Geo log i cal Quar terly, 2017, 61 (1): 239–250 DOI: http://dx.doi.org/10.7306/gq.1334
Geo chem i cal sig na tures of per va sive me te oric diagenesis of Early Mio cene syn-rift car bon ate plat form, Red Sea, NW Saudi Ara bia
Khalid AL-RAMADAN1, *
1 King Fahd Uni ver sity of Pe tro leum & Min er als, Geosciences De part ment, 3126, P.O. Box 1400, Dhahran, Saudi Ara bia
Al-Ramadan, K., 2017. Geo chem i cal sig na tures of per va sive me te oric diagenesis of Early Mio cene syn-rift car bon ate plat - form, Red Sea, NW Saudi Ara bia. Geo log i cal Quar terly, 61 (1): 239–250, doi: 10.7306/gq.1334
Dif fer ent diagenetic en vi ron ments have been rec og nized in the Early Mio cene car bon ate plat form of Musayr For ma tion in the Red Sea rift area. Early ma rine diagenesis in cludes micritisation that oc curs as thin en ve lope around skel e tal and non-skel e - tal grains in low-en ergy mud dom i nated fa cies and isopachous fi brous cal cite in high-en ergy grain-dom i nated fa cies. Per va - sive me te oric wa ter diagenesis re sulted in ce men ta tion of the car bon ates by coarse-crys tal line blocky-drusy cal cite and me nis cus ce ments. De ple tion of ox y gen (avg. –9.08‰), car bon (avg. –1.6‰) iso topes and trace el e ments con cen tra tions (avg. val ues of Fe: 1387 ppm; Mn: 1444 ppm; Sr: 419 ppm; Na: 1194 ppm) in con junc tion with neg a tive cor re la tion be tween Mn2+ and ox y gen iso tope data sug gest vari able de grees of fluid-rock in ter ac tions and per va sive me te oric diagenesis. The for ma tion of me te oric diagenesis in the Musayr For ma tion can be ex plained by two sub se quent mech a nisms: (1) the pres - ence of me te oric lenses dur ing the time of de po si tion might have been as so ci ated with ac tive fresh wa ter in put from the hin - ter land (NE) due to fall in the rel a tive sea level; (2) later up lift ep i sode dur ing Plio-Pleis to cene may have also con trib uted to the per va sive me te oric diagenetic al ter ations of the car bon ates of the Musayr For ma tion. The first mech a nism is sup ported by the ce ment stra tig ra phy where the blocky-drusy ce ments post date the me nis cus ce ment. The lat ter mech a nism seems to have more pro nounced ef fect on the al ter ation of Musayr car bon ate se quence by ob serv ing the occurence of late ce ments such as blocky cal cite in most of the sam ples. The im pact of me te oric diagenesis on the stud ied sam ples sug gest that dis so - lu tion is less se vere than ce men ta tion, hence the vis i ble po ros ity is very low. Un der stand ing the tim ing of me te oric diagenesis pro vides use ful in for ma tion about the res er voir qual ity dis tri bu tion in syn-rift car bon ate se quences.
Key words: me te oric diagenesis, car bon ate, sta ble iso topes, trace el e ments, Red Sea, Mio cene.
INTRODUCTION
Con strain ing the dis tri bu tion pat tern of car bon ate diagenesis has been a ma jor re search topic for de cades. Many re search ers (e.g., Bathurst, 1975; Allan and Matthews, 1982;
Tucker, 1986; Bishop et al., 2014; Swart, 2015) have high - lighted the sen si tiv ity of metastable ar agon ite and high-Mg cal - cite (HMC) to the changes of pore wa ter chem is try. The me te - oric en vi ron ment, in par tic u lar, plays a key role in the po ros ity de vel op ment in car bon ate res er voirs (e.g., Smalley et al., 1994;
Bishop et al., 2014; Dick son and Kenter, 2014; Ma jor, 2014).
How ever, such stud ies have en coun tered many dif fi cul ties in rec og niz ing the fea tures and es tab lish the im pact of me te oric diagenesis from petrographic ev i dence solely. There fore, re - cent stud ies (e.g., Melim et al., 2002; Heba et al., 2009; Swart, 2015) have sug gested in te gra tion be tween petrographic data
and sta ble ox y gen and car bon iso topes, in con junc tion with trace el e men tal anal y sis (Fe, Mn, Sr, Na) of the bulk car bon ate sam ples can be used to de ci pher the pres ence of me te oric diagenesis realm in an cient car bon ate se quences.
The dis tri bu tion and con cen tra tion of some trace el e ments (e.g., Fe, Mg, Sr, Mn) in car bon ate sed i ments which are strongly in flu enced by depositional and diagenetic pro cesses, are of ten used as en vi ron men tal in di ca tors. For ex am ple, Sr which is sen si tive to depositional and diagenetic en vi ron ments (Veizer and Demovic, 1974), has been used as a fa cies in di ca - tor show ing in creas ing trend from backreef to forereef fa cies (Mor row and Mayers, 1978).
Sev eral stud ies on car bon and ox y gen iso to pic sig na tures have been con ducted, par tic u larly on Ho lo cene car bon ate de - pos its (Gross, 1964; Land and Ep stein, 1970; Allan and Matthews, 1977; Videtich, 1982; Melim et al., 2002). These stud ies have shown that the ox y gen iso to pic com po si tion of these calcitised me te oric ce ments is in vari ant that may re flect a nar row range of both rain wa ter sig na ture and tem per a ture. Car - bon iso tope sig na tures are en riched with in creas ing depth.
This study aims at us ing the geo chem i cal sig na tures to rec - og nize the me te oric diagenesis and at tempts to con strain its tim ing. The find ings of this study can be used as an a logue for other subsurface synrift car bon ate se quence else where.
* E-mail: ramadank@kfupm.edu.sa
Received: April 30, 2016; accepted: November 23, 2016; first published online: December 13, 2016
STRATIGRAPHIC FRAMEWORK SETTINGS
The stud ied car bon ate plat form suc ces sions of Musayr For - ma tion are well ex posed in the study ar eas of Midyan Pen in sula (Fig. 1). Midyan Pen in sula is lo cated within an ac tive tec tonic re gion that is greatly in flu enced by Late Oligocene mar i time Red Sea rift ing and Late Miocence left lat eral strike slip of Aqaba Fault (Fig. 1). Con ver gent plate mar gin? be gan in the Early Oligocene time at the Zagros su ture due to the NE move - ment of Af ri can–Ara bian Plate. This move ment led to crustal stretch ing and weak en ing along the proto Red Sea and Gulf of Aden (Hughes and John son, 2005), which re sulted in the for - ma tion of rift ba sin and de po si tion of rift se quences that span from Late Oligocene to Re cent.
The tectono-strati graphic evo lu tion of the study area can be sum ma rized in Fig ure 2. Dur ing the Late Oligocene, rift ing was well un der way and re sulted in the de po si tion of the first syn-rift sed i ments, the im ma ture fluvio-lac us trine de posit Al-Wajh For -
ma tion. Dur ing the tec tonic re or ga ni za tion, small-re stricted bas - ins were formed and al lowed the de po si tion of hypersaline basinal evaporites of the Yanbu For ma tion in the Early Mio cene time. Syn chro nous with this evaporites, the first ma jor car bon - ate suc ces sion was com menced in the study area rep re sented by the Musayr For ma tion which was de pos ited in the dif fer ent part of the half graben. This car bon ate suc ces sion was de pos - ited un der nor mal sa line to brack ish wa ter con di tions (Hughes and John son, 2005). Rapid sub si dence was sub se quently taken place af ter the de po si tion of car bon ate unit that led to the in creas ing ac com mo da tion space cre ation. It al lows de po si tion of deep wa ter turbidites of the Burqan For ma tion (Al-Ramadan et al., 2013). Tec tonic re ad just ment dur ing late Early Mio cene, which was as so ci ated with mid-clysmic events on the Gulf of Suez (Bosworth and McClay, 2001) had fa cil i tated the de vel op - ment of palaeohighs bounded by nor mal faults, al lowed the Wadi Waqb car bon ates of the Jabal Kibrit For ma tion to nu cle - ate (Tubbs et al., 2014). The Musayr For ma tion is strati gra - Fig. 1. Generalized geological map of the Midyan Peninsula (modified after Clark, 1986)
The Musayr Formation has been studied in two different locations as indicated in the map by number:
1 – Maqna locality, 2 – Wadi Al-Hamd locality
Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform... 241
Fig. 2. Lithostratigraphic column of the Red Sea area showing the rift phases and tectonic events from Late Cretaceous to present (modified after Tubbs et al., 2014)
phically equiv a lent to the syn rift, Gharamul Mem ber (reefal lime stones) of the Nukhul For ma tion in the Gulf of Suez (Saoudi and Khalil, 1986; Hughes and John son, 2005).
METHODOLOGY
A to tal of 40 rep re sen ta tive car bon ate sam ples from Musayr For ma tion were col lected along two strati graphic sec tions in two dif fer ent lo ca tions (Fig. 3). Thin sec tions pre pared for sam - ples im preg nated with blue ep oxy were stud ied us ing stan dard petrographic tech nique. The thin sec tions were stained with Aliz a rin Red S (Dick son, 1966) to dis crim i nate be tween cal cite and do lo mite. Scan ning elec tron mi cros copy (SEM) and X-ray Dif frac tion (XRD) were per formed to ana lyse the crys tal mor - phol ogy and bulk min er al ogy of the se lected sam ples. In duc - tively Cou pled Plasma Spec trom e try (ICP-OES) was con - ducted on fif teen rep re sen ta tive car bon ate sam ples in or der to de ter mine the con cen tra tion of trace el e ments (Fe, Mn, Sr and Na). Sim i lar sam ples were sub jected to car bon and ox y gen sta - ble iso topes anal y ses on bulk sam ples at Sta ble Iso tope Lab o - ra tory (SIL), Uni ver sity of Mi ami. These sam ples were ana lysed us ing a com mon acid bath in ter faced to a Finnigan-MAT 251 mass spec trom e ter on whole rock of rep re sen ta tive car bon ate sam ples. Data are re ported us ing a con ven tional no ta tion (VPDB) and cor rected for the usual in ter fer ence. The pre ci sion for this method is ±0.08‰.
RESULTS
LITHOFACIES
The Musayr For ma tion is com posed of eleven lithofacies (Ta ble 1), which are dis trib uted through out the hang ing wall dipslope car bon ate ramp (Koeshidayatullah et al., 2016). The in ner ramp en vi ron ment is char ac ter ized by the pres ence of basal arkosic sand stones with some peb ble lenses, stromatolitic boundstone and bioclastic packstones, which are interbedded with the siliciclastic de pos its (Fig. 4A). The ooidal and peloidal grainstones have been de pos ited in the shoal en vi - ron ment (Fig. 4B). Mid dle ramp en vi ron ment is the most dom i - nant en vi ron ment be ing mostly com posed of oys ter rudstone-float stone, bio-intraclastic packstones and foraminiferal packstone (Fig. 4C–F).
DIAGENESIS
Var i ous diagenetic pro cesses are rec og nized and dis tin - guished by the petrographic ob ser va tions of Musayr car bon ate sam ples from the two stud ied sec tions. The pe trog ra phy study shows that all diagenetic pro cesses, the most im por tant are linked with early diagenetic en vi ron ments, i.e. diagenetic al ter - ations caused by ma rine and me te oric wa ters. This in cludes a suite of al ter ations such as dolomitisation, ma rine ce men ta tion, leach ing and ce men ta tion, which can be ob served in the dif fer - ent parts of the Musayr For ma tion. Of less im por tance are burial pro cesses that are un der the realm of me chan i cal and chem i cal com pac tion, with stylolite and frac ture gen er a tion.
Micritisation. This pro cess oc curs at the ear li est stage of ma rine diagenesis and found at the bor der of both skel e tal and non-skel e tal grains (Fig. 5A). The micrite mostly ap pears in the form of a thin, micrite en ve lope in the outer rim of grains. Gen er - ally, the micrite helps the for ma tion of ce ment to nu cle ate and also pre serve the grains from sub se quent dis so lu tion. This pro -
cess is ob served in the Musayr For ma tion es pe cially in the bioclastic wackestone microfacies in low en ergy la goon en vi - ron ment.
Ce men ta tion. Dif fer ent types of car bon ate ce ments in the Musayr For ma tion were dis tin guished based on their mor phol - ogy, fab rics and tex ture. They are di vided into two groups of ma rine and me te oric ce ments as de scribed be low:
1. Ma rine ce ments
There are two type of ma rine ce ments that can be ob served in the stud ied sam ples: (1) isopachous fi brous cal cite hav ing size up to 100 mm in length and usu ally forms prior to com pac - tion (Fig. 4B). This ce ment com monly found oc cu py ing the pore spaces in grainstones and grain-dom i nated packstone. Al - though in each case pri mary tex tures can be well-pre served, they have un doubt edly un der gone var i ous de grees of recrystallisation (e.g., from metastable to sta ble mineralogies) and geo chem i cal al ter ation; (2) mo saic cal cite ce ment char ac - ter ized with a polycristalline na ture with xenomorphic pat tern.
This type of ce ment has size up to 200 mm in width and is mostly as so ci ated with frac tures and also fill the intra- and inter gra nu lar pore spaces. The pres ence of mo saic ce ment has sig nif i cantly caused po ros ity re duc tion (Fig. 4F).
2. Me te oric ce ments
Meteroic ce ments are found in many sam ples with three dif - fer ent forms: (1) drusy cal cite ce ment hav ing coarse crys tal (size up to 400 mm in width) and pro gres sively be comes coarser to wards the cen tre of the pore (Fig. 5B); and (2) blocky ce ment with var i ous crys tal size rang ing from 200 to 400 mm and has xenotopic crys tal fab ric. This ce ment oc curs mostly as pore fill - ing (Fig. 5C); (3) me nis cus ce ment hav ing fine crys tal size and is mostly rec og nized near grain to grain con tacts (Fig. 5D).
Dis so lu tion. The dis so lu tion pro cess took place in two dif - fer ent stages: (1) early stage of micrite and skel e tal grains dis - so lu tion that re sulted in for ma tion of intragranular and moldic po ros ity (Fig. 5E). This stage pre dates the me chan i cal com pac - tion where dis solved shells are readily and pref er en tially com - pacted; (2) late stage dis so lu tion of blocky-mo saic cal cite and do lo mite ce ments with po ros ity en hance ment (Fig. 4F).
Com pac tion. Me chan i cal and chem i cal compactions were ob served in the study sam ples. Me chan i cal com pac tion is rep - re sented by re or ga ni za tion, de for ma tion, and frac tur ing of bioclastic grains such as coralline-al gal frag ments and brachi o - pod/bi valve shells (Fig. 5F). Chem i cal com pac tion is ex em pli - fied by microstylolites (grain-to-grain su tured con tacts; Fig. 5D), and, lo cally, by more per sis tent sty lo lites and cross-cut ting grains (Fig. 5G).
Dolomitisation. Do lo mite can be ob served in out crop scale as a dark brown col our. In thin sec tion, it oc curs as a coarse crys tal that has mostly fill ing fea tures (Figs. 3A and 5H).
STABLE ISOTOPE AND TRACE ELEMENTS
The sta ble iso tope val ues of the Musayr car bon ate se quences in both lo cal i ties are pre sented in Ta ble 2. The d18O val ues range be tween –10.08 and –6.91‰ (avg. –9.08‰,), and d13C val ues range be tween –4.72 and 3.09‰ (avg. –1.60‰; Ta ble 2).
Stron tium. The Sr con cen tra tion in the Musayr car bon ate ranges from 29.5 to 2032 ppm (avg. 347 ppm). Maqna lo ca tion shows rel a tively higher stron tium con cen tra tion in com par i son with Wadi Al-Hamd area.
So dium. The Na con cen tra tion in the Musayr car bon ate ranges from 277 to 4474 ppm (avg. 1180 ppm). Higher so dium con cen tra tion is ob served in the lower part of Musayr For ma tion suc ces sion in Maqna area.
Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform... 243
Fig. 3. Measured stratigraphic sections from two different localities:
A – in Maqna (cir cle no. 1 in Fig ure 1): 1 – pan oramic view of the stud ied sec tion, 2 – the light grey stromatolite fa cies in truded by dark brown frac tures filled do lo mite, 3 – interbedded bioclastic wacke- to pack-stone lay ers; B – in Wadi Al-Hamd (cir cle no. 2 in Fig ure 1): 1 – pan oramic view of the stud ied sec tion, 2 – oys ter-rich beds that are com monly found in the up per sec tion of Musayr For ma tion, 3 – the pres ence of mi nor faults that rep re sent post-depositional tec tonic events af fect ing the Musayr car bon ate se quence
Iron and man ga nese. The Fe and Mn con cen tra tions in the Musayr car bon ates range from 302 to 3560 ppm (avg. 1388 ppm) and from 422 to 2489 ppm (avg. 1444 ppm), re spec tively.
DISCUSSION
PARAGENETIC SEQUENCE
Based on the petrographic char ac ter is tics and their el e - men tal com po si tions, the tim ing of diagenetic events and their en vi ron men tal evo lu tion of the Musayr For ma tion can be in - ferred. Three dif fer ent diagenetic en vi ron ments (ma rine, me te - oric and burial) are rec og nized and dis cussed be low (Fig. 6).
Ma rine diagenesis. Ma rine diagenesis is rep re sented by the pres ence of micrite en ve lope, mi cro bial micritisation and isopachous rim. The micrite en ve lope and isopachous rim are mostly as so ci ated with high-en ergy grain-dom i nated fa cies whereas the micritisation is more abun dant in the low en ergy mud-dom i nated fa cies. The long and point con tacts na ture of the grains and the pre-com pac tion na ture of the ce ment may sug gest con cur rent oc cur rence of ma rine ce men ta tion and early com pac tion (Jadoul and Galli, 2008; Ronchi et al., 2011).
Trace el e men tal cross plots in di cate ar agon ite-high mag ne -
sium cal cite as the orig i nal min er al ogy be fore it un der gone se - vere me te oric diagenesis (Fig. 7).
Burial diagenesis. This stage is char ac ter ized by chem i cal com pac tion and in ten sive dis so lu tion (Fig. 5F, G). These fea - tures are rec og nized un der thin sec tion by su ture grain con - tacts, grain interpenetration and cre ation of moldic pores. Ad di - tion ally, coarse crys tal line with un du la tory ex tinc tion of do lo mite min eral may also sug gest late burial, high-tempreture flu ids or i - gin. The po si tion of Musayr plat form that was lo cated on the palaeohighs of Midyan Ba sin may ex plain lit tle in flu ence of burial diagenesis.
Me te oric diagenesis. This diagenetic realm is the most dom i nant in the Musayr For ma tion in both lo cal i ties and char ac - ter ized by clear blocky (Fig. 5C) and drusy mo saic ce ments, fol - lowed by se lec tive dis so lu tion by us ing frac tures as the path - ways. Most of the me te oric ce ments formed as low mag ne sium cal cite (LMC) and pro gres sively oc cluded pri mary pore spaces in the grain-dom i nated car bon ate fa cies and filled the frac tures.
The av er age value of stron tium con cen tra tion (347 ppm) in the LMC may be at trib uted to the dis so lu tion of the un sta ble ma rine car bon ates (ar agon ite and HMC) that re leases stron tium to the pore wa ters.
Me te oric diagenesis oc curs dur ing eodiagenesis via me te - oric wa ter flux en hanced by subaerial ex po sure of car bon ate se quence (e.g., Fheed et al., 2015). The pres ence of me nis cus ce ment at the grain con tacts has been in ter preted to oc cur in Lithofacies
code Lithofacies Fa cies as -
so ci a tion De scrip tion Po ros ity type Hy dro dy namic
en ergy
F1 Foraminiferal
packstone FA1
Typ i cally found interbedded with cal car e ous sand - stone. Com posed mostly of miliolids foraminifera
(Quinqueloculina spp) and other skel e tal frag - ments, such as bi valves and gas tro pods.
Inter gra nu lar,
intragranular Low
F2 Cal car e ous
sand stone FA1
Com posed of mostly quartz and plagioclase grains. The grains are subangular to subrounded.
Lenses of peb bles are com monly as so ci ated with this fa cies. Bur rows are com mon. Cross-strat i fi ca -
tion found in some cases.
Inter gra nu lar,
frac ture Mod er ate to high
F3 Bioclastic
wacke-packstone FA1 Nor mally graded and well strat i fied. Com posed of a mix ture of skel e tal frag ments, such as rhodoliths,
miloilids foraminifera, gas tro pods, and bi valves.
Inter gra nu lar,
fracture Low to moderate
F4 Stromatolitic
boundstone FA1 Com mon fenestral fab ric. Most of the grains are
recrystallised. Fenestral Low to mod er ate
F5 Ooidal grainstone FA2
Well strat i fied and ex ten sive lat eral con ti nu ity. Well sorted ooid grains with con cen tric cor tex ar range - ments. The nu clei are mostly com posed of quartz
frag ments.
Inter gra nu lar,
intragranular High
F6 Peloidal grainstone FA2
Typ i cally found as so ci ated with ooidal grainstone.
Mas sive and with lime mud pel lets, micritic intraclasts, vari able amounts of silt-sized quartz,
mi nor bioclasts (gas tro pods, brachi o pods).
Intercrystalline
po ros ity Mod er ate
F7 Bioclastic-
packstone FA3
Nor mal grad ing, mostly com posed of var i ous bioclast frag ments (ben thic foraminifera,
rhodoliths) and intraclasts. In ten sive bioturbation. Inter gra nu lar Mod er ate to high F8 Oys ter float stone FA3 Typ i cally found as so ci ated with the rudstone and
bioclast-intraclast packstone fa cies. Float ing oys - ter frag ments within packstone ma trix.
Inter gra nu lar,
vug gy High
F9 Oys ter rudstone FA3 Typ i cally found as so ci ated with the float stone fa - cies (F8). Oys ter frag ments more dom i nant, ma trix
<10%. Pla nar to low-an gle cross strat i fi ca tion.
Inter gra nu lar,
vug gy High
F10 Foraminiferal
packstone FA3
This fa cies mostly com prise ramp-de rived frag - ments. Abun dant ben thic forams (Miogypsina sp) and mi nor bioclasts (bi valves, gas tro pods) char ac -
ter ize this fa cies.
Inter gra nu lar Mod er ate to high
F11 Bioclastic
wackestone FA3
Di verse fos sil con tent and abun dant micrite. As so - ci ated with the oys ter rudstone and found com -
monly at the base of cy cles. Bioturbation is ob served in this fa cies.
Inter gra nu lar Low T a b l e 1 Sum mary of Musayr carbonates
Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform... 245
Fig. 4. Thin section micrograph of the Musayr Formation display different microfacies
A – stromatolitic boundstone with siliciclastic grains en trapped within the laminae, equant and coarse cal cite crys tals are ev i dent from the thin sec tion; B – ooid grains with tan gen tial cor ti ces, grain interpenetration can be ob served as an ev i dence of deep burial com pac tion; C – rhodolith encrustation; D – cal cite re place ment of ben thic foraminifera (Miogypsina sp.); E – par tial dis so lu tion on peloidal and bioclastic grains and coarse, blocky ce ment fill ing the pore space; F – equant and coarse cal cite ce ments that pro found in the Musayr For ma tion
Fig. 5. Thin section images of the Musayr Formation show various diagenetic features
A – in ten sive mi cro bial micrttisation on the skel e tal grains; B – drusy ce ment in the peloidal grainstone fa cies; C – blocky ce ment ob served as re plac ing and pore-fill ing ma te rial; D – me nis cus ce ment within grains, an in di ca tor of me te oric vadose en vi ron ment; E – inter gra nu lar and moldic po ros ity as a prod uct of in ten sive dis so lu tion, sec ond stage of dis so lu tion that dis solved late do lo mite ce ment; F – long and point con tacts be tween grain sug gest ing me chan i cal com pac tion; G – grain interpenetration sug gesting in ten sive burial com pac tion; H – late do lo mite ce ment that fill ing the frac tures
me te oric vadose en vi ron ment (Tucker and Wright, 1990). This may be ex plained by the footwall up lift dur ing rift phase prior to post-rift sub si dence phase. This has been re ported in Gulf of Suez, where lo cal but im por tant phase of subaerial ex po sure al - lows lat eral in flux of me te oric wa ters into the Mio cene car bon - ate of Abu Shaar Plat form (Aissaoui et al., 1986). Our stud ied car bon ate se quence has been pre ceded by post Mio cene clastics and this sug gests pe riph eral up lift and in flux of me te oric wa ter that re sulted in dolomitisation of the pre-ex it ing Mio cene car bon ate se quence.
ORIGIN OF METEORIC FLUID
The pe trog ra phy and geo chem i cal data were used to de ci pher the pres ence of me te oric diagenesis realm in the stud ied car bon - ate se quence. Cross-plots be tween var i ous pa ram e ters (trace el -
e ments and sta ble iso topes) and re la tion ships have been uti lized to ex plain the for ma tion of Musayr car bon ate se quences.
GEOCHEMICAL SIGNATURES OF METEORIC DIAGENESIS
Pre vi ous stud ies have dem on strated that shal low ma rine car bon ates d13C val ues are not readily af fected by diagenetic al ter ations (Ban ner and Hanson, 1990). Con versely, d18O val - ues of ma rine car bon ate rocks are eas ily in flu enced by burial and me te oric diagenesis (Brand and Veizer, 1981; Brand, 2004). The in te gra tion of car bon and ox y gen iso topes with trace el e men tal anal y sis have un rav eled spe cific trends that may sug gest the pres ence of me te oric diagenesis in the stud ied car - bon ate plat form (Figs. 7 and 8).
Car bon iso tope val ues of the Musayr For ma tion in the Maqna lo ca tion (Fig. 7) ex hibit a marked de ple tion rel a tive to Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform... 247
Lo ca - tion
Sam ple
ID Ca
[%] Mg
[%] Fe
[ppm] Mn
[ppm] Na
[ppm] Sr
[ppm] d13C [‰] d18O
[‰]
Do lo mite
[%] Cal cite [%]
Wadi Alhamd
WH-4 10.98 0.11 732.6 891.3 700.6 42.1 –2.68 –9.41 0 100
WH-5 31.75 0.13 1033.6 1949.1 365.0 110.6 –2.71 –9.74 0 100
WH-6 10.09 0.08 302.3 664.3 580.9 31.5 –2.74 –10.08 0 100
WH-7 27.55 0.28 2008.5 1578.3 753.0 112.9 –2.57 –9.11 0 100
WH-8 11.55 0.18 1394.3 422.8 1097.2 29.5 –0.45 –7.58 0 100
WH-9 28.09 0.12 392.3 1167.7 571.4 136.0 –0.74 –9.09 0 100
WH-10 42.71 0.16 306.2 1215.6 393.4 152.7 –0.47 –9.81 0 100
Maqna
M-3 42.04 0.26 990.4 1939.6 277.1 280.4 –0.56 –6.91 0 100
M-4 24.43 0.30 1629.9 2489.8 4474.3 134.2 –4.72 –9.93 0 100
M-5 21.02 0.51 3556.5 1506.8 3397.5 136.3 –4.72 –9.14 0 100
M-6 15.71 0.56 3560.3 1776.8 2919.5 170.5 –2.89 –9.39 0 100
M-7 35.56 0.34 1404.5 2429.4 980.0 201.5 –2.59 –9.72 0 100
M-8 37.35 0.36 1366.4 1962.5 470.5 294.5 –0.98 –8.47 0 100
M-9 32.80 0.29 1684.5 873.6 367.9 1347.8 1.77 –9.42 0 100
M-10 44.06 0.23 453.0 796.9 356.5 2032.0 3.09 –8.41 0 100
T a b l e 2 Per cent age of cal cite and do lo mite ob tained from XRD and the car bon and ox y gen iso topes
of the Musayr For ma tion
Fig. 6. Paragenetic sequence of Musayr Formation shows a dominant meteoric diagenesis
Fig. 7. Cross-plot between d18O and different trace elements (Fe, Mn, Sr, Na) of the carbonate sequence of Musayr Formation in the two studied areas
Note that d18O depleted calcites have high Na concentrations
Fig. 8. Cross-plot between various trace elements
Fe, Mn, and Sr have been uti lized be cause their sen si tiv ity to diagenetic fluid changes, es pe cially me te oric diagenesis. Three of four dif fer ent cross-plots ex hibit pos i tive cor re la tion that can be used to in di cate per va sive me te oric al ter ation to the Musayr car bon ate se quences
ma rine car bon sig na tures at ex po sure sur faces and be come more en riched with depth. Such trends rep re sent an ini tial in put of 13C-poor bi car bon ate from soil or ganic pro cesses and pro - gres sive ad di tion of 13C-rich bi car bon ate from in ter ac tion with the car bon ate host rock (e.g., Allan and Matthews, 1982).
Cross-plots be tween trace el e ments (Fe, Mn, Sr and Na) and d18O val ues (Fig. 7) have shown gen eral trends that also sup port the in ter pre ta tion of per va sive me te oric diagenesis in the Musayr For ma tion, such as neg a tive cor re la tion be tween d18O and man ga nese (Mn). This neg a tive cor re la tion may sug - gest higher in cor po ra tion of Mn el e ments de rived by fresh wa ter into the car bon ate se quence (Whitaker et al., 2006).
Dif fer ent cross-plots con firmed the se vere ef fect of me te oric diagenesis such as the in creas ing iron and man ga nese con - cen tra tions and pos i tive cor re la tion be tween them (Fig. 8).
These el e ments can be eas ily in cor po rated into cal cite struc - ture where the re duc tion con di tion is pro longed. Pos i tive cor re - la tions be tween manganase and stron tium as well as man ga - nese and iron (Fig. 8) sug gest me te oric diagenesis on the Musayr For ma tion (Winefield et al., 1996).
It is un der stood that high man ga nese con cen tra tion in cal cite is at trib uted to me te oric diagenesis be cause of the high man ga - nese con tent in the me te oric wa ters (Brand and Veizer, 1980;
Rao, 1991). Be side, the in creas ing con cen tra tion of man ga nese may also be used to in fer par tially open sys tem and re duc tion diagenetic realm con di tions. An other sup port ing ev i dence of me - te oric diagenesis is low stron tium con cen tra tion be cause its low con tent on the me te oric wa ters (Winefield et al., 1996).
The in verted J-curve de lin eated by Musayr sam ples is in dic - a tive of diagenetic al ter ation gov erned by vari able de grees of
fluid-rock in ter ac tion and me te oric or i gin of diagenetic flu ids (e.g., Meyers and Lohmann, 1985; Lohmann, 1988; Ban ner and Hanson, 1990). This in verted trend may be as so ci ated with dif fer ent iso to pic equil i bra tion be tween car bon ate com po nents and diagenetic flu ids at low fluid rock in ter ac tions (Bishop et al., 2014; Fig. 9). There is no dis tinct trend or re la tion ship be tween Dun ham’s tex ture and de ple tion of iso to pic com po nents. This may sug gest that the trend is gov erned by microscale struc ture such as microporosity that can not be cap tured by Dun ham tex - ture clas si fi ca tion (Batt et al., 2008; Bishop et al., 2014).
CONCLUSIONS
The petrographic, sta ble iso tope and trace el e ment re sults in di cate that:
1. Per va sive me te oric diagenesis is ev i dent fea ture in the Musayr car bon ate se quence and it is mainly char ac ter - ized by the pres ence of coarse and equant cal cite ce - ments.
2. De ple tion of sta ble iso topes val ues of both ox y gen (avg.
–9.08‰) and car bon (avg. –1.6‰) and trace el e ments (avg. val ues of Fe: 1387 ppm; Mn: 1444 ppm; Sr:
419 ppm; Na: 1194 ppm) in con junc tion with neg a tive cor re la tion be tween Mn2+ and ox y gen iso tope data sug - gest that these cal cite ce ments were pre cip i tated from flu ids of me te oric or i gin.
3. The me te oric diagenesis have been in ter preted to be as a prod uct of early diagenesis (eodiagenesis) that is as - so ci ated with the in ter play be tween ac tive fresh wa ter in - put from the hin ter land (NE) that brought siliciclastic sed i ments and sev eral ep i sodes of rel a tive sea level lowstands dur ing the time of de po si tion in the Early Mio - cene.
4. Un der stand ing the tim ing of me te oric diagenesis and its re lated diagenetic prod ucts is im por tant be cause it may pro vide in for ma tion on the pres ence and ex tent of these fea tures in the subsurface and hence the in flu ence on res er voir qual ity dis tri bu tion through out this study area and other syn-rift car bon ate se quences in other parts of the world.
Ac knowl edge ments. The au thor would like to ac knowl - edge the sup port pro vided by King Ab dul Aziz City for Sci ence and Tech nol ogy (KACST) through the Na tional Sci ence and the Tech nol ogy Ini tia tive Pro gram at King Fahd Uni ver sity of Pe tro leum and Min er als (KFUPM) for fund ing this pro ject (NSTIP # 11-OIL2145-04). I ac knowl edge also con struc tive re - views from the Geo log i cal Quar terly ref er ees, R. Aubrecht and anon y mous one, and T.M. Peryt that im proved this pa per.
REFERENCES
Aissaoui, D.M., Coniglio, M., James, N.P., Purser, B.H., 1986.
Diagenesis of a Mio cene reef-plat form: Jebel Abu Shaar, Gulf of Suez, Egypt. In: Reef Diagenesis (eds. J.H. Schroeder and B.H.
Purser): 112–31, Springer, Berlin.
Allan, J.R., Matthews, R.K., 1977. Car bon and ox y gen iso topes as diagenetic and strati graphic tools: sur face and subsurface data, Bar ba dos, West In dies. Ge ol ogy, 5: 16–20.
Allan, J.R., Matthews, R.K., 1982. Iso tope sig na tures as so ci ated with early me te oric diagenesis. Sedimentology, 29: 797–817.
Al-Ramadan, K., Dogan, A., Senalp, M., 2013. Sedimentology and diagenesis of the Mio cene Nutaysh Mem ber of the Burqan For - ma tion in the Midyan Area, north west ern Saudi Ara bia. Geo log i - cal Quar terly, 57 (1): 165–174.
Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform... 249
Fig. 9. Cross-plot between d18O and d13C showing inverted J-curve that may suggest meteoric diagenesis
Explanations as in Figure 7
Ban ner, J., G. Hanson, 1990. Cal cu la tion of si mul ta neous iso to pic and trace el e ment vari a tions dur ing wa ter-rock in ter ac tion with ap pli ca tions to car bon ate diagenesis. Geochimica et Cosmochimica Acta, 54: 3123–3137.
Bathurst, R.G.C., 1975. Car bon ate Sed i ments and Their Diagenesis. 2nd edi tion, Elsevier, New York.
Batt, L.S., Montañez, I.P., Isaacson, P., Pope, M.C., Butts., S.H.
Ablplanalp, J., 2008. Multicarbonate com po nent re con struc - tion of mid-Car bon if er ous (Chesterian) sea wa ter d13C. Palaeo - ge ogra phy, Palaeoclimatology, Palaeo ec ol ogy, 256: 298–318.
Bishop, J.W., Osleger, D.A., Montañez, I.P., Sum ner, D.Y., 2014.
Me te oric diagenesis and fluid-rock in ter ac tion in the Mid dle Perm ian Capitan backreef: Yates For ma tion, Slaugh ter Can yon, New Mex ico. AAPG Bul le tin, 98: 1495–1519.
Bosworth, W., Mcclay, K., 2001. Struc tural and strati graphic evo lu - tion of the Gulf of Suez rift, Egypt: a syn the sis. Mémoires du Muséum na tional d’histoire naturelle, 186: 567–606.
Brand, U., 2004. Car bon, ox y gen and stron tium iso topes in Pa leo - zoic car bon ate com po nents: an eval u a tion of orig i nal sea wa - ter-chem is try prox ies. Chem i cal Ge ol ogy, 204: 23–44.
Brand, U., Veizer, J., 1980. Chem i cal diagenesis of a multicomponent car bon ate sys tem – 1: trace el e ments. Jour nal of Sed i men tary Re search, 50: 1219–12136.
Brand, U., Veizer, J., 1981. Chem i cal diagenesis of a multicomponent car bon ate sys tem – 2: sta ble iso topes. Jour nal of Sed i men tary Re search, 51: 987–997.
Clark, M.D., 1986. Ex plan a tory Notes to the Geo logic Map of the Al Bad’ Quad ran gle, sheet 28A, King dom of Saudi Ara bia. Saudi Ara bian Dep uty Min is try for Min eral Re sources Geoscience Map Se ries GM-81A, C, scale 1:250,000, with text, 46 p.
Dick son J.A.D, 1966. Car bon ate iden ti fi ca tion and gen e sis as re - vealed by stain ing. Jour nal of Sed i men tary Pe trol ogy, 36:
441–505.
Dick son, J.A.D., Kenter, J.A.M., 2014. Diagenetic evo lu tion of se - lected parasequences across a car bon ate plat form: Late Pa leo - zoic, Tengiz Res er voir, Kazakhstan. Jour nal of Sed i men tary Re - search, 84: 664–693.
Fheed, A., Swierczewska, A. and Krzy¿ak, A., 2015. The iso lated Wuchiapingian (Zechstein) Wielichowo Reef and its sed i men - tary and diagenetic evo lu tion, SW Po land. Geo log i cal Quar terly, 59 (4): 762–780.
Gross, M.G., 1964. Vari a tions in the 18O/16O and 13C/12C ra tios of diagenetically al tered lime stones in the Ber muda Is lands. Jour - nal of Ge ol ogy, 72: 170–194.
Heba, G., Prichonnet, G., El Albani, A., 2009. Me te oric diagenesis of Up per Cre ta ceous and Paleocene–Eocene shal low-wa ter car bon ates in the Kruja plat form (Al ba nia): geo chem i cal ev i - dence. Geologica Carpathica, 60: 165–179.
Hughes, G.W., John son, R.S., 2005. Lithostratigraphy of the Red Sea Re gion. GeoArabia, 10: 49–126.
Jadoul, F., Galli, M.T., 2008. The Hettangian shal low wa ter car bon - ates af ter the Tri as sic/Ju ras sic biocalcification cri sis: the Albenza For ma tion in the West ern South ern Alps. Rivista Italiana di Paleontologia e Stratigrafia, 114: 453–470.
Koeshidayatullah, A., Al-Ramadan, K., Col lier, R., Hughes, G.W., 2016. Vari a tions in ar chi tec ture and cyclicity in fault-bounded car bon ate plat forms: Early Mio cene Red Sea Rift, NW Saudi Ara bia. Ma rine and Pe tro leum Ge ol ogy, 70:
77–92.
Land, L.S., Ep stein, S., 1970. Late Pleis to cene diagenesis and dolomitization, North Ja maica. Sedimentology, 14: 187–200.
Lohmann, K.C., 1988. Geo chem i cal pat terns of me te oric diagenetic sys tems and their ap pli ca tion to stud ies of paleo -
karst. In: Paleokarst (eds. N.P. James and P.W. Choquette):
58–80. Springer, Berlin.
Ma jor, R.P., 2014. Early Me te oric Diagenesis and Po ros ity Pres er - va tion: Strati graphic Traps in Car bon ate Grainstones. Con fer - ence pa per in In ter na tional Pe tro leum Tech nol ogy Con fer ence, Kuala Lumpur, Ma lay sia, https://www.onepetro.org/down - load/con fer ence-pa per/IPTC-17924-MS?id=con fer ence-pa - per%2FIPTC-17924-MS
Melim, L.A., Westphal, H., Swart, P.K., Eberli, G.P., Munnecke, A., 2002. Ques tion ing car bon ate diagenetic par a digms: ev i - dence from the Neo gene of the Ba ha mas. Ma rine Ge ol ogy, 185:
27–53.
Meyers, W.J., Lohmann, K.C., 1985. Iso tope geo chem is try of re - gion ally ex ten sive cal cite ce ment zones and ma rine com po - nents in Mis sis sip pian lime stones, New Mex ico. SEPM Spe cial Pub li ca tion, 26: 223–239.
Mor row, D.W., Mayers, I.R., 1978. Sim u la tion of lime stone diagenesis: a model based on stron tium de ple tion. Ca na dian Jour nal of Earth Sci ence, 15: 376–396.
Rao, C.P., 1991. Geo chem i cal dif fer ence be tween sub trop i cal (Or - do vi cian) tem per ate (Re cent and Pleis to cene) and sub po lar (Perm ian) car bon ates, Tas ma nia, Aus tra lia. Car bon ates and Evaporites, 6: 83–106.
Ronchi, P., Jadoul, F., Ceriani, A., Di Giulio, A., Scotti, P., Ortenzi, A., Previde Massara, E., 2011. Mul ti stage dolomitization and dis tri bu tion of dolomitized bod ies in Early Ju - ras sic car bon ate plat forms (South ern Alps, It aly).
Sedimentology, 58: 532–565.
Saoudi, A., Khalil, B., 1986. Dis tri bu tion and Hy dro car bon Po ten tial of Nukhul Sed i ments in the Gulf of Suez. Pro ceed ings of the Sev enth Ex plo ra tion Sem i nar, Cairo: 75–96.
Smalley, P.C., Bishop, P.K., Dick son, J.A.D., Em ery, D., 1994.
Wa ter-rock in ter ac tion dur ing me te oric flush ing of a lime stone:
im pli ca tions for po ros ity de vel op ment in karstified pe tro leum res er voirs. Jour nal of Sed i men tary Re search, 64: 180–189.
Swart, P.K., 2015. The geo chem is try of car bon ate diagenesis: the past, pres ent and fu ture. Sedimentology, 62: 1233–1304.
Tubbs, R.E., Fouda, A.H., Afifi, A.M., Raterman, N.S., Hughes, G.W. Fadolalkarem, Y.K., 2014. Midyan Pen in sula, north ern Red Sea, Saudi Ara bia: Seis mic im ag ing and re gional in ter pre - ta tion. GeoArabia, 19: 165–184.
Tucker, M.E., 1986. For merly aragonitic lime stones as so ci ated with tillites in the late Pro tero zoic of Death Val ley, Cal i for nia. Jour nal of Sed i men tary Pe trol ogy, 56: 818–830.
Tucker, M.E., Wright, V.P., 1990. Car bon ate Sedimentology.
Blackwell Sci en tific Pub li ca tions.
Videtich, P.E., 1982. Or i gin, ma rine diagenesis, and early fresh-wa - ter diagenesis of lime stones and dolomites (Ter tiary-Re cent):
sta ble iso to pic, elec tron microprobe, and petrographic stud ies.
Ph.D. the sis, Prov i dence, Rhode Is land, Brown Uni ver sity.
Veizer, J., Demovic, R., 1974. Stron tium as a tool in fa cies anal y sis.
Jour nal of Sed i men tary Pe trol ogy, 44: 93–115.
Whitaker, F.F., Pat er son, R.J., Johnston, V.E., 2006. Me te oric diagenesis dur ing sea-level lowstands: ev i dence from mod ern hydrochemical stud ies on north ern Guam. Jour nal of Geo chem i - cal Ex plo ra tion, 89: 420–423.
Winefield, P.R., Nel son, C.S., Hodder, A.P.W., 1996. Dis crim i nat - ing tem per ate car bon ates and their diagenetic en vi ron ments us - ing bulk el e men tal geo chem is try, a re con nais sance study based on New Zea land Ce no zoic Lime stone. Car bon ates and Evaporites, 11: 19–31.
