . In o t h e r c a s e s , it f o r m s s p h e r i c a l c o n c r e t i o n s . A n o t h e r c o m m o n f o r m is s p h e r i c a l w i t h i n t e r n a l , relatively t h i c k , g r o w t h b a n d s t h a t a re r o u g h l y c o n c e n t r i c . T h e s e b a n d s r e p r e s e n t
z o n e s of d e n s e r c e m e n t a t i o n b y i r o n o x i d e s a r r a n g e d a l t e r n a t e l y w i t h o t h e r , l ess c e m e n t e d bands. G o e t h i t e o c c u r s m a i n l y a s a p o r e fill. B o t r y o i d a l t e x t u r e is c l e a r l y o b s e r v e d in t h e c e n t r a l p a r t s of t h e voi ds (Fig. 8F]. A wide r a n g e of f e a t u r e s is d i s p l a y e d b y d e t r i t a l g r a i n s r e p l a c e d b y goethite. A
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
353
Fig. 8. A. Authigenic filite in a halr-like, delicate form coaUng the detrital grains. SEM,EI-Anbagl area. B. Authigenic fllite in the form ofboxwork or cellular textures as pore-lining normal to detrltal grain surfaces. SEM, EI-Anbagi area. C. Interstitial pore-filling and pore-lining hematite displaying botryoidal and cellular texture. SEM,EI-Anbagi area. D. Crystalline hematite (AHM) of tiny fibrous shape developed as pore-fill. SEM, EI-Wekala EI-Hamra area. E. Laths of elongate shape, hematite (AHM) coaUng the authigenic quartz (AQ) and oriented parallel to the grain surface. SEM, Wadi Hammamat area. F. A typical botryoidal texture of goethite showing two or more generations formed during later stages of diagenesis. SEM, uppermost part of Wadi Hammamat area. m u s c o v i t e g r a i n replaced by goethite is one clear example of s u c h r e p l a c e m e n t (Fig. 9A). Replacem e n t t o o k place along t h e cleavage p l a n e s of the mica. R e p l a c e m e n t of detrital gains, s u c h as felds p a r a n d quartz, w a s also observed.
Titanium oxide Authigenic t i t a n i u m oxides are c o m m o n in the Nubia F o r m a t i o n a n d o c c u r m a i n l y as e u h e d r a l prismatic c r y s t a l s or a s small discrete c r y s t a l s of a n a t a s e or rutfle. The t i t a n i u m ions m a y have b e e n released d u r i n g t h e m a r t i t i z a t i o n of detrital titano-
354
ANTARA, ABDEL-WAHABand P. TURNER
D
,
5Oum
b
~
Fig. 9. A. Authlgenic goethite (GOE) replacing detrital mica (M), in reflected llght,off. PPL, uppermost part ofWadl Hammamat area. ]3. Authigenle carbonates developed as pore -filldisplaying a perfec t crystal rhombohedral form. SEM, Wadi Hammamat area. C. An advanced stage of calcite replacement (Ca), relices of quartz (Q) are left. El-Wekala EI-Hamra area. D. Carbonate pore-filling (Ca) which took place post-datlng the authigenic quartz (AQ). E. Corroded grains previously replaced by carbonate and infllled later by hematite. XN, El- Wekala EI-Hamra area. F. Abnormal large pores as a result of carbonate replacement a n d / o r cement which subsequently dissolved and produced oversized pores.PPL, Wadi Hammamat area.
magnetite as suggested by Ixer et al. (1979) or by the alteration of other titanium-bearing minerals, s u c h as sphene. Carbonates
Authigenic poikflotopic a n d sparry calcite cem e n t (Fig. 9b) h a s a p a t c h y distribution in the Nubia Formation. ranging from 0 to 8% a n d
averaging 2.5%. Various quartz grains display corroded a n d e m b a y e d borders s u r r o u n d e d by patchy carbonate (Figs 5C a n d 9C), as weU as carbonate pore flU post-datlng well-formed quartz overgrowths (Fig. 9D). C e m e n t stratigraphy indicates there were two generations of calcite. Nonferroan calcite took place at a very early stage of diagenesis a n d hindered complete pore filling by
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
355
originated w h e n originany adjacent grains of feldspar and other unstable minerals were removed by dissolution; 4. more t h a n 80 % of the grains have less t h a n four contacts, and more t h a n 92 % of the grain contacts are floating, tangential,~or long; 5. shallow, early cementation is'suggested by the relatively low values of contact strength (1.55) and the consolidation factor (48.6 % }. In comparison, Triassic sandstones in Nottinghamshire, England, that u n d e r w e n t intermediate burial depths have a consolidation factor of 52.5 and a contact strength of 1.83 and contact index of 1.56 (Serf EI-Dein, 1983); 6. the mild effect of compaction suggested by absence of extensively fractured grains, strongly bent micas, or squeezed clay clasts. However, local, u n c o m m o n compactional features are prePorosity and l>lagen~is sent locally; 7. the large variation in quartz cement, which The porosity of Nubia sandstone samples ranges from 5 % to 39 % of rock volume; most of it is m a y have been controlled by earlier calcite cement; 8. e m b a y m e n t s in quartz overgrowths that interpreted to be of secondary origin. Low porosity samples have a b u n d a n t authigenic calcite, quartz, are attributed to be the result of corrosion by and iron oxides. calcite cement. The identification of secondary porosity in the The development of secondary porosity in the Nubia Formation is based on the following criteria: Nubia Formation during the different diagenetic 1. fractured silicate grains that formed as a stages is summarized in a schematic diagram result of compaction u p o n s u d d e n leaching of (Fig. I0). carbonate cement; 2. s a n d s t o n e s with patchy r e m n a n t s of SUMMARYANDCONCLUSIONS corroded calcite c e m e n t and with corroded quartz grains. The texture suggests t h a t a once-extensive Facies and paleocurrent analyses identified four calcite cement, which corroded quartz grains, has lithofacies. They are: root-turbated and biobeen dissolved. Many of these secondary pores turbated clayey sfltstone (paleosol), trough crosswere later infilled later by hematite (Fig. 9E); bedded pebbly sandstone, planar-tabular cross3. partial to complete dissolution of detrital bedded sandstone offluviatile origin, and goethitegrains. Most of the leached grains were probably c e m e n t e d t a b u l a r c r o s s - b e d d e d s a n d s t o n e of feldspar and volcanic rock fragments (Fig. 2C-F). coastal and shallow-marlne origin. The first three In places the grain boundaries are only represent- facies are identified at most of the localities, but ed by clay or iron oxide rims; the last is recognized only in the u p p e r m o s t part 4. oversized pores derived from either of the Wadi H a m m a m a t locality. detrital grain dissolution or from dissolution of The climate was hot and humid, a n d intense carbonate-replaced grains (Fig. 9F): weathering was responsible for the formation of a 5. patchy carbonate c e m e n t with undis- soft horizon a few m e t e r s thick at the base of the solved r e m n a n t s (Fig. 5C). trough cross-bedded sandstones. Extensive The Nubia Formation was c e m e n t e d by weathering is indicated by the scarcity of feldspars calcite before burial, during a very early diagenetic and absence of ferromagnesian minerals, the prestage, followed shortly by quartz overgrowth. This ponderance of kaolinite in the matrix, and repeata s s u m p t i o n is derived from the following observa- ed weathering profiles on the s a n d bodies. tions: Paleocurrent data indicate t h a t a unimodal 1. the high pre-cement porosity (25 to 40 %) pattern is the predominant type, generally directed and secondary porosity; toward the north, northeast, or northwest. Poly2. the very low value of s u t u r e d contacts (2.7 modal trends, which have been found in the upper0/6) and the scarcity of concavo-convex contacts most part associated with p l a n a r - t a b u l a r cross(1.5 %); bedding, probably formed as a result of the 3. the a b u n d a n c e of floating grains (7.6 %) is T e t h y a n m a r i n e t r a n s g r e s s i o n d u r i n g Late exceptionally large for a sandstone that has been Cretaceous time. buried at least 1400 m. Some floating grains The diagenetic history of the Nubia Formation quartz, whereas, ferroan calcite post-dates the quartz overgrowths and was probably produced at a very late stage. The a b u n d a n c e of large, oversize pores, floating grains, corroded and etched quartz grains in the Nubia suggest t h a t calcite cement was once m u c h more a b u n d a n t but was subsequently dissolved. Many sandstones without calcite c e m e n t are semi-friable. The replacement of the margins ofdetrital quartz grains and some quartz c e m e n t by carbonate is a c o m m o n feature. F r i e d m a n e t a/, (1976) described various stages of quartz dissolution, initially with a V-shape a n d prismatic etch pattern. Advanced dissolution of SiO 2 and s y n c h r o n o u s precipitation of CaCO s encourages the formation of pseudom o r p h o u s carbonate after quartz (Fig. 9C) on a small scale.
356
AWrARA. ABDEL-WAHABand P.
%.
Eodiagenesis
%1 °..
~ m
ml~a,:~I/,.,.,.~:
Carbonam
Silica rele~xl
/~ovettiz~ poreprodu~on)
Mesod~genesis
(mature)
2
[,am generation of carbonate Fig. 10. Schematic diagram Indicating the developmentof secondary porosity during the different stages ofdlagenesis. took place in four stages: a} the eodm___genetlcstage = changes t h a t took place soon after the rock was deposited: i) early polkflotoplc calcite cement, tO quartz overgrowth, Ill} m i n o r dissolution of feldspar and formation of kaolinite, iv) coloration by iron oxides, which started early and took place over a long period of time. The net product was release of ions (SI, AI. K, Ca, Mg, Fe) to pore waters: b) the i m m a t u r e m e s o d l a g e n e t l c stage {mechanical compaction) had a m i n o r role. Although the effect of compaction was minimized
because of early cementation, observed features of compaction include: fracturing of brittle material, bending of ductile grains, mtcrostylolites, and concavo-convex contacts. Minor pressuresolution effects, and minor reduction of porosity continued: c} mature mcsodiagcncsis involved the production of vast secondary porosity through mesogenetic decarbonaUzation. Dissolution and breakdown of remaining feldspar and rock fragments took place at this stage. A well-developed authlgenlc kaolinite was probably produced. During the
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt g e n e r a t i o n of s e c o n d a r y porosity, c a r b o n a t e s were dissolved b y acidic w a t e r w i t h consequent rise m pH, w h i c h c a u s e d s u p e r s a t u r a t i o n w i t h r e s p e c t to kaolinite if d i s s o l v e d A1 a n d S i w e r e present. It also included: i) c o n v e r s i o n of kaolinite to a highly cryst_anlne form (dickite), ii) highly acidic w a t e r w h i c h led to v e r m l c u l a r kaolinite, iii) in s i t u illite, followed b y a u t h l g e n i c K-feldspar, followed b y a s e c o n d g e n e r a t i o n of illite, iv) martite, w h i c h m a y h a v e f o r m e d a s a r e s u l t of oxidized magnetite, a n d v} c o n v e r s i o n of s m e c t i t e to tllite. The i m p o r t a n c e of t h e llllte-smectite t r a n s i t i o n a s a s o u r c e of w a t e r a n d c a t i o n s in t h e N u b i a F o r m a t i o n is difficult to establish. Although there are no substantial m a r i n e m u d r o c k s in t h e s t u d y area, a d j a c e n t a r e a s m o r e deeply b u r i e d c o u l d have b e e n a s u i t a b l e s o u r c e . T h e p r e s e n c e of s m e c t i t e a n d fllitesmectite suggests that the Nubia Formation has n o t s u f f e r e d v e r y d e e p b u r i a l or m a y have f o r m e d after t h e b u r i a l process; d) t h e telodiagenetic (post-burial) stage = all t h e p r o c e s s e s f r o m b u r i a l u p to t h e present. The m a r i n e t r a n s g r e s s i o n w o u l d h a v e p r o d u c e d significant c h a n g e s in p H a n d E h of t h e circulating pore solutions, d u e to t h e mixing of t h e alkaline, hypersaline m a r i n e w a t e r s . This mixing m a y have led to t h e n e o f o r m a t i o n of a u t h i g e n i c minerals: i} devel o p m e n t of a u t h l g e n i c p y r a m i d a l or b i p y r a m i d a l p r i s m a t i c quartz, ii) smectite, chlorite, or v e r m i c u lite d e v e l o p m e n t , a n d ill) late c a r b o n a t e generation. This n e o f o r m a t i o n w a s a r e s u l t of t h e e x c e s s of Ca ~÷, HCOs. a n d Fe with CO s liberated b y t h e r e d u c t i o n of organic m a t t e r , p o s t - d a t i n g t h e late b i p y r a m i d a l q u a r t z plugging of t h e available pore s y s t e m . S e c o n d a r y p o r o s i t y is slightly r e d u c e d d u e to late c a r b o n a t e g e n e r a t i o n , a n d goethite a n d h e m a t i t e pore filling. Calcite w a s p r o b a b l y derived from t h e overlying C r e t a c e o u s m a r i n e l i m e s t o n e s of t h e Q u e s i r F o r m a t i o n . A c k n o w l e d g e m e n t s - The first author is greatly indebted to Prof. Earle F. McBride for his review and critical comments. Rosemary Brant is thanked for typing an early draft. REFERENCES
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357
m/nera/s. 19, 49-54. Dickson J. A. D. 1966. Carbonate identification and genesis as revealed by staining, J. Sed. Petro/ogy, 38, 491-505. EI-Hinnawi, E. E., Kabesh M. L. and Zahran, I. 1973. Mineralogy and chemistry of Nubla sandstone from the central Eastern Desert of Egypt. N. Jb. Miner. Abb., 1 1 8 , 211-234. Fanning, D. S. and Keramlds, V. Z. 1977. Micas, In: Dixon, J. B. and Weed, S. B., eds., 9, 195-258. Friedman, G. M., All, S. A. and Krinsley, D. H. 1976. Dissolution of quartz accom-panying carbonate preclpitaUon and cementation in reefs: example from the Red Sea. J. Sed. Petrv&~y, 46, 970-973. Garrels, R. M. and Christ, C. L. 1965. Solutions, minerals, and equilibria. NewYork, Harper and Row, 450 p. Hawkins, P. J. 1978. Relationship between diagenesis, porosity reduction, and oil emplacement in Late Carboniferous sandstone reservoirs. J. Geo/. Sac. Lond., 135, 7-24. Helgeson, H,, Garrels, R. and McKenzie, F. 1969. EvaluaUon of irreversible reactions in geochemical processes involving minerals and aqueous solutions. II - AppllcaUon. Geochlm. Cosmochim~ Acta, 33, 455481. Hemley, J. J. and J o n ~ , W. R. 1964. Chemical aspects ofhydrothermal alteration with emphasis on hydrogen metasomatism. Econ. Geo/ogy, 59, 538-589. Hutcheon, I. 1983. Aspects of the dlagenesls of coarsegrained sillciclastlc rocks. Sc/. Canada, 10, 4-14. Issawi, B. 1968. The geology of Kurkur Dungul area. GeoL Surv. Egypt, 48, 102 p. Issawi, B. 1972. Review of Upper Cretaceous-Lower Tertiary stratigraphy in central and southern Egypt. Am.Assoc. Petrol Geo/ogtsts Bu/l-, 56, 1148-1463. Ixer, R., Turner, P. and Waugh, B. 1979. Authigeniciron and titanium oxides in Triassic red beds: St. Bees Sandstone, Cumbria, Northern England. Geo/. Jour., 14, 179-192. Kantorowicz, J. 1983. Diagenetic modelling of Middle Jurassic clastic sediments from the Ravenscaw Group in Yorkshire and Brent Group, northern North Sea. Ph.D. Thesis, Hull Univ., United Kingdom, 363p. Kastner, M. and Siever, R. 1979. Low-temperature feldspars in sedimentary rocks. Am. J. Science, 279, 435479. Klitzsch, E., Harms, J. C., Lejal-Nicol, A. and List, F. K. 1979. Major sub-divisions and deposiUonal environments of Nubia strata, southwestern Egypt. Am. Assoc. Petrol Geologists Bull, 63, 967-974. Omara, S., Bishara, W. W. and Nasr, M. 1974. Grain-size parameters and paleo-environments of the Nubia Sandstone. J. Seal. Petro/ogy, 44, 136-144. PetUjohn, F. J., Potter, P. E. and Siever, R. 1972. Sand and Sandstone. New York, Springer, 618 p. Pittman, E. D. 1972. Diagenesls of quartz in sandstone a s revealed by scanning electron microscopy. J. Seal. Petro/ogy, 42, 507-519. Shukri, N. M. 1945. Geology of the Nubian Sandstone. Nature, 156, 116. SeffEI-Dein, S. 1983. Diagenesls and aquifer characteristics of the Sherwood Sandstone Group in north Nottinghamshire. unpub. Ph.D. thesis, Aston Univ., Birmingham, 206 p.
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A~rr~,~ A. ABDEL-WAHABand P.
Sippel, R. F. 1968. Sandstone petrology, ev/dence from luminescence petrography. J. SeeL Petro/ogy, 3S, 530554. Stapleln, N. K. and Dapples, E. C. 1977. Feldspars of the Tunnel City Group {Cambrian}, eastern Wiscsonin. J. Sed. Petro/ogy, 47, 1512-1538. Turner, P. 1980. Cont/nental red beds: Development in Sed/mentology, 29, Amsterdam, Elsevier, 562 p. Van Houten, F. B. and Bhattacharyya, D. P. 1979. Late Cretaceous Nubia Format/on at Aswan, south Eastern Desert, F_~ypL Ann. Geo/. Surv. Egyp~ 9, 408-431. Van Houten, F. B., Bhattacharyya, D. P. and Mansour, S. E. I. 1984. Cretaceous Nubia Formation and correlaUve deposits, eastern ~ t : major regress/vetransgressive complex. GeoL Soc. Amerk~ Bu//., 95, 397-405. Walker, T. I~ 1967. FormaUonofredbedsln modernand ancient deserts. GeoL Soc.Amerk~Bull., 72, 353-368. Walker, T. 11. 1976. Diagenet/c origin of continental red beds. In: The C o ~ a / P e r m / a n / n Central West, and SouthEurope. (ed. byH. Flake}, Dordrecht, D. Radel, p. 240-282. Walker, T. R., Waugh, B. and Crone, ,~ J. 1978. Diagenesis in first-cycle alluvium of Cenozoic age,
southwestern U. S. and northwestern Mexico. GeoL Soc. Amerk-aBu//., Sg, 19-32. Ward, W. C. and McDonald, K. C. 4~7g. N u b i a FormaUon of central Eastern Desert: major subdivisions and depositlonal sett/ng. Amer. Assoc./~troL Geo/og/sts Bu/L, 63, 975-983. Waugh, B. 1970~ Format/on of quartz overgrowth in the Pemrith Sandstone [Lower Permian] of northwest England as revealed by scanning electron microscopy. Sedtme~, 14, 309-320. Waugh, B. 1970b. Petrology, provenance, and silica diagenesis of the Penr/th Sandstone (Lower Penn/an} of northern England. J. See/. Petro/ogy, 40, 1226-1240. Waugh, B. 1978. Authigenlc K-feldspar in Br/tish Permo-Triasslc sandstones. J. GeoL Soc. Lond., 1515, 51-56. Whlteman, A. J. 1971, The Geology of the Sudan Republ/c. London, Oxford University Press, 290 p. Wilson, M. D. and Pittman, E. D. 1977. Auth~erdc clays in sandstone: recognIUon and influence on reservoir properties and paleoenvironmental analysis. J. SecL Petro/ogy, 47, 3-31. Youssef, M. I. 1957. Upper Cretaceous rocks in Quseir area. Bull. Inst. Desert, Egyp~ 7, 35-53.
0899-5362/91 $3.00+0.00 © 1991 PergamonPmu pie
Printedin Thailand
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt ANTAZA. ABDEL-WAHABand *P. TUaN~ Departmentof GeologicalSciences.Universityof Texas at Austin.Austin TX 78713 *Schoolof Earth Sciences,BirminghamUniversity,Birmingham,England (Acceptedfor publication 14 May, 1990) Abstract - Four lithofacies are recognized in Late Cretaceous sandstones that crop out on both sides of the basement complex in the central EamtemDesert.Bar,ed on lithologicalclmr~teristi~ and paleocurrentanalysis. these facies can be described as follows, starting with the oldest: a) paleosols, b) Irough cross-beddedpebbly sandstone,c) planar-tabularcross-beddedsandstone,and d) goethite-ccmealcdtabularcross-beddedsandstone. These sandstoneshave be4msubjectedto a variety of diageneficalterationswhich stronglyaffected the original compositionofthesediments. Fourdiageneticstagesme recorded: 1) eodiagenesis,2)immature mesodiagenesis (mechanicalcompaction),3) maturemesodiagenesis,and4) telodiagenesis.Themaindiageneficfeaturesidentified are: 1) mechanical infdtration of clay, 2) dissolution of feldspars and unstable ferromagnesiansilicates, and replacementby clays, 3) neomorphismof kaolinitefrom feldsparand mica, and 4) formationof an authigenicsuite ofminerals,whichincludesquartz,K-feldspar,clayminerals,carbonate,hematite,andgoethite. Secondaryporosity is dominant and is closely related to the decarbonizationprocess of the me.sodiagenesisstage.
INTRODUCTION The s e q u e n c e of t h i c k s a n d s t o n e b e d s exposed o n b o t h s i d e s of t h e Nile Valley in s o u t h e r n Egypt, n o r t h e r n S u d a n , a n d c e n t r a l Sinai h a s a t t r a c t e d t h e a t t e n t i o n of a g r e a t n u m b e r of w o r k e r s since t h e last c e n t u r y . The t e r m Nubia F o r m a t i o n w a s given to t h e b r o w n i s h , highly dissected, a l m o s t horizontal, e x t e n s i v e l y c r o s s - b e d d e d a n d s p a r s e l y fossiliferous s a n d s t o n e b e d s t h a t a r e widely d i s t r i b u t e d over t h e s o u t h e r n p a r t s of Egypt. It r e s t s u n c o n formably on the peneplalned basement complex and underlies marine Upper Cretaceous sedimenta r y rocks (Quseir Variegated Shale). According to Y o u s s e f (1957), N u b i a s a n d s t o n e s s h o u l d have b e e n d e p o s i t e d u n d e r similar conditions a n d t h e i r age s h o u l d be r e s t r i c t e d to t h e C a m p a n i a n .
Study a r e a T h r e e localities of t h e Nubia F o r m a t i o n were c h o s e n for detailed studies. Two sections were s a m p l e d f r o m t h e e a s t e r n f l a n k of t h e P r e c a m b r l a n belt a r o u n d t h e Q u s e i r a r e a at EI-Beida, EIAnbagi, a n d EI-Wekala E I - H a m r a . T h e o t h e r s e c t i o n w a s s a m p l e d f r o m t h e w e s t e r n f l a n k at Wadi H a m m a m a t (Fig. 1). T h e s e t h r e e stratig r a p h i c s e c t i o n s w e r e c h o s e n b e c a u s e t h e Nubia F o r m a t i o n is c o n c o r d a n t o n b o t h sides of t h e Precambrian rocks and the stratigraphic s e q u e n c e s a r e t h e s a m e at e a c h locality.
343
]Previotm w o r k A l t h o u g h t h e Nubia F o r m a t i o n h a s b e e n s t u d i e d in g e n e r a l t e r m s b y m a n y workers, few detailed s t u d i e s h a v e b e e n p u b l i s h e d on its sedimentology, petrology, m i n e r a l o g y or g e o c h e m i s t r y . In particular, t h e r e h a v e b e e n no ddtafled d e s c r i p t i o n s of t h e diagenetic h i s t o r y of t h e N u b i a Formation. The Nubia F o r m a t i o n h a s b e e n s t u d i e d in different p a r t s of Egypt b y a n u m b e r of workers, i n c l u d i n g S h u k r i (1945), Youssef ( 1957), Issawi ( 1968, 1972), W h i t e m a n {1971), E I - H i n n a w i et al. (1973), B i s h a r a a n d Nasr (1973), O m a r a et al. (1974), W a r d a n d McDonald (1979), V a n H o u t e n a n d B h a t t a c h a r y y a (1979), Klitzseh et al. (1979), a n d V a n H o u t e n e t a/. (1984).
S c o p e a n d l l m of t h e p r e s e n t work There is c u r r e n t l y no satisfactory c o m p r e h e n s i v e s t u d y of t h e Nubia F o r m a t i o n t h a t c a n be r e g a r d e d as definitive. O u r p r e s e n t knowledge of t h e s e s a n d s t o n e s is still in its infancy. The age of t h e s e q u e n c e is poorly u n d e r s t o o d a n d is d e b a t a b l e in m o s t areas, a n d t h e deposiUonal e n v i r o n m e n t s of t h e s e s a n d s t o n e s h a v e n o t b e e n fully interpreted. The a i m of t h i s p a p e r is to describe the lithofacies and diagenetic history of the Nubia F o r m a t i o n in t h e c e n t r a l E a s t e r n D e s e r t of Egypt.
METHODS Field w o r k i n c l u d e d g r a p h i c logging of t h e sections, s a m p l e collection a n d m e a s u r e m e n t s of
344
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Fig. 1. Location map and measured straUgraphic secUons at the present studied localities (The geological map is simplified and modified after the EgypUan Geological Survey Map, 198 I). The first three facies are identified at all localities u n d e r c o n s i d e r a t i o n , b u t t h e last facies is recognized only in the u p p e r m o s t part of the Wadi H a m m a m a t section (Fig. 1). The lowest unit (soft horizon), or kaolinitized zone, is clearly identified Just above Precambrian rocks displaying the typical characteristics of laterite. This facies w a s d e s c r i b e d by m a n y workers in the early and middle parts of this century. Moreover, it h a s been recently identified and d e s c r i b e d b y W a r d and McDonald (1979) and Van Houten et aL (1984). This unit is 3 to 6 m thick. At m o s t localities a trough cross-bedded sandstone lies conformably on the soil horizon. This unit corresponds to the "Lower Nubia" of Attia (1955) in the vicinity of Aswan. This unit is also equivalent to Unit A described by Ward and RESULTS McDonald (1979), a n d equivalent to Facies i identified by Van Houten et al. (1984) in the vicinity of The Iden¢!fied f a c i e s Aswan. It is 6 to 35 m thick. In the p r e s e n t studied sections, four lithofacies The p l a n a r - t a b u l a r c r o s s - b e d d e d s a n d s t o n e are identified. They are: facies is ubiquitous in all the areas u n d e r study. 1. soft horizon {rootlet bed); This lithofacies is the one m o s t commonly identi2. trough cross-bedded pebbly sandstone; fied as the Nubia Facies. This unit corresponds to 3. planar-tabular cross-bedded sandstone offlu- the "Upper Nubia" of Attla (1955) and the Taref viatile origin; a n d Sandstone of Awad a n d Ghobrial (1965). It is Unit 4. goethite-cemented planular-tabular sandstone B of Ward a n d McDonald (1979) and Facies 2 of of coastal to shallow-marine origin. Van Houten et al. (1984). It is 22 to 33 m thick. the orientation of cross-bedding. The a m o u n t and direction of cross-bedding dip were corrected for tectonic dip a n d strike of strata. About 150 thin sections impregnated with a green dye (araldite CY 212 and h a r d e n e r HY 951) were prepared. Carbonate c e m e n t s were stained using a combination of p o t a s s i u m ferricyanide and Alizarin Red-S (Dickson, 1966). A s c a n n i n g electron microscope (SEM) with energy dispersive X-ray analyzer (EDAX) was also used to interpret textures and identify minerals. Luminescence characteristics of sandstone (CL) were s t u d i e d u s i n g a n electron-probe microanalyzer (EPMA) and a conventional cold cathode instrument. Clay minerals was identified using Xray diffraction.
Diagonosis of the Nubia Formation, Central Eastern Desert, Egypt The g o e t h i t e - c e m e n t e d t a b u l a r c r o s s - b e d d e d s a n d s t o n e facies h a s b e e n recognized only at the Wadi H a m m a m a t locality. This facies is quite different from t h e facies below it. It h a s a polymodal p a l e o c u r r e n t direction t h a t is t h e m o s t characteristic feature of this facies. The facies is probably equivalent to Unit D of Ward a n d McDonald (I 979) a n d m a y be equivalent to Facies 3 of Van H o u t e n et al. (1984). It is approximately 100 m thick.
Environmental significance O u r p a l e o c u r r e n t d a t a from cross-bedding is d o m i n a n t l y unlrnodal, especially at the e a s t e r n flank of t h e P r e c a m b r i a n belt. The u n i m o d a l p a t t e r n is compatible (PettiJohnet a/., 1972} with t h e interpretation t h a t the m a j o r part of t h e Nubia Formation is fluviatile. The m e a n direction of crossb e d d i n g is 22 °, 56 °, a n d 44 ° at El-Wekala ElHamra, EI-Anbagi, a n d EI-Beida respectively; this indicates t h a t t h e p a l e o c u r r e n t flow w a s generally toward t h e n o r t h e a s t , a n d t h a t t h e m a i n source of detritus was h i g h l a n d s to t h e s o u t h a n d southeast. In t h e w e s t e r n area (Wadi H a m m a m a t ) , unim o d a l c r o s s - b e d d i n g is s e e n in t h e lower twot h i r d s of t h e succession. A polymodal or bipolar t r e n d h a s b e e n n o t e d in t h e u p p e r m o s t t a b u l a r c r o s s - b e d d e d facies. This facies m a y have suffered s o m e m a r i n e influence associated with T e t h y a n t r a n s g r e s s i o n d u r i n g Late C e n o m a n i a n - T u r o n i a n time. F u r t h e r east, s t r e a m s c o n t i n u e d to t r a n s p o r t s e d i m e n t s from a h i g h region t h a t roughly parallelled t h e p r e s e n t Red Sea graben. Sandstone composition S a n d s t o n e s of t h e Nubia from b o t h flanks of t h e P r e c a m b r i a n belt in t h e Central E a s t e r n Desert are f n e - to m e d i u m - g r a i n e d , a n d m o s t are brown to r e d d i s h b r o w n b e c a u s e of iron oxide pigment. T h i n sections s h o w t h a t today t h e s a n d s t o n e s have the c o m p o s i t i o n of quartzarenites, b u t t h a t considerable a m o u n t s of u n s t a b l e grains have b e e n lost by dissolution. Today t h e average s a n d s t o n e h a s 62 % detrltal quartz. 4 % rock f r a g m e n t s {clay clasts. c h e r t a n d mica schists) a n d 1 % each of feldspar a n d heavy minerals. C e m e n t a n d r e p l a c e m e n t minerals are variable in a b u n d a n c e , b u t are chiefly clays (kaolinite. dickite, chlorite, illite w h i c h total 16 9/0). quartz {6 %). iron oxides (3 %) a n d calcite (3 %). Porosity r a n g e s from 5 to 39 % {avg. = 18 %). a n d m u c h of it is secondary. OUTLINE OF DIAGENETIC PROCESSES General C o m m e n t s The Nubia F o r m a t i o n in t h e central E a s t e r n Desert of Egypt s h o w s various diagenetic features. These include: 1. m e c h a n i c a l infiltration of clay;
345
2. dissolution of u n s t a b l e detrital silicate grains, s u c h as feldspar a n d f e r r o m a g n e s i a n silicate, a n d carbonates; 3. clay replacement of unstable detritalgrains: 4. n e o m o r p h i s m of k a n d i t e s (dickite a n d kaolinite) from K-feldspar a n d mica, a n d neoformation of o t h e r clay minerals; 5. formation of a n authigenic m i n e r a l suite t h a t i n c l u d e s calcite, quartz, K-feldspar, clay minerals, martite, hematite, and younger c a r b o n a t e a n d goethite; 6. compacUonal features s u c h as m e c h a n i c al fracturing, deformation, a n d b e n d i n g of ductile a n d cleavable grains. These diagenetic features are c o m m o n to a large n u m b e r of c o n t i n e n t a l red b e d s (Walker 1967, 1976; Walker e t a L , 1978; Turner, 1980). M e c h a n i c a l Infiltration o f Clay Mechanically infiltrated clay r e p r e s e n t s the earliest diageneUc process in the Nubia Formation, particularly in the lowest part of the sections. It is p r e s e n t locally at o t h e r horizons. Using the SEM, there is no difficulty in identiflng clay of this origin a n d it c a n be easily d i s t i n g u i s h e d from replacem e n t or authlgenic clay. Three textural varieties of mechanically infiltrated clay have b e e n detected. The first texture is m a r k e d by clay material t h a t occurs in pores a n d m a y fill or nearly fill interstitial voids (Fig. 2A) or occurs as seepages w h i c h percolate t h r o u g h the s e d i m e n t with or w i t h o u t a preferred orientation, d e p e n d i n g on t h e textural a n d petrophysical properties. The second texture occurs as grain coatings with different clay-sized platelets t h a t are oriented parallel to t h e grain surface. The third texture h a s oriented coatings or "skins" (Fig. 2B), a texture previously identified by Walker et aL (1978). Mechanical infiltration of clay into p o r o u s a n d permeable s e d i m e n t s as grain coatings is a ubiquit o u s feature in alluvial red beds. Most coarsegrained alluvium is essentially free of interstitial clay w h e n first deposited (Walker eta/., 1978} d u e to sorting by s t r e a m flow. The soil horizon at t h e base of the Nubia F o r m a t i o n m a y have served as a s u p p l y for clay. D i s s o l u t i o n o f F r a m e w o r k Silicates In the Nubia Formation, s e c o n d a r y porosity p r o d u c e d by the dissolution of detrital grains a n d possibly authigenic m i n e r a l s is extensive. The complete a b s e n c e of pyroxene, amphibole, a n d other u n s t a b l e heavy m i n e r a l s is probably t h e result of extensive dissolution, b e c a u s e t h e source rock is s u p p o s e d to be either igneous or m e t a m o r phic. However, t h e relics or complete dissolution of grains t h a t have b e e n observed frequently (Fig. 2C,
346
Am'AR A. AaDm -W,~q,~n and P. A
4
Fig. 2 ~L pore-filling infllterated clay. PPL, EI-Anbagi area. B. Clay skin coating the framework grains and oriented parallel to the grain surface. SEM, EI-Anbagl area. C. Oversized pores attributed to complete dissolution (DG), Wadi Hammamat area. D. Oversized pores due to partial dissolution, PPL, EI-Wekala EI-Hamra area. E. Complete dissolution ofdetrital grains with iron oxides and/or clay minerals indicating the original boundary. PPL, El-Wekala EI-Hamra area. F. Feldspar (F) partially dissolved along the cleavage planes. PPL, EI-Wekala EI-Hamra area. D, a n d E) m a y b e a t t r i b u t e d to a m p h i b o l e a n d p y r o x e n e dissolution. The dissolution t h a t t a k e s place in this m a n n e r m a y leave no clue to t h e n a t u r e of t h e original grains. T h e d i s s o l u t i o n of f e l d s p a r s in t h e N u b i a F o r m a t i o n is a crucial feature. Chemical a n a l y s i s a s well a s m o d a l a n a l y s i s indicates t h a t t h e s e s a n d s t o n e s are p o o r i n alkalies, w h i c h s u g g e s t s t h a t m o s t of t h e f e l d s p a r s were dissolved. Petro-
graphic o b s e r v a t i o n s indicate t h a t m o s t of t h e r e m n a n t s are strongly altered. The d i s s o l u t i o n of silicate g r a i n s t a k e s place b y hydrolysis. Na, Ca, a n d Mg are r e l e a s e d from t h e silicate f r a m e w o r k r a t h e r easily, w h e r e a s Fe, AI, a n d Ti t e n d to r e m a i n b e h i n d (Turner 1980). The a c t u a l n a t u r e of t h e hydrolytic r e a c t i o n d e p e n d s u p o n the c o m p o s i t i o n of the m i n e r a l involved. For p o t a s h f e l d s p a r it m a y b e written:
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
347
>KAIsSLtO2o(OH)4+4K÷ grains, b u t also authigenic K-feldspar. The released K, Al, a n d Si p r o d u c e d during t h e break+ 8H4SiO 4 down of feldspars m a y be responsible for the K-feldspar inite f o r m a t i o n of a u t h i g e n i c p o t a s s i u m m i n e r a l s K-feldspar grains t h a t have b e e n etched by dis- including illite (Fig. 3B). Microcline is the m o s t frequent K-feldspar mineSolution are a very c o m m o n feature (Figs. 2F a n d 3A). In general, t h e dissolution t a k e s place prefer- ral due to its high stability. However, it shows high entially along the cleavage plane of the grains degrees of alteration embracing various degrees of (Fig. 2F). The dissolution involves not only detrital c r o s s h a t c h twining obliteration (Fig. 3C). 5KA1ShOs+ 4H'+ 16H20
Fig. 3 A. Feldspar dissolution having deep surface voids. SEM. B. Authigenic K-feldspar (AKF) post-datlng authigenie fllite (AIL}.SEM,Wadi Hammamat. C. alteration ofmlcrocllne (Mic)displayingvarlous degrees of cross-hatched twinning obliteration. XN, EI-Anbagi area. D. The dissolution of siderite (Sid). SEM, Wadl Hammamat area, E. The dissolution ofauthigenic calcite ACa). SEM, Wadl Hammamat. F. The dissolution of quartz overgrowth and/or the detrltal grain and partial replacement by clay. (CI), Wadi Hammamat area.
348
ANTARA. ABDEL-WAHABand P. TURNER
It is n o t e w o r t h y t h a t m o s t of t h e m e t a s t a b l e a n d u n s t a b l e heavy m i n e r a l s have b e e n dissolved or strongly altered to opaques, a n d t h e heavy mineral suites are r e p r e s e n t e d by t h e ZTR group. This m a y indicate t h e s t r o n g effect of i n t r a s t r a t u m solutions. The dissolution of c a r b o n a t e minerals (siderite a n d calcite) is a very c o m m o n feature (Fig. 3D, E). Its effect is considerable in producing large a m o u n t s of s e c o n d a r y pores. The features previously m e n t i o n e d indicate t h a t t h e dissolution p r o c e s s started early, before t h e formation of authlgenic minerals, a n d c u l m i n a t e d d u r i n g shallow or i n t e r m e d i a t e burial (mesodiagenesis).
Replacement by Clay The clay r e p l a c e m e n t process is closely related to the dissolution process. As a result, different elements released into t h e interstitial solution were deposited as authigenic minerals. The two processes are responsible for c h a n g i n g the extent of textural a n d mineralogical m a t u r i t y as well as the b u l k chemical composition of the sediment. In the Nubia Formation, clay r e p l a c e m e n t is very c o m m o n in individual mineral grains (Fig. 3F) a n d rock fragments. Many features reported herein are very similar to t h o s e described by Walker e t al. (1978). S u c h r e p l a c e m e n t s are considered authigenic (Wilson a n d Pittman, 1977). Very little ferromagnesian minerals (mostly hornblende) are present, b e c a u s e m o s t have been completely dissolved leaving oversized pore spaces. Using t h e EDAX analyzer with SEM, differences in chemical c o m p o s i t i o n were noted along a traverse of a n altered h o r n b l e n d e (Fig. 4). Ca, Fe, a n d Mg progressively decrease toward t h e alteration product, a c c o m p a n i e d by a n increase in b o t h Al a n d Si. A m a r k e d increase in iron oxide n e a r the original h o r n b l e n d e grain w a s also seen. Clay replacement is not confined to detrital grains, b u t is also p r e s e n t in authigenic K-feldspar. The r e p l a c e m e n t of authigenic K-feldspar by authigenic illite is a c o m m o n feature (Fig. 5A).
Neomorphism The t e r m n e o m o r p h i s m is u s e d here in the sense of Kantorowicz (1983) as the in situ s t r u c t u r a l alteration of one p h a s e to a n o t h e r involving t h e loss a n d / o r addition of n o n - s t r u c t u r a l or mobile cations w i t h o u t dissolution a n d reprecipitation of the original p h a s e . This t e r m s h o u l d not be confused with neoformation, in w h i c h a new p h a s e is formed from the interstitial solution. Neomorphism a n d dissolution are closely associated. Kaolinite, w h i c h formed as a result of mica a n d feldspar alteration, is c o m m o n (Fig. 5B) a n d quite different in s h a p e from t h e neoformed type. The
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(I}
(2}
% % SiO2 42.9 44.3 AI203 II.2 19.3 TiOz 1.46 1.02 Fe203 16.3 II.2 MgO I0.4 6.32 CaO 12.1 7.21 NazO 0.97 0.42 KzO 0.76 0.38
(3)
(4}
(5}
% 45.1 24.2 0.63 6.31 3.11 2.15 0.II 0.19
% 46.8 39.5 0.12 1.31 0.13 0.41 0.00 0.07
% 44.7 12.1 1.31 15.9 II.2 11.3 1.21 0.69
Flg. 4. Compositional variation during the replacement of ferromagnesian minerals by clay (kaolinite). The data based on EDAX analysis.
former still preserves t h e p r e c u r s o r grain habit, a n d t h e alteration c o n t a c t c a n be seen. Moreover, t h e perfect crystalline forms were missing.
Authigenic Minerals A u t l ~ e n l c m i n e r a l s identified in t h e p r e s e n t s t u d y Include two categorles: non-clayey mlnerals s u c h as quartz, feldspar, a n d iron oxides; a n d authlgenlc clay minerals. Authlgenic quartz Quartz overgrowth in the Nubia F o r m a t i o n is hlghlyvarlable. We believe t h a t the variation of quartz c e m e n t a t i o n c a n be u n d e r s tood a n d interpreted t h r o u g h p o r e - s p a c e availability within the rock before c e m e n t a t i o n . T h e s e pores were c e m e n t e d before c o m p a c t i o n by carbon a t e at a very early stage of diagenesis. The developed overgrowth m a y be c o n s i d e r e d as a f u n c t i o n of dissolved c a r b o n a t e or unfllled pores (Fig. 5C). Many quartz overgrowth f e a t u r e s m e n t i o n e d
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
349
Fig. 5 A. The replacement ofau thigenic K-feldspar(AKF)by authigenic illlte(AIL}.SEM, El-Wekala EI-Hamm area. B. Neomorphosls of feldspar to vermiform Kaolinite. SEM, Wadi H a m m a m a t area. C. Patches of calcite a s pore-filllng which is still incompletely dissolved. XN, El-WekalA El-Hamra area. D. Quartz overgrowth with euhedral faces grown on detrltal grains coated with clay skin. SEM, Wadi H a m n m m a t area. E. The quartz overgrowths merge forming incomplete crystal faces. SEM, EI-Wekala EI-Hamra area. F. Overgrowth of prismatic projections on monocrystalline grains forming a large prismatic form. SEM, EI-Anbagi area
previously by W a u g h (1970a, b), Pittman (1972), and others were identified in the present samples. Quartz overgrowth with euhedral faces has occurred on detrital grains, and is a c o m m o n feature (Fig. 5D). Due to saturation conditions, two stages of the overgrowth on the grain surface merge forming incomplete crystal faces (Fig. 5E). At first glance, s u c h a feature m a y lead to its interpreta-
tion as dissolution features. An advanced stage in the formation of euhedral, prismatic crystals (Fig. 5F) is represented, from which perfect crystal faces of authigenic quartz cement developed locally, having hexagonal pyramidal form (Fig. 6A). A lot of work has been reported on the origin of secondary silica (Sippel, 1968; Hawkins, 1978;
350
AbrrARA. ABDEL-WAHABand P.
C
AI
Si
EDAX Analysis NA 0~5 K AL 12.2 SI 67.4 ~A 18.9 0.17 BA 0.18
7
Fig. 6 A. A perfect crystal ofauthigenlc quartz having hexagonal pyramidal termination s to prism faces. SEM, El-Wekala El-Hamra area. B. Further stage of K-feldspar (AKF) forming large crystal faces (adularla). SEM, Wadl Hammamat area. C. EDAX spot analysis {x}indicating K-feldspar composition (marked point in B). D. K-feldspar in the form of tabular prismatic crystals showing signs of dissolution. SEM, Wadi Hammamat area. E. Authlgenlc K-feldspar (AIG~developed on a grain surface coated with authigenic fllite (AIL).SEM, El-Wekala EI-Hamra area. F. K-feldspar (AKF)altered to authigenic illlte (AIL).SEM.
and Hutcheon, 1983). The dissolution of feldspar and rock fragments and their replacement by clays, as well as the replacement of quartz, mica, and feldspar, are the most reasonable potential sources of silica.
hedron-like crystals of authigenic monoclinic Kfeldspars (term of Baskin,1956). The second is tabular prismaUc crystals (Fig. 6D). These forms were previously described by Stablein and Dapples (1977). Authlgenic K-feldspars also occur as pore fillings, Authlgenlc Feldspars Two different habits of locally forming from authigenic fllite (they postauthigenic K-feldspar are present in the Nubia. date mite) (Fig. 6E). A later stage of authigenic fllite The first is adularia (Fig. 6B, C), tiny rhombo- formed from authigenlc K-feldspar (Fig. 6F).
Diagenesis of the Nubia Formation, CentralEastern Desert,Egypt
351
The growth of authlgenic K-feldspar in sedimen- kaolinite/dickite as both neoformed and neotary deposits requires t h a t the diagenetic environ- morphosed particles requires substantial acid water m e n t be enriched with K, Al, and Si ions (Waugh flux ( H e l g e s o n et al., 1969; F a n n i n g a n d (1978)). These ions could be released by the disso- Keramides, 1977). Smectite a n d mixed-layer fllite-smectite interlution of detrital alkali feldspar and other silicate framework grains from alkaline intrastrata solu- preted to be ofauthigenic origin are present but not tions. The chemical conditions required for the abundant. These phases display flake-like crystals precipitation ofauthlgenic feldspars are controned which often curl up from the grain surface at high by the pH and relative a m o u n t s of various cations: angles (Fig. 7E). Thls curly appearance m a y result K, Na, Ca, a n d Si. A relatively high Na*/H ÷ or K÷/ from dehydration, perhaps during the SEM coatH ÷ ratio associated with high H4SiO4 activities is ing process. Their presence in small a m o u n t s needed for feldspar authigenesis. The alkali m e t a l / supports the belief that the Nubia Formation h a s hydrogen-ion ratio, with a m i n i m u m a m o u n t of not undergone very deep burial. Chlorite is less a b u n d a n t and is present in the H4SiO4in solution, is indicative of feldspar stability with respect to a solution and t h u s indicative form of pore-fill rosettes growing normal to detrital of its possible precipitation (Garrels and Christ, grains (Fig. 7F). Honeycomb chlorite is another 1965; PettiJohn et aL, 1972). Hemley and J o n e s c o m m o n variety. Authlgenic fllite is present locally, generally as (1964) suggested t h a t slightly elevated temperatures with moderate to deep burial are convenient pore-linlng t h a t coats detrital grains. A hair-llke, conditions for feldspar precipitation. However, delicate form is c o m m o n (Fig. 8A). A flake-like illite Kastner and Siever (1979) stated that temperature that is commonly curled is also present. A boxwork is not the m o s t important consideration for feld- or cellular texture of authigenic fllite is recognized spar authlgenesis and t h a t there is no minimal in a few samples (Fig. 8B). depth of burial t h a t would correspond to m i n i m u m In the present study, two s u b s e q u e n t authlgenic geothermal heating before authlgenesis can occur. illites are recognized. The first was formed at a n The authlgenic feldspars (mainly K-feldspars) in early stage of diagenesis before authigenic K-feldthe Nubia Formation seem to have formed at low spar (Fig. 6E). The other post-dates the authigenic temperatures; stratigraphic data suggest the Nubia K-feldspar and formed from adularia dissolution (Fig. 6F). Formation h a s not been deeply buried.
Clay Mlneralm Authigenic kaolinite (including dickite) is present in nearly all samples of the Nubia Formation in a m o u n t s up to several percent. However, there are local occurrences of expandable clays (smectite and mixed-layer lnite-smectite), several morphologic types of fllite, and traces of chlorite and mixedlayer chlorite-vermiculite. Kaolinite is present as both pore-fill and porelining. P s e u d o h e x a g o n a l plates are a r r a n g e d normal to the grain surface {Fig. 7A). A pseudohexagonal pore-filling crystal with monoclinic habit and twinning typical of dickite is also present. Dickite consists of m a n y t h i n n e r lamellae {Fig. 713). Thick packets lacking parallel lamellae are also present. Bohor and Hughes {1971) could not identify the factor controlling packet thickness. Kaolinite with a delicate crystalline habit in the form of rosettes (Fig. 7C) is also present. It is also pore-fill, and displays typical vermicular form. It consists of stacked, long sequences ofpseudohexagonal plates growing between detrital grains and exhibiting bridges (Fig. 7D). Most authigenic kaolinite herein is neoformed, precipitated from interstitial pore water or neomorphosed, formed from a precursor silicate framework. The identification of dickite is based on SEM coupled with the XRD analysis. The dissolution of feldspars and formation of
Iron Oxide Minerals Iron oxide minerals include hematite, martite and goethite. Hematite is ubiquitous in the Nubia Formation. Botryoidal and cellular textures are found in the form of pore lining and interstitial matrix (Fig. 8C), where they are closely associated with clay minerals. Crystalline hematite ofacicular habit, fringing the pore spaces and coating detrital quartz (Flg. 8D), is very c o m m o n in the uppermost part of the Wadi H a m m a m a t section. Another c o m m o n form is elongate laths which coat quartz overgrowths and are oriented parallel with the grain surface (Fig. 8E). In m a n y samples, more t h a n one generation of hematite is present, which suggests the persistence of reddening for a long time. It is not clear w h e t h e r hematite (alpha Fe~Os) forms directly from magnetite or maghemite (gamma Fe20 s) or a phase intermediate between Fe2Osand Fe30 ~ is involved (Turner, 1980). Martite is c o m m o n in the u p p e r m o s t part of the Wadi H a m m a m a t section, where it is associated with hematite and is clearly a low-temperature authlgenic mineral. It probably formed from the oxidation of magnetite or titanomagnetite. The apparent absence of magnetite in the samples studied suggests t h a t it m a y have been the precursor of hematite (cf. Turner, 1980). Authigenic goethite was identified only in the
352
Am'As A. ABDEL-WAHABand P. TURNER
i
A
Fig. 7/k Pseudo-hexagonal plates of kaolinlte arranged normal to the grain surface.SEM, Wadi Hammamat area. B. A typical dickite(DK)ofbook shape displaying pseudohexagonal, stacked plates with characteristic twinning and parallel lamellae. SEM, EI-Anbagi area. C. Pseudo-hexagonal idiomorphic form with a delicate crystalline habit in the form of rosettes. SEM, Wadi Hammamat area. D. A kaolinite pore-fill displaying typical vermlcular form, consisting of pseudo-hexagonal plates, growing between the detrital grains and exhibiting bridges. SEM, EI-Anbagiarea. E. Flake-like smectlte crystals (Sm)which curl up from the grain surface. SEM, El-Wekala EI-Hamra area. F. Authlgenic chlorite in the form of rosettes developed as pore-llning. SEM, EI-Anbagi area. u p p e r m o s t h o r i z o n of W a d i H a m m a m a t . It w a s observed mainly along the cross-bedding planes, h i g h l y c e m e n t e d a n d w i t h a d a r k b r o w n metallic a p p e a r a n c e . In o t h e r c a s e s , it f o r m s s p h e r i c a l c o n c r e t i o n s . A n o t h e r c o m m o n f o r m is s p h e r i c a l w i t h i n t e r n a l , relatively t h i c k , g r o w t h b a n d s t h a t a re r o u g h l y c o n c e n t r i c . T h e s e b a n d s r e p r e s e n t
z o n e s of d e n s e r c e m e n t a t i o n b y i r o n o x i d e s a r r a n g e d a l t e r n a t e l y w i t h o t h e r , l ess c e m e n t e d bands. G o e t h i t e o c c u r s m a i n l y a s a p o r e fill. B o t r y o i d a l t e x t u r e is c l e a r l y o b s e r v e d in t h e c e n t r a l p a r t s of t h e voi ds (Fig. 8F]. A wide r a n g e of f e a t u r e s is d i s p l a y e d b y d e t r i t a l g r a i n s r e p l a c e d b y goethite. A
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
353
Fig. 8. A. Authigenic filite in a halr-like, delicate form coaUng the detrital grains. SEM,EI-Anbagl area. B. Authigenic fllite in the form ofboxwork or cellular textures as pore-lining normal to detrltal grain surfaces. SEM, EI-Anbagi area. C. Interstitial pore-filling and pore-lining hematite displaying botryoidal and cellular texture. SEM,EI-Anbagi area. D. Crystalline hematite (AHM) of tiny fibrous shape developed as pore-fill. SEM, EI-Wekala EI-Hamra area. E. Laths of elongate shape, hematite (AHM) coaUng the authigenic quartz (AQ) and oriented parallel to the grain surface. SEM, Wadi Hammamat area. F. A typical botryoidal texture of goethite showing two or more generations formed during later stages of diagenesis. SEM, uppermost part of Wadi Hammamat area. m u s c o v i t e g r a i n replaced by goethite is one clear example of s u c h r e p l a c e m e n t (Fig. 9A). Replacem e n t t o o k place along t h e cleavage p l a n e s of the mica. R e p l a c e m e n t of detrital gains, s u c h as felds p a r a n d quartz, w a s also observed.
Titanium oxide Authigenic t i t a n i u m oxides are c o m m o n in the Nubia F o r m a t i o n a n d o c c u r m a i n l y as e u h e d r a l prismatic c r y s t a l s or a s small discrete c r y s t a l s of a n a t a s e or rutfle. The t i t a n i u m ions m a y have b e e n released d u r i n g t h e m a r t i t i z a t i o n of detrital titano-
354
ANTARA, ABDEL-WAHABand P. TURNER
D
,
5Oum
b
~
Fig. 9. A. Authlgenic goethite (GOE) replacing detrital mica (M), in reflected llght,off. PPL, uppermost part ofWadl Hammamat area. ]3. Authigenle carbonates developed as pore -filldisplaying a perfec t crystal rhombohedral form. SEM, Wadi Hammamat area. C. An advanced stage of calcite replacement (Ca), relices of quartz (Q) are left. El-Wekala EI-Hamra area. D. Carbonate pore-filling (Ca) which took place post-datlng the authigenic quartz (AQ). E. Corroded grains previously replaced by carbonate and infllled later by hematite. XN, El- Wekala EI-Hamra area. F. Abnormal large pores as a result of carbonate replacement a n d / o r cement which subsequently dissolved and produced oversized pores.PPL, Wadi Hammamat area.
magnetite as suggested by Ixer et al. (1979) or by the alteration of other titanium-bearing minerals, s u c h as sphene. Carbonates
Authigenic poikflotopic a n d sparry calcite cem e n t (Fig. 9b) h a s a p a t c h y distribution in the Nubia Formation. ranging from 0 to 8% a n d
averaging 2.5%. Various quartz grains display corroded a n d e m b a y e d borders s u r r o u n d e d by patchy carbonate (Figs 5C a n d 9C), as weU as carbonate pore flU post-datlng well-formed quartz overgrowths (Fig. 9D). C e m e n t stratigraphy indicates there were two generations of calcite. Nonferroan calcite took place at a very early stage of diagenesis a n d hindered complete pore filling by
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt
355
originated w h e n originany adjacent grains of feldspar and other unstable minerals were removed by dissolution; 4. more t h a n 80 % of the grains have less t h a n four contacts, and more t h a n 92 % of the grain contacts are floating, tangential,~or long; 5. shallow, early cementation is'suggested by the relatively low values of contact strength (1.55) and the consolidation factor (48.6 % }. In comparison, Triassic sandstones in Nottinghamshire, England, that u n d e r w e n t intermediate burial depths have a consolidation factor of 52.5 and a contact strength of 1.83 and contact index of 1.56 (Serf EI-Dein, 1983); 6. the mild effect of compaction suggested by absence of extensively fractured grains, strongly bent micas, or squeezed clay clasts. However, local, u n c o m m o n compactional features are prePorosity and l>lagen~is sent locally; 7. the large variation in quartz cement, which The porosity of Nubia sandstone samples ranges from 5 % to 39 % of rock volume; most of it is m a y have been controlled by earlier calcite cement; 8. e m b a y m e n t s in quartz overgrowths that interpreted to be of secondary origin. Low porosity samples have a b u n d a n t authigenic calcite, quartz, are attributed to be the result of corrosion by and iron oxides. calcite cement. The identification of secondary porosity in the The development of secondary porosity in the Nubia Formation is based on the following criteria: Nubia Formation during the different diagenetic 1. fractured silicate grains that formed as a stages is summarized in a schematic diagram result of compaction u p o n s u d d e n leaching of (Fig. I0). carbonate cement; 2. s a n d s t o n e s with patchy r e m n a n t s of SUMMARYANDCONCLUSIONS corroded calcite c e m e n t and with corroded quartz grains. The texture suggests t h a t a once-extensive Facies and paleocurrent analyses identified four calcite cement, which corroded quartz grains, has lithofacies. They are: root-turbated and biobeen dissolved. Many of these secondary pores turbated clayey sfltstone (paleosol), trough crosswere later infilled later by hematite (Fig. 9E); bedded pebbly sandstone, planar-tabular cross3. partial to complete dissolution of detrital bedded sandstone offluviatile origin, and goethitegrains. Most of the leached grains were probably c e m e n t e d t a b u l a r c r o s s - b e d d e d s a n d s t o n e of feldspar and volcanic rock fragments (Fig. 2C-F). coastal and shallow-marlne origin. The first three In places the grain boundaries are only represent- facies are identified at most of the localities, but ed by clay or iron oxide rims; the last is recognized only in the u p p e r m o s t part 4. oversized pores derived from either of the Wadi H a m m a m a t locality. detrital grain dissolution or from dissolution of The climate was hot and humid, a n d intense carbonate-replaced grains (Fig. 9F): weathering was responsible for the formation of a 5. patchy carbonate c e m e n t with undis- soft horizon a few m e t e r s thick at the base of the solved r e m n a n t s (Fig. 5C). trough cross-bedded sandstones. Extensive The Nubia Formation was c e m e n t e d by weathering is indicated by the scarcity of feldspars calcite before burial, during a very early diagenetic and absence of ferromagnesian minerals, the prestage, followed shortly by quartz overgrowth. This ponderance of kaolinite in the matrix, and repeata s s u m p t i o n is derived from the following observa- ed weathering profiles on the s a n d bodies. tions: Paleocurrent data indicate t h a t a unimodal 1. the high pre-cement porosity (25 to 40 %) pattern is the predominant type, generally directed and secondary porosity; toward the north, northeast, or northwest. Poly2. the very low value of s u t u r e d contacts (2.7 modal trends, which have been found in the upper0/6) and the scarcity of concavo-convex contacts most part associated with p l a n a r - t a b u l a r cross(1.5 %); bedding, probably formed as a result of the 3. the a b u n d a n c e of floating grains (7.6 %) is T e t h y a n m a r i n e t r a n s g r e s s i o n d u r i n g Late exceptionally large for a sandstone that has been Cretaceous time. buried at least 1400 m. Some floating grains The diagenetic history of the Nubia Formation quartz, whereas, ferroan calcite post-dates the quartz overgrowths and was probably produced at a very late stage. The a b u n d a n c e of large, oversize pores, floating grains, corroded and etched quartz grains in the Nubia suggest t h a t calcite cement was once m u c h more a b u n d a n t but was subsequently dissolved. Many sandstones without calcite c e m e n t are semi-friable. The replacement of the margins ofdetrital quartz grains and some quartz c e m e n t by carbonate is a c o m m o n feature. F r i e d m a n e t a/, (1976) described various stages of quartz dissolution, initially with a V-shape a n d prismatic etch pattern. Advanced dissolution of SiO 2 and s y n c h r o n o u s precipitation of CaCO s encourages the formation of pseudom o r p h o u s carbonate after quartz (Fig. 9C) on a small scale.
356
AWrARA. ABDEL-WAHABand P.
%.
Eodiagenesis
%1 °..
~ m
ml~a,:~I/,.,.,.~:
Carbonam
Silica rele~xl
/~ovettiz~ poreprodu~on)
Mesod~genesis
(mature)
2
[,am generation of carbonate Fig. 10. Schematic diagram Indicating the developmentof secondary porosity during the different stages ofdlagenesis. took place in four stages: a} the eodm___genetlcstage = changes t h a t took place soon after the rock was deposited: i) early polkflotoplc calcite cement, tO quartz overgrowth, Ill} m i n o r dissolution of feldspar and formation of kaolinite, iv) coloration by iron oxides, which started early and took place over a long period of time. The net product was release of ions (SI, AI. K, Ca, Mg, Fe) to pore waters: b) the i m m a t u r e m e s o d l a g e n e t l c stage {mechanical compaction) had a m i n o r role. Although the effect of compaction was minimized
because of early cementation, observed features of compaction include: fracturing of brittle material, bending of ductile grains, mtcrostylolites, and concavo-convex contacts. Minor pressuresolution effects, and minor reduction of porosity continued: c} mature mcsodiagcncsis involved the production of vast secondary porosity through mesogenetic decarbonaUzation. Dissolution and breakdown of remaining feldspar and rock fragments took place at this stage. A well-developed authlgenlc kaolinite was probably produced. During the
Diagenesis of the Nubia Formation, Central Eastern Desert, Egypt g e n e r a t i o n of s e c o n d a r y porosity, c a r b o n a t e s were dissolved b y acidic w a t e r w i t h consequent rise m pH, w h i c h c a u s e d s u p e r s a t u r a t i o n w i t h r e s p e c t to kaolinite if d i s s o l v e d A1 a n d S i w e r e present. It also included: i) c o n v e r s i o n of kaolinite to a highly cryst_anlne form (dickite), ii) highly acidic w a t e r w h i c h led to v e r m l c u l a r kaolinite, iii) in s i t u illite, followed b y a u t h l g e n i c K-feldspar, followed b y a s e c o n d g e n e r a t i o n of illite, iv) martite, w h i c h m a y h a v e f o r m e d a s a r e s u l t of oxidized magnetite, a n d v} c o n v e r s i o n of s m e c t i t e to tllite. The i m p o r t a n c e of t h e llllte-smectite t r a n s i t i o n a s a s o u r c e of w a t e r a n d c a t i o n s in t h e N u b i a F o r m a t i o n is difficult to establish. Although there are no substantial m a r i n e m u d r o c k s in t h e s t u d y area, a d j a c e n t a r e a s m o r e deeply b u r i e d c o u l d have b e e n a s u i t a b l e s o u r c e . T h e p r e s e n c e of s m e c t i t e a n d fllitesmectite suggests that the Nubia Formation has n o t s u f f e r e d v e r y d e e p b u r i a l or m a y have f o r m e d after t h e b u r i a l process; d) t h e telodiagenetic (post-burial) stage = all t h e p r o c e s s e s f r o m b u r i a l u p to t h e present. The m a r i n e t r a n s g r e s s i o n w o u l d h a v e p r o d u c e d significant c h a n g e s in p H a n d E h of t h e circulating pore solutions, d u e to t h e mixing of t h e alkaline, hypersaline m a r i n e w a t e r s . This mixing m a y have led to t h e n e o f o r m a t i o n of a u t h i g e n i c minerals: i} devel o p m e n t of a u t h l g e n i c p y r a m i d a l or b i p y r a m i d a l p r i s m a t i c quartz, ii) smectite, chlorite, or v e r m i c u lite d e v e l o p m e n t , a n d ill) late c a r b o n a t e generation. This n e o f o r m a t i o n w a s a r e s u l t of t h e e x c e s s of Ca ~÷, HCOs. a n d Fe with CO s liberated b y t h e r e d u c t i o n of organic m a t t e r , p o s t - d a t i n g t h e late b i p y r a m i d a l q u a r t z plugging of t h e available pore s y s t e m . S e c o n d a r y p o r o s i t y is slightly r e d u c e d d u e to late c a r b o n a t e g e n e r a t i o n , a n d goethite a n d h e m a t i t e pore filling. Calcite w a s p r o b a b l y derived from t h e overlying C r e t a c e o u s m a r i n e l i m e s t o n e s of t h e Q u e s i r F o r m a t i o n . A c k n o w l e d g e m e n t s - The first author is greatly indebted to Prof. Earle F. McBride for his review and critical comments. Rosemary Brant is thanked for typing an early draft. REFERENCES
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