Origin and timing of siderite cementation in Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya

Marine and Petroleum Geology 23 (2006) 459–471 www.elsevier.com/locate/marpetgeo

Origin and timing of siderite cementation in Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya M.A.K. El-ghali a,b,*, K.G. Tajori b, H. Mansurbeg a, N. Ogle c, R.M. Kalin c a Department of Earth Science, Uppsala University, Villava¨gen 16, SE 75236 Uppsala, Sweden Department of Earth Science, Faculty of Science, Al-Fateh University, P.O. Box 13696, Tripoli, Libya c School of Civil Engineering, Environmental Engineering Research Centre, The Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, Northern Ireland b

Received 15 July 2005; received in revised form 8 February 2006; accepted 10 February 2006

Abstract The origin and timing of siderite cementation have been constrained in relation to depositional facies and sequence stratigraphy of Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya. Optical microscope, backscattered electron imagery, and carbon and oxygen stable isotope analysis have revealed that siderite is of eo- and mesogenetic origin. Eogenetic siderite is Mg-poor with a mean composition of (Fe91.7Mg1.5Ca0.3Mn6.5)CO3, and occurs in paraglacial, tide-dominated deltaic highstand systems tract (HST) sandstones, in paraglacial, foreshore to shoreface HST sandstones and in postglacial, Gilbert-type deltaic lowstand systems tract (LST) sandstones. This siderite is typically of meteoric water origin that influxed into the LST and HST sandstones during relative sea level fall and basinward shift of the strandline. Mesogenetic siderite, which engulfs and thus postdates quartz overgrowths and illite, is Mg-rich with a mean composition of (Fe72.2Mg21.7Ca0.8Mn5.3)CO3 and occurs in the paraglacial, tide-dominated deltaic HST sandstones, in paraglacial foreshore to shoreface HST sandstones, in glacial, tide-dominated estuarine transgressive systems tract (TST) sandstones, in postglacial, Gilbert-type deltaic LST sandstones, and in postglacial, shoreface TST sandstones. d18OV-PDB values of this siderite, which range between K22.6 and K13.8‰, suggest that precipitation has occurred from evolved formation waters with d18O values between K14.0 and C1.0‰ and was either meteoric, mixed marine– meteoric and/or marine in origin by assuming postdating quartz overgrowths and illite temperature between 80 and 130 8C. q 2006 Elsevier Ltd. All rights reserved. Keywords: Siderite; Glaciogenic sandstone diagenesis; Sequence stratigraphy; Depositional facies; Upper Ordovician; the Murzuq basin; SW Libya

1. Introduction The origin, elemental and isotopic composition, and distribution patterns of siderite cement in sandstones from a wide variety of depositional environments and diagenetic regimes have been the focus of numerous studies (Matsumoto and Iijima, 1981; Curtis et al., 1986; Mozley, 1989; Pye et al., 1990; Morad et al., 1994; Huggett et al., 2000). Siderite typically precipitates from reducing, non-sulphidic pore waters that have evolved in suboxic, methanogenic geochemical conditions (Garrels and Christ, 1965; Froelich et al., 1979; * Corresponding author. Present address: Department of Earth Science, Uppsala University, Villava¨gen 16, SE 752 36, Uppsala, Sweden. Tel.: C46 18 4712552; fax: C46 18 4712591. E-mail addresses: [email protected], [email protected] (M.A.K. El-ghali).

0264-8172/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.02.002

Berner, 1981; Hem, 1985; Morad, 1998). Siderite chemistry has been used to unravel the origin of pore waters which can be either marine, mixed marine–meteoric or meteoric in composition (Curtis and Coleman, 1986; Bahrig, 1989; Mozley, 1989; Mozley and Wersin, 1992; Baker et al., 1996) and all can be influenced by either transgression or regression events (Morad et al., 2000). Defining the geochemistry and distribution of siderite in a sequence stratigraphic context, which is adopted in this study, allows a better understanding of the parameters that control its chemical composition and formation. The depositional facies and sequence stratigraphic framework of the Upper Ordovician glaciogenic sandstones (i.e. glacial, paraglacial, and postglacial), outlined by El-ghali (2005), made this study feasible. The diagenetic regimes used in this study are: (i) eodiagenesis (0–2 km depth and at less 70 8C), which includes alterations that have occurred where the pore water chemistry was influenced by surface conditions (depositional waters and climate), (ii) mesodiagenesis (depths over 2 km and at

460

M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471 17˚

09˚

10

N

14

18

22

Mediteranian Sea

Tunisia Tripoli

28˚

Banghazi 30

Ghadamis Basin

Al Fuqaha

Brak

Algeria

Idri

26

Awbari Sand Sea

Al Qarqaf

30

Jabal As Sawda

Arch

Al Haruj Al-Aswad

Egypt 26

Murzuq Basin

l-Aswad

Sabha

B1

Al Haruj A

Al-Hasawnah Mountain

Tibisti

Jadu Basin

22

Awbari

22

Niger

F2

Zuaylah

Al Awaynat

NC 174

0

Scale in

10

Murzuq

Km

Chad

Sudan

600

14

18

22

Tahalah

Legend Al Qatrun

Ghat

Murzuq Sand Sea

Ordovician

Tajarhi Anay

Eolian,Wadi and Fluvio/eolian deposits

Al Wigh

Undifferentiated Ordovician / Camberian Upper cambrian Older Granite , Granodiorite

Ayn Az Zan

23˚

Twon and Villages Unsellected Oil wells Sellected Oil wells

0

Scale in Kilometres

500

Fig. 1. Location map of the study area in the Murzuq basin, SW Libya. Samples used in this study were collected from wells 1 and 2 and from outcrops in areas A–C, which refer to the Ghat, Al-Qarqaf and Wadi Anlaline areas, respectively.

temperatures above 70 8C), which is strongly controlled by increases in temperature and mediated by evolved formation waters (Morad et al., 2000). 2. Geological setting The Murzuq basin is composed of a huge Paleozoic intracratonic basin that covers most of southwestern Libya (Fig. 1) with an area in excess of 400,000 km2. The basin contains up to 4000 m of Paleozoic marine deposits truncated by Mesozoic to Quaternary continental deposits (e.g. Davidson et al., 2000; Echikh and Sola, 2000; Hallett, 2002). The studied stratigraphic interval includes part of the Melaz Shuqran and Mamuniyat formations (ca. 250 m thick; Fig. 2) that were deposited during the Upper Ordovician (Hirnancian) glaciation of Gondwana (e.g. Sutcliffe et al., 2000) when the Murzuq basin was lying along the continental margin of west Gondwana, close to the ice cap of the South Pole (Kent and Van der Voo, 1990; Scotese and Barett, 1990; Smith, 1997; Davidson et al., 2000). Deposition of the Upper Ordovician glaciogenic sediments has occurred during glacial, paraglacial, and postglacial periods (El-ghali, 2005) with alternating cold and warm climatic conditions (Scotese et al., 1999). The burial history curve, which was constructed from analyzing shale velocities, suggested that the Upper

Ordovician glacial and glacial-related deposits in one of the studied wells (Fig. 1) reached a maximum burial depth (ca. 2.6 km) during the late Jurassic to early Paleocene, which was 300 m deeper than present day (Davidson et al., 2000; Fig. 3). This maximum burial depth corresponded to a maximum bottom hole temperature of ca. 130 8C, which was ca. 30 8C higher than present day (Davidson et al., 2000; Fig. 3). 3. Depositional facies and sequence stratigraphy The short-lived (ca. 0.5–1 million years) Upper Ordovician glaciation event (Beuf et al., 1971; Deynoux, 1980; Vaslet, 1990; Blanpied et al., 2000) has resulted in the deposition of the Melaz Shuqran and Mamuniyat formations in the Murzuq basin, SW Libya (Fig. 2). Detailed depositional facies and sequence stratigraphy of these deposits were described by El-ghali (2005), who recognized three depositional sequences (Fig. 2). These sequences include: (i) depositional sequence 1, which corresponds to the entire Melaz Shuqran Formation (Fig. 2) that was deposited during a period of overall transgression. Glacial advance, loading of the continental shelf and subsequent glacial retreat caused sea-levels to rise and thus transgression to occur (Sutcliffe et al., 2000; El-ghali, 2005). This sequence contains: (a) a transgressive systems tract (TST) with glacial, shoreface to offshore deposits containing

M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471

Lithology

Scale in (m)

Sequence Strata

Glacial cycles

Formation

Murzuq Basin, SW - Libya

Legend

Grain size Sedimentary structure F & Dm

Facies Codes S

C

Lithofacies

Tanzuft Formation

Isostatic rebound cycle

461

diamictite (Dm)

CS TST

conglomerate (C)

300

ts

sandstone (S) fine grained (F) LST

Sedimentary structure

SB

massive planar bedding

parallel lamination current ripples HST

Second glacial/paraglacial cycle

Mamuniyat

250

wave ripples trough cross-stratification hummocky cross-stratification

200 flaser bedding lenticular bedding mfs

syn-sedimentary deformation structures

TST

outsized clastic

ts

SB

First glacial/paraglacial cycle

Melaz Shuqran

100

fine grained

8 = granules 9 = pebbles 10 = cobbles 11 = bouldrers

sandstone

LST

1 = clay 2 = silt 3 = very fine sand 4 = fine sand 5 = medium sand 6 = coarse sand 7 = very coarse sand

conglomerate

Grain size

150

Sequence stratigraphy HST = Highstand systems tracts

HST

CS = Condensed section mfs = Maximum flooding surface I

I

I

TST = Transgressive systems tracts ts = Transgressive surface

I

LST = Lowstand systems tracts SB = Sequence boundary

50 mfs CS

I

Others Loading pebbles

TST

Bioturbation

fining and deepeing

Striation SB

I

Cambrian-Ordovician

Iron nodules mud drapes

0

Coarsing and shallowing

1 2 3 4 5 6 7 8 9 10 11

Fig. 2. Schematic sequence stratigraphic, depositional facies and siderite distribution summary for the Upper Ordovician Melaz Shuqran and Mamunyiat formations in the Murzuq basin (modified after El-ghali, 2005).

M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471

500

400

300

200

100

0

50

1000

Temperature ˚C

Depth in metres

0

2000 100 130 Fig. 3. Burial-thermal history curve of the Upper Ordovician in one of the studied wells in the Murzuq basin (modified after Davidson et al., 2000).

5. Results 5.1. Sandstone composition The sandstones are dominantly medium- to very coarsegrained, poorly to well-sorted, quartz arenites with a mean modal composition of Q97.8F1.3L0.9 (Fig. 4; Table 1). Monocrystalline quartz was the dominant detrital constituent (52– 80%; av. 71 vol%) and was more abundant than polycrystalline quartz (trace-18%; av. 2%). Detrital feldspars (trace to 5%; av. 1%) include K-feldspar and plagioclase. The lithic fragments (trace-2%; av. 1%) are mainly volcanic, low-grade Quartz 5 Subarkose Lithic sub-arkose

Sublitharenite

5

5

Subarkose Sublitharenite

ite

Sandstone samples were collected from outcrops in which the studied Upper Ordovician stratigraphic intervals are exposed along the basin margins and from two wells drilled in the basin center (Figs. 1 and 2). The collected samples included: (i) paraglacial, tide-dominated deltaic HST

25

ren

4. Sampling and analytical procedures

Quartzarenite

25

Lithic arkose Feldspar 10

Quartz

5

ha Lit

ice-rafted debris, and (b) a highstand systems tract (HST) with paraglacial, tide-dominated deltaic deposits. The HST sandstones were deposited when the sediment supply exceeded the rate of relative sea level rise during glacial retreat (El-ghali, 2005). (ii) Depositional sequence 2 corresponds to the lower and middle parts of the Mamuniyat Formation (Fig. 2) and contains: (a) glacial, incised-valley, fluvial deposits of lowstand systems tract (LST), (b) Glacial, tide-dominated estuarine and shoreface to offshore TST deposits, and (c) Paraglacial, foreshore to shoreface HST deposits. The HST sandstones were deposited when the sediment supply exceeded the rate of relative sea level rise during glacial retreat (El-ghali, 2005). (iii) Depositional sequence 3 comprises the upper part of the Mamuniyat Formation (Fig. 2), which was deposited during a period of isostatic rebound as a result of glacial retreat and subsequent sea-level fall and formation of a sequence boundary, SB (Sutcliffe et al., 2000; El-ghali, 2005). The SB is covered by postglacial, Gilbert-type deltaic (LST) deposits and postglacial, upper shoreface (TST) deposits. The transgressive surface (TS) is marked by the presence of transgressive lag deposits at the base of the upper shoreface deposits, whereas the maximum flooding surface (MFS) is recognized by the presence of a condensed section (CS) on top of the TST deposits in the outer shelf. The CS signifies distal sediment starvation, which in association with global sea level rise during the Silurian, marks the boundary between the Upper Ordovician deposits and the lower Silurian hot shale in the Murzuq basin (Fig. 2).

sandstones, (ii) paraglacial, foreshore-shoreface HST sandstones, (iii) glacial, tide-dominated estuarine TST sandstones, (iv) postglacial, Gilbert-type deltaic LST sandstones, and (v) postglacial, shoreface TST sandstones. Detailed petrographic examination was performed on thin sections from selected samples, which were prepared subsequent to impregnation with blue epoxy under vacuum. The modal compositions were obtained from 20 representative samples by counting 300 points in each thin section. Stable carbon and oxygen isotope analysis was carried out on 24 siderite-cemented sandstone samples; each sample contained a certain dominant siderite cement type. Isotope data is presented in the d notation and was measured relative to the Vienna PDB (PeeDee Belemnite) standard. Analytical precision was found to be better than G0.04‰. Chemical analysis of the siderite cements was performed on 24 carboncoated, polished thin sections using a Cameca Camebax BX50 microprobe (EMP), equipped with a backscattered electron detector (BSE). Operational values were such that accelerating voltageZ20 kV, beam currentZ10–15 nA, and beam spot sizeZ1–5 mm. Analytical totals of siderites were normalized to 100% for comparison purposes.

se

Age in millions of years

Ar ko

462

Feldspatic litharenite 50

10

Lithic fragments

Fig. 4. Framework grain (i.e. quartz, feldspars and lithic fragments) composition of 20 representative Upper Ordovician sandstone samples from paraglacial, tide-dominated HST sandstones, paraglacial, foreshore-shoreface HST sandstones, glacial, tide-dominated estuarine TST sandstones, postglacial, Gilbert-type deltaic LST sandstones, and postglacial, shoreface TST sandstones plotted on McBride’s (1963) classification. The sandstones are quartzarenite in composition and show no significant variation between depositional facies and systems tracts.

Table 1 Modal composition (maximum, minimum, average and standard deviation) of 20 representative sandstone samples from paraglacial, tide-dominated HST sandstones, paraglacial, foreshore-shoreface HST sandstones, glacial, tide-dominated estuarine TST sandstones, postglacial, Gilbert-type deltaic LST sandstones, and postglacial, shoreface TST sandstones of the Upper Ordovician Melaz Shuqran and Mamunyiat formations in the studied area Depositional facies and systems tract

Postglacial shoreface TST (nZ4)

Postglacial Gilbert-type deltaic LST (nZ5)

Paraglacial shoreface HST (nZ6)

Glacial tide-dominated estuarine TST (nZ4)

Paraglacial tide-dominated delatic HST (nZ4)

Min

Max

Mean

Min

Max

Mean

SD

Min

Max.

Mean

SD

Min

Max

Mean

SD

Min

Max

Mean

SD

70.3 0.0 0.0 0.0 0.0 0.0 0.0

78.7 0.7 0.0 0.0 0.0 0.0 0.0

74.9 0.3 0.0 0.0 0.0 0.0 0.0

3.7 0.3 0.0 0.0 0.0 0.0 0.0

52.0 0.3 0.0 0.0 0.0 0.0 0.0

76.0 18.0 2.0 0.3 0.7 0.0 0.7

68.8 5.7 0.8 0.1 0.1 0.0 0.2

9.8 7.3 1.0 0.2 0.3 0.0 0.3

60.0 0.3 0.0 0.0 0.0 0.0 0.0

80.0 2.3 3.0 2.3 1.3 0.3 0.7

67.0 1.1 0.8 0.7 0.4 0.1 0.2

7.1 0.9 1.2 1.1 0.5 0.1 0.3

65.7 1.0 0.3 0.0 0.0 0.0 0.0

75.0 3.0 1.0 1.3 2.0 0.3 0.3

71.3 1.7 0.7 0.7 0.8 0.1 0.1

4.4 0.9 0.3 0.8 1.0 0.2 0.2

67.0 0.0 0.0 0.0 0.3 0.0 0.0

80.0 2.3 0.7 1.0 1.7 0.0 0.0

74.2 1.0 0.3 0.5 1.0 0.0 0.0

5.7 1.2 0.4 0.6 0.6 0.0 0.0

0.0

0.0

0.0

0.0

0.0

1.3

0.3

0.6

0.0

1.0

0.2

0.4

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.3 0.0

1.0 0.0 10.0 0.7

0.3 0.0 5.7 0.1

0.4 0.0 4.4 0.3

0.0 0.0 0.0 0.0

1.0 0.0 3.3 1.3

0.3 0.0 0.9 0.4

0.4 0.0 1.4 0.5

0.0 0.0 0.0 0.7

2.7 0.0 4.3 1.0

1.3 0.0 1.4 0.8

1.1 0.0 2.0 0.2

0.0 0.0 0.0 0.0

2.3 0.0 3.3 1.0

1.4 0.0 1.0 0.4

1.0 0.0 1.6 0.4

Diagenetic minerals Kaolinite Clay coating Illite Chlorite Pyrite Quartz overgrowths Glauconite Albitized feldspars Siderite Ankarite Dolomite Calcite replacement Calcite pore filling Iron oxide

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.3 0.0

0.0 1.3 0.0 0.0 0.0 1.0 0.0 0.0 3.0 0.0 0.0 0.0 14.0 20.0

0.0 0.7 0.0 0.0 0.0 0.5 0.0 0.0 0.8 0.0 0.0 0.0 11.0 5.0

0.0 0.8 0.0 0.0 0.0 0.6 0.0 0.0 1.5 0.0 0.0 0.0 2.6 10.0

4.0 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

7.7 3.7 1.3 0.3 3.0 8.7 0.0 0.0 2.3 0.0 0.0 0.0 2.3 1.3

5.8 2.1 0.5 0.1 0.7 4.7 0.0 0.0 0.6 0.0 0.0 0.0 0.5 0.3

1.8 1.0 0.7 0.2 1.3 3.3 0.0 0.0 1.0 0.0 0.0 0.0 1.0 0.6

1.7 0.0 0.0 0.0 0.0 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

6.3 2.7 0.7 0.7 0.0 8.7 0.7 1.7 12.0 0.0 1.7 0.0 16.7 1.7

4.6 0.9 0.2 0.1 0.0 3.6 0.2 0.6 3.7 0.0 0.6 0.0 3.9 0.4

1.7 1.2 0.3 0.3 0.0 3.7 0.3 0.8 4.5 0.0 0.7 0.0 6.6 0.7

1.0 0.0 0.0 0.0 0.0 1.3 0.0 0.0 2.3 0.0 0.0 0.0 0.0 0.0

4.3 0.0 0.0 5.3 0.7 3.7 1.0 3.0 14.3 0.0 0.0 0.0 3.7 0.0

2.0 0.0 0.0 1.3 0.3 2.4 0.6 0.8 7.1 0.0 0.0 0.0 1.7 0.0

1.6 0.0 0.0 2.7 0.3 1.0 0.4 1.5 5.3 0.0 0.0 0.0 1.9 0.0

2.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

5.0 0.7 0.3 0.0 3.7 11.3 0.0 0.3 15.0 11.7 1.0 0.0 6.0 0.0

3.5 0.2 0.1 0.0 0.9 4.8 0.0 0.1 4.8 2.9 0.3 0.0 1.5 0.0

1.1 0.3 0.2 0.0 1.8 4.8 0.0 0.2 6.9 5.8 0.5 0.0 3.0 0.0

Porosity Intergranular porosity Intragranular porosity Moldic porosity

0.0 0.0 0.0

11.7 0.0 0.0

7.3 0.0 0.0

5.0 0.0 0.0

0.0 0.0 0.0

6.7 0.0 2.7

2.3 0.0 0.6

3.0 0.0 1.2

3.3 0.0 0.0

15.0 0.7 6.0

7.2 0.1 2.2

4.2 0.3 2.3

3.3 0.0 0.0

7.7 0.0 0.0

5.3 0.0 0.0

2.2 0.0 0.0

0.0 0.0 0.0

3.0 0.0 0.0

1.2 0.0 0.0

1.5 0.0 0.0

SD

Detrital composition

M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471

Monocrystalline quartz Polycrystalline quartz K-feldspars Plagioclase Volcanic lithic fragments Plutonic lithic fragments Metamorphic lithic fragments Sedimentary lithic fragments Muscovite Chert Matrix Hevy minerals

LST, TST, and HST refer to lowstand, transgressive, and highstand systems tracts, respectively.

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M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471

metamorphic and sedimentary sandstones, with trace amounts of granitic fragments. Mica content (trace-3%; av. 1%) includes muscovite and a trace amount of biotite. Heavy minerals (trace-2%; av. !0.5%) includes zircon, apatite, and epidote. Trace amounts of glauconite grains (trace-1%; av. ! 0.5% and z100–300 mm in diameter) are rounded to subrounded in shape and are either fresh or reveal various degree of oxidation. Matrix (trace-10%; av. 2%) is composed mainly of mud and silt-sized quartz grains, being more abundant in the postglacial, Gilbert-type deltaic LST sandstones (av. 5%). There is no significant variation in detrital composition with respect to the depositional facies and systems tracts (Table 1). 5.2. Siderite: petrography, elemental and stable isotopic composition, and distribution within depositional facies and sequence stratigraphy Siderite cement in the sandstones occurs in trace to significant amounts (trace-15 vol%; av. 3 vol%; Table 1). Siderite exhibits coarse-crystalline and, less commonly, microcrystalline textural habits and displays considerable variation in chemical composition as revealed by BSE imaging. Coarse-crystalline siderite is either chemically homogenous and/or inhomogeneous and occurs as intergranular cement that fills pores varying widely in size (ca. 100– 600 mm across; Fig. 5A and B). Homogenous coarse-crystalline siderite is either Mg-poor or Mg-rich, which henceforth referred to as Types I and II, respectively (Figs. 5C and D, 6A

and B; Table 2). Type I siderite, which is Mg-poor with a mean composition of (Fe91.7Mg1.5Ca0.3Mn6.5)CO3 (Table 2; Fig. 5C and D), tends to fill large intergranular pores (ca. 250–600 mm across) in loosely packed framework grains (Fig. 5A) and, in some cases, is closely associated with kaolinite (Fig. 5D). Type II siderite, which is Mg-rich with a mean composition of (Fe72.2Mg21.7Ca0.8Mn5.3)CO3 (Table 2; Fig. 6A), tends to fill relatively small intergranular pores (ca. 100–200 mm across; Fig. 5B). Type II siderite engulfs, and thus postdating, Type I siderite (Fig. 6B), quartz overgrowths (Figs. 5B and 6C), and illite (Fig. 6D). Inhomogeneous coarse-crystalline siderite, which henceforth referred to as Type III, is characterized by mottled texture under BSE images (Fig. 6E and F). Type III siderite occurs as scattered patches, which is characterized by Mg-rich composition (Fe78.6Mg16.9Ca2.2Mn2.3)CO3 embedded in Mg-poor composition (Fe93.8Mg1.9Ca0.5Mn3.8)CO3 (Table 2; Fig. 6E and F), which displays some corrosions. Type III siderite tends to fill relatively smaller intergranular pores compared with Type I siderite (ca. 100–200 mm across), and is associated with kaolinite (Fig. 6E) and engulfs, and thus postdates, Type I siderite crystals (Fig. 6F). Microcrystalline siderite (!10 mm), which referred to as Type IV, is chemically homogenous and is Mg-rich with a mean composition of (Fe70.5Mg22.1Ca0.9Mn6.5)CO3 (Table 2). Type IV siderite occurs as tiny crystals that fringe detrital grains, and/or fills intercrystalline micropores between and dickite crystals. In some cases, the dickite is displaced entirely.

Fig. 5. Photomicrographs (Crossed Nichols) showing( (A) Coarse-crystalline siderite cement that fills large, intergranular pores, suggesting an early, precompactional origin. (B) Coarse-crystalline siderite cement that fills smaller intergranular pores and engulfs, and thus postdates, quartz overgrowths, and hence suggests mesogenetic origin. Back scattered electronic image showing: (C) homogeneous Mg-poor coarse-crystalline (Type I) siderite, which fills large pores in paraglacial, tide-dominated HST sandstones, paraglacial, foreshore-shoreface HST sandstones, and postglacial, Gilbert-type deltaic LST sandstones, indicating precipitation from meteoric waters that influxed into the sandstones during relative sea level fall and basinward shift of the shoreline. (D) Homogeneous Mg-poor coarse-crystalline (Type I) siderite associated with kaolinite (arrow) supports precipitation from meteoric waters.

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465

Fig. 6. Back scattered electronic image showing: (A) homogeneous Mg-rich coarse-crystalline (Type II) siderite in paraglacial, tide-dominated HST sandstones, paraglacial, foreshore-shoreface HST sandstones, glacial, tide-dominated estuarine TST sandstones, postglacial, Gilbert-type deltaic LST sandstones, and postglacial, shoreface TST sandstones, which is interpreted to have been precipitated from evolved formation water relative to the contemporaneous Upper Ordovician meteoric waters. (B) Homogenous Mg-poor (Type I) siderite (black arrow) engulfed by, and thus pre-dating, homogenous Mg-rich (Type II) siderite (white arrow). SEM image showing: (C) homogeneous Mg-rich coarse-crystalline (Type II) siderite engulfing quartz overgrowths. (D) Homogeneous Mg-rich coarse-crystalline (Type II) siderite engulfing illite. Back scattered electronic image showing: (E) inhomogeneous mottled (Type III) siderite associated with kaolinite (arrow). (F) Homogenous Mg-poor (Type I) siderite (black arrow) engulfed by, and thus pre-dating, inhomogeneous mottled (Type III) siderite (white arrow).

Siderite cement occurs in various depositional facies and systems tracts (Fig. 2), including: (i) paraglacial, tidedominated deltaic HST sandstones, (ii) paraglacial, foreshore to shoreface HST sandstones, (iii) glacial, tide-dominated deltaic TST sandstones, (iv) postglacial, Gilbert-type deltaic LST sandstones, and (v) postglacial, shoreface TST sandstones. The Type I and the Type III siderite cements are restricted to the paraglacial, tide-dominated deltaic HST sandstones, paraglacial, foreshore to shoreface HST sandstones, and postglacial, Gilbert-type deltaic LST sandstones close to the sequence boundary (Fig. 2). The Type II siderite occurs in the paraglacial, tide-dominated deltaic HST

sandstones, paraglacial, foreshore to shoreface HST sandstones, glacial, tide-dominated estuarine TST sandstones, postglacial, Gilbert-type deltaic LST sandstones, and postglacial, shoreface TST sandstones (Fig. 2). The Type IV siderite occurs in the glacial, shoreface to offshore TST and postglacial, shoreface TST sandstones and along the maximum flooding surface (Fig. 2). The stable isotope composition of siderite cements (Table 2) exhibit depleted d18OV-PDB values that range between K22.6 and K13.8‰. d13CV-PDB values range between –14.5 and K6.0‰. d18OV-PDB and d13CV-PDB composition of siderite cements are positively correlated (rZC0.83; Fig. 7).

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Table 2 Chemical (mole%) and carbon (d13CPDB‰) and oxygen (d13CPDB‰) stable isotope composition of siderite Samples

Texture

Mg(CO3)

Ca(CO3)

Mn(CO3)

Fe(CO3)

d13CV-PDB‰

d18OV-PDB‰

F2-4 F2-5 F2-7 F2-8-1 p1 F2-8-2 p1 F2-9 F2-14 F2-15 F2 16 F2 17 F1-21 p1 F1-21 p2 F1-21 p3 F2-22 F2-33 F2-38 F2-41 F2-45 B1-3 B1-4 B1-5-1 p1 B1-5-1 p2 B1-5-1 p3 B1-5-2 p1 B1-5-2 p2 B1-5-3 p1 B1-5-3 p2 B1-5-4 p1 B1-5-4 p2 B1-5-4 p3 B1-5-4 p4 B1-5-6 p1 B1-5-6 p2 B1-5-5 p1 B1-6 B1-7 B1-8 B1-11 B1-12-1 p1 B1-12-1 p2 B1-12-2 p1 B1-12-3 p1 B1-12-4 p1 B1-12-4 p2 B1-12-5 p1 B1-12-6 p1 B1-12-6 p2 B1-18

Type IV Type II Type II Type IV Type I Type II Type II Type II Type IV Type II Type III Type III Type III Type II Type II Type II Type II Type II Type II Type II Type II Type II Type I Type III Type III Type IV Type II Type III Type III Type II Type II Type II Type II Type II Type II Type II Type IV Type II Type IV Type III Type II Type I Type III Type III Type II Type II Type IV Type IV

20.1 18.4 19.4 18.4 3.2 25.6 18.1 18.4 19.7 22.1 0.5 11.4 4.7 24.9 23.1 22.7 21.3 25.2 21.5 20.3 25.5 22.4 1.2 0.1 19.4 15.1 16.3 0.3 19.4 12.3 26.8 25.9 24.2 22.1 19.9 22.4 26.2 18.5 0.2 13.9 24.0 0.1 5.4 20.3 22.9 22.3 25.7 26.1

0.9 0.5 0.1 0.2 0.0 0.5 1.2 0.1 1.3 0.3 1.0 0.3 0.2 0.2 0.5 1.8 2.1 0.6 0.1 bdl 0.6 2.4 0.3 0.3 1.6 1.2 0.8 0.6 0.1 0.6 0.5 1.3 0.5 0.4 1.5 bdl bdl bdl 0.2 5.6 0.5 0.5 0.6 3.4 0.3 0.3 1.4 0.9

4.2 1.7 2.7 5.4 13.2 6.8 10.2 6.2 9.1 5.3 0.8 2.8 14.9 8.4 5.3 6.6 8.4 4.1 4.2 8.1 4.6 7.4 5.4 5.3 1.0 2.0 2.4 1.5 7.1 3.2 5.6 6.0 6.0 2.2 7.4 3.5 3.2 1.1 0.4 0.4 5.9 1.0 0.4 0.3 5.3 4.6 6.2 6.1

74.8 79.4 77.8 76.0 83.5 67.1 70.5 75.3 69.9 72.3 97.8 85.6 80.2 66.5 71.1 68.9 68.2 70.1 74.2 71.6 69.3 67.8 93.1 94.2 78.0 81.7 80.5 97.6 73.4 83.9 67.1 66.8 69.4 75.3 71.2 74.1 70.6 80.4 99.2 80.2 69.6 98.5 93.7 76.0 71.4 72.8 66.7 66.9

K8.8 K14.3 K11.2 K10.2

K19.7 K22.6 K18.0 K18.5

K9.6 K14.5 K10.6 K8.5 K8.4 K8.5

K17.6 K21.4 K19.7 K14.3 K13.8 K14.3

K8.3 K8.7 K12.6 K8.9 K8.6 K8.9 K9.1 K7.1

K13.9 K14.2 K20.7 K16.0 K14.2 K14.7 K14.9 K14.7

K11.1 K8.9 K8.2 K6.0 K7.6

K14.4 K14.7 K14.5 K15.3 K15.4

K9.2

K14.4

Three varieties of siderite cements have been recognized according to their elemental composition; including homogeneous, Mg-rich (Types II and IV) siderite with a mean composition of (Fe72.2Mg21.7Ca0.8Mn5.3)CO3 and (Fe70.5Mg22.1Ca0.9Mn6.5)CO3, homogeneous, Mg-poor (Type I) siderite with a mean composition of (Fe91.7Mg1.5Ca0.3Mn6.5)CO3, and inhomogeneous mottled (Type III) siderite with Mg-rich [(Fe78.6Mg16.9Mn2.2Ca2.3)CO3] patches embedded in Mg-poor [(Fe93.8Mg1.9Mn0.5Ca3.8)CO3]. Stable isotopic composition represents the samples dominated by Mg-rich and mottled (Types II–IV) siderite cements and characterized by d18O from K22.6 to K13.8‰ and d13C from K14.5 to K6.0‰.

6. Discussion 6.1. Paragenesis and origin of siderite cements Although it is not possible to determine the precise timing of the various types of diagenetic siderite cement in relation to other diagenetic alterations, an overall paragenetic sequence

was established based on the textural relationships and oxygen isotopic data (Fig. 8). Petrographic observations and elemental composition of siderite cements suggest that the siderite formed under various diagenetic conditions. Type I siderite that fills large intergranular pores in loosely packed framework grains (Fig. 5A) is believed to have an early, pre-compactional origin, being formed subsequent to kaolinite,

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–4 R = +0.83 (n = 24) –6

d13CV-PDB‰

–8 –10 –12 –14 –16 –24

–22

–20

–18

–16

–14

–12

d18OV-PDB‰ Fig. 7. Cross plot of d13CPDB versus d18OPDB values of siderite cements showing a positive correlation (rZC0.83), which is attributed to increasing input of 12C from thermal alteration of organic matter during progressive burial and increasing temperature.

which is typically of eogenetic regime (Meisler et al., 1984; McAulay et al., 1994; Morad et al., 2000; Ketzer et al., 2003). Conversely, the precipitation of Type II siderite, which fills smaller pores and engulfs Type I siderite, quartz overgrowths and illite (Figs. 5B and 6B–D), is believed to have occurred during deeper burial. Although the exact timing of Type III siderite is not entirely clear, it engulfs Type I siderite (Fig. 6F) and this would suggest a later diagenetic origin. Type IV siderite, which occurs between and, in some cases, displaces dickite crystals along the maximum flooding surface suggests later formation during burial diagenesis. The elemental composition of siderite is obviously influenced by the concentration of Fe2C, Mg2C, Mn2C, and Ca2C ions in the pore waters (Matsumoto and Iijima, 1981; Curtis and Coleman, 1986; Mozley, 1989). The Mg-poor nature of the eogenetic, Type I siderite (Table 2) suggests precipitation from meteoric pore waters (Mozley, 1989). Conversely, Types II and IV siderite indicates precipitation from pore waters with high Mg concentrations, in other words

Diagenetic minerals

it is either of marine (Mozley, 1989) or from evolved formation water (Curtis and Coleman, 1986; Morad et al., 1994; Rezaee and Schulz-Rojah, 1998; Rossi et al., 2001). Types II and IV siderite either precipitate and engulf quartz overgrowths and illite (i.e. Type II) or precipitated between dickite crystals (i.e. Type IV). This suggests a mesodiagenetic origin from evolved formation waters. Type III siderite, which occurs as Mg-rich patches embedded in Mg-poor siderite with corrosions, suggests that this siderite formed via partial dissolution and replacement of Mg-poor siderite by Mg-rich siderite owing to an increase of Mg ions in the pore waters during subsequent burial. The presence of different siderite types in each analyzed sandstone sample prevents isotopic analysis of individual siderite cement types. The majority of bulk samples contain three dominant siderite cements. Although most isotopic analyses are bulk analysis, it is dominated by Types II–IV siderite cement. The results can still be used to explain conditions of siderite precipitation. In order to calculate the temperature under which the mesogenetic siderite (Types IIIV) has precipitated, it is important to understand if the evolved formation waters were originally marine, mixed marine– meteoric or meteoric. However, precise knowledge of d18OS-MOW composition of these pore waters is difficult to achieve. Nevertheless, using the bulk d18OV-PDB values of these siderite (K22.6 to K13.8‰), the fractionation equation of Carothers et al. (1988), and assuming d18OS-MOW values (0 to C2‰; Lundegard and Land, 1986) for the formation waters evolved from the contemporary Upper Ordovician sea water (K5 to 0‰; Marshall et al., 1997), siderite would have precipitated at temperatures ranging from 120 to 315 8C. The uppermost temperatures are unreasonable for siderite precipitation in the Upper Ordovician sandstones because this would exceed the maximum burial temperature achieved by the sediments, which is approximately 130 8C (Davidson et al., 2000; Fig. 3). Therefore, this method of assuming the d18OSMOW of the pore waters in order to calculate formation temperature leads to inconclusive results. As petrographic observations show that Types II–IV siderite postdate quartz overgrowths and illite, it is therefore possible to deduce the d18OV-PDB values of pore waters by assuming the 70 °C 290(my)

10-25 °C 450(my) eodiagenesis

130 °C 0 mesodiagenesis

kaolinite

kaolin Type I

467

dickite Type III

Type II and IV

sidertie pyrite quartz overgrowths iron-oxide Fig. 8. Paragenetic sequence of the diagenetic siderite and associated diagenetic minerals in the Upper Ordovician sandstones of the Melaz Shuqran and Mamunyiat formations based on petrographic observation and oxygen isotopic composition and burial history curve. The boundary between eodiagenesis and mesodiagenesis is according to Morad et al. (2000).

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200

20

Tempreature (°C)

150

10

0

100

50

0 –20

–10 d18O

water

0

10

20

(S-MOW) ‰

Fig. 9. Range of temperature and d18OS-MOW values of pore waters calculated based on oxygen isotope values of siderite. Siderite, which is dominated by homogenous Mg-rich (Types II and III) and inhomogeneous mottled (Type IV) type, has d18OV-PDB values between K22.6 and K13.8‰. This siderite is believed to have precipitated during mesodiagenesis at temperatures of 70 to 130 8C from pore waters with d18OS-MOW from K14.0 to C1.0‰. This is equivalent to evolved formation waters that were originally meteoric, mixed marine–meteoric and marine.

temperature range typical for quartz overgrowths and illite formation (80–130 8C; McBride, 1989; Worden and Morad, 2000). Precipitation would have occurred from pore waters with d18OS-MOW values between K14.0 and C1.0‰ (Fig. 9). This wide range of pore water composition suggests that siderite precipitation has occurred from pore waters varying in origin from evolved meteoric and mixed marine–meteoric to marine waters relative to the contemporary Upper Ordovician meteoric waters (K17‰; Craig and Gordon, 1965), and marine waters (K5 to 0‰; Marshall et al., 1997), respectively. The d13CV-PDB values (K14.5 to K6.0‰) of Types II–IV siderite are consistent a major source of carbon derived from the thermal decarboxylation of organic matter, which produces a strong 12C-enriched carbon (Hudson, 1977; Irwin et al., 1977; Carothers and Kharaka, 1980; Kharaka et al., 1983; Morad, 1998). The positive correlation between d18OV-PDB and d13CV-PDB values of siderite suggests an increase of 12C from the thermal alteration of organic matter during progressive burial and an increase in temperature (Irwin et al., 1977; Morad et al., 1990). 6.2. Summary model for the distribution of siderite Integration of petrographic observations and elemental/ isotopic composition of various types of siderite cement in sandstones have helped to explain its spatial and temporal distribution patterns. Variations in the chemical composition of siderite cements are strongly controlled, primarily, by the chemistry of pore waters, which, in turn, can be linked to

depositional environments (Mozley, 1989). The restriction of Mg-poor siderite (i.e. Type I) to the paraglacial, tidedominated deltaic HST, paraglacial, foreshore to shoreface HST, and postglacial, Gilbert-type deltaic LST sandstones close to the SB as well as its close association with eogenetic kaolinite are interpreted to indicate precipitation during eodiagenesis by meteoric waters (Fig. 10). The Mg-poor nature of siderite (i.e. Type I) corresponds to precipitation from meteoric waters (Mozley, 1989). Meteoric waters were presumably fluxed into the LST and HST sandstones as a consequence of relative sea level fall, which was aided by isostatic rebound due to glacial retreat and subsequent prograding, and thus basinward shift of the shoreline (El-ghali, 2005). The glacial, tide-dominated estuarine TST and postglacial, shoreface TST sandstones, which remained poorly cemented by eogenetic Mg-poor siderite (i.e. Type I), as well as the paraglacial, tide-dominated deltaic HST, paraglacial, foreshore to shoreface HST, and postglacial, Gilbert-type deltaic LST sandstones have all been subjected to cementation by mesogenetic homogeneous Mg-rich and inhomogeneous siderite (i.e. Types II–IV; Fig. 10) derived from evolved formation waters, which were either originally meteoric, mixed marine–meteoric or marine in composition. 7. Conclusion Petrographic characteristics and the elemental/stable isotopic compositions of siderite cements from Upper Ordovician, glaciogenic sandstones have allowed interpretation of the important factors controlling their precipitation and distribution within depositional facies, sequence stratigraphy and diagenetic evolution. This study has also explained the impact of changes in the pore water chemistry on the distribution of eo- and mesogenetic siderite cement. Important findings of the study include: (1) Eogenetic Mg-poor (Type I) siderite, which fills large pores in loosely packed framework grains and is in close association with eogenetic kaolinite, occurs in paraglacial, tide-dominated deltaic highstand systems tract sandstone, paraglacial, foreshore to shoreface highstand systems tract sandstone, and postglacial, Gilbert-type deltaic lowstand systems tract sandstone close to the sequence boundary. This siderite precipitated during eodiagenesis by influx of meteoric waters into the lowstand systems tracts during relative sea level fall and basinward shift of the shoreline and progradation of the highstand systems tracts. (2) Homogeneous Mg-rich (Types II and IV) and inhomogeneous (Type III) siderite cements dominate in the glacial, tide-dominated estuarine transgressive systems tract sandstone and postglacial, shoreface transgressive systems tract sandstone that remained poorly cemented by eogenetic Mg-poor siderite. They are also prominent in the paraglacial, tide-dominated deltaic highstand systems tract, paraglacial, foreshore to shoreface highstand systems tract, and postglacial, Gilbert-type deltaic lowstand systems tract sandstones. This siderite is of mesogenetic

M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471

landward

469

basinward postglacial shoreface

postg

lacial

gla

cia

Gilbe

1

rt-typ

l flu

via

3

e delt

l in

aic 2

cise

d-v

Silurian hot shale sequence MFS TS SB

2 paraglacial fores hore-shoreface 3 glacial tide-dominated estuarine and offshore

alle

y

MFS TS

isostatic rebound

2

highstand systems tract

SB

paraglacial tide-dominated deltaic

1

MFS TS+SB

glacial shoreface/offshore

transgressive systems tract

Pre-Upper Ordovician sequences

lowstand systems tract MFS maximum flooding surface TS transgressive surface SB sequence boundary

kaolintized feldspar

iron-oxide

Qtz

Qtz

Qtz

feld

Qtz

Qtz

Qtz

Qtz Qtz

Qtz

porosity Qtz

Qtz Qtz

Qtz Qtz

Qtz

Qtz

Qtz

Mg-poor (Type I) siderite

Qtz

Qtz

Eodiagenesis

3

2

1

kaolinitized feldspar

Qtz

Qtz Qtz

Qtz

Qtz

Qtz

mottled (Type III) siderite Qtz

Qtz

dickitized kaolinite

dickitized kaolinite Mg-rich (Type II) siderite Qtz

Qtz Qtz

Qtz

Qtz Qtz

Qtz

Mesodiagenesis

Qtz Qtz

Qtz

Mg-rich (Type IV) siderite

Qtz

Qtz Qtz

Qtz Qtz

Qtz

Qtz

quartz overgrowths

Qtz - detrital quartz grains feld- detrital feldspar grains Fig. 10. Schematic model showing the spatial and temporal distribution of diagenetic siderite and associated diagenetic minerals as well as variations in the diagenetic evolution pathways of the Upper Ordovician sandstones within a sequence stratigraphic framework.

origin as it fills relatively small pores as well as envelope quartz overgrowths and illite.

Acknowledgements M.A.K. El-ghali and K. Tajori dedicate this work to Dr S. Lagha, Al-Fateh University, Geology Department, Tripoli,

Libya who passed away while he was doing his job, field geology. The authors thank Dr S. Morad for reading and commenting on the manuscript. Thanks also go to the Petroleum Research Center, Tripoli, Libya, especially to Professor A. Sbeta, Dr A. Bourima and Dr A. El-Harbi for supporting the fieldwork. The National Oil Corporation, especially B. El-Mejrab, E. Hamuni and data section staff, are acknowledged for giving access to the drill core samples.

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