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 b 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 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 (Fe 91.7 Mg 1.5 Ca 0.3 Mn 6.5 )CO 3 , 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 (Fe 72.2 Mg 21.7 Ca 0.8 Mn 5.3 )CO 3 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. d 18 O V-PDB values of this siderite, which range between K22.6 and K13.8‰, suggest that precipitation has occurred from evolved formation waters with d 18 O 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; 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 compo- sition (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 post- glacial), 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 Marine and Petroleum Geology 23 (2006) 459–471 www.elsevier.com/locate/marpetgeo 0264-8172/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.02.002 * 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).
Transcript
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, Villavagen 16, SE 75236 Uppsala, Swedenb 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
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 betweenK14.0 andC1.0‰ 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.
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.
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471460
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
3 = very fine sand4 = fine sand5 = medium sand6 = coarse sand
fine grained
sand
ston
eco
nglo
mer
ate
7 = very coarse sand
8 = granules
9 = pebbles
10 = cobbles11 = bouldrers
Grain size
LST = Lowstand systems tracts
TST = Transgressive systems tracts
HST = Highstand systems tracts
CS = Condensed section
mfs = Maximum flooding surface
ts = Transgressive surface
SB = Sequence boundary
Sequence stratigraphy
Others
lenticular bedding
flaser bedding
mud drapes
Coarsing and shallowing
fining and deepeing
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) 459–471 461
5 5
SublithareniteSubarkose
Quartz
Quartzarenite
Litharenite
Feldspatic litharenite
Lithic arkose
Ark
ose
Lithic sub-arkose
25
5 5
Subarkose
25
Quartz
Sublith- arenite
Tem
pera
ture
˚C
Age in millions of years
Dep
th in
met
res
0100200300400500
100
501000
2000
130
0
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).
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471462
ice-rafted debris, and (b) a highstand systems tract (HST) with
paraglacial, tide-dominated deltaic deposits. The HST sand-
stones 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 low-
stand 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).
5010 10Feldspar Lithic fragments
Fig. 4. Framework grain (i.e. quartz, feldspars and lithic fragments)
composition of 20 representative Upper Ordovician sandstone samples from
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 carbon-
coated, 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.
5. Results
5.1. Sandstone composition
The sandstones are dominantly medium- to very coarse-
grained, poorly to well-sorted, quartz arenites with a mean
modal composition of Q97.8F1.3L0.9 (Fig. 4; Table 1). Mono-
crystalline 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
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
postglacial, shoreface TST sandstones, which is interpreted to have been precipitated from evolved formation water relative to the contemporaneous Upper
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‰.
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471466
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,
–24 –22 –20 –18 –16 –14 –12–16
–14
–12
–10
–8
–6
–4
R = +0.83 (n = 24)
d18OV-PDB‰
d13C
V-P
DB
‰
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.
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471 467
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 eodiagenesis
kaolin
sidertie
pyrite
quartz overgrowths
iron-oxide
450(my)10-25 °C
kaolinite
Type I
Fig. 8. Paragenetic sequence of the diagenetic siderite and associated diagenetic min
formations based on petrographic observation and oxygen isotopic composition and
according to Morad et al. (2000).
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 II-
IV) 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
toC2‰; 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 precipi-
tation 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 d18OS-
MOW 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
mesodiagenesis
290(my) 070 °C 130 °C
dickite
Type II and IVType III
erals in the Upper Ordovician sandstones of the Melaz Shuqran and Mamunyiat
burial history curve. The boundary between eodiagenesis and mesodiagenesis is
–20 –10 0 10 20
0
50
100
150
200T
empr
eatu
re (
°C)
d18Owater (S-MOW) ‰
01020
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.
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471468
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, tide-
dominated 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 iso-
topic compositions of siderite cements from Upper Ordovician,
glaciogenic sandstones have allowed interpretation of the
important factors controlling their precipitation and distri-
bution 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 inhomo-
geneous (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
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.
M.A.K. El-ghali et al. / Marine and Petroleum Geology 23 (2006) 459–471 469
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.
