Diagenesis

The Florisbad spring site has produced a large collection of fossil bones dating to the Middle Pleistocene and several assemblages of Middle Stone Age (MSA) artifacts. The species featured in the faunal collection define the Florisian Land Mammal Age, characterized mainly by grazing ungulates that reflect an open grassland environment. Early MSA artifacts found within the basal layers represent the earliest evidence of human presence at Florisbad, followed by an intact MSA occupation horizon characterized by stone tools and animal bones, in direct association and primary context. These fossils and artifacts were found embedded within sand and peat layers. Previous studies investigated the genesis of such deposits using field descriptions and bulk sedimentological analyses. However, sediments were never studied within their original context, thus leading to difficulties in determining the formation processes of the deposits. Using a multi-analytical micro-geoarchaeological approach including sediment micromorphology, FTIR and XRD, we were able to show that the site is characterized by alternating aeolian sand layers reworked by lacustrine water during the wet phases of the Pleistocene, and peat layers formed in marshes during dry phases. The results confirm most of the previous paleoenvironmental reconstructions and provide a refined stratigraphic sequence of the site.

Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence along the margin of Great Bahama Bank (ODP Leg 166). These cycles consist of decimetre- to metre-scale alternations between light-grey and dark-grey/black wackestones/packstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The dark-grey wackestones/packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera and fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80% carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5%. Light δ13C and slightly negative δ18O values were observed in the dark uncemented layers while the cemented intervals show heavy δ13C values and slightly more positive δ18O values. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45‰. Both δ13C and δ18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy. The internal stacking pattern of the cycles is closely tied to sea level. The dark layers are deposited during rising sea level, while the light layers reflect sediment production and export during highstand and falling sea level. These cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat-topped platforms. Most measured parameters suggest that primary sediment composition played an important role in the cementation process. The primary differences in composition were enhanced during shallow burial diagenesis. The initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite-rich layers leading to the formation of the light beds. Dissolution of metastable high-Mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite. Diagenesis was enhanced by the coarse grain size of the primary aragonite-rich layers. In contrast the more pelagic, dark calcite-rich layers were only slightly affected by diagenesis and could preserve their aragonite content. The uncemented layers subsequently were subject to strong mechanical compaction, whereas the cemented beds stayed nearly uncompacted.

Geochemical characterization and numerical modelling of surface water and ground water, combined with hydrological observations, provide quantitative estimates of meteoric diagenesis in Pleistocene carbonates of the northern Bahamas.Meteoric waters equilibrate with aragonite, but water- rather than mineral-controlled reactions dominate. Dissolutional lowering of the undifferentiated bedrock surface is an order of magnitude slower than that within soil-filled topographic hollows, generating small-scale relief at a rate of 65–140 mm ka−1 and a distinctive pocketed topography. Oxidation of organic matter within the subsoil and vadose zones generates an average P of 4·0 × 10−3 atm, which drives dissolution during vadose percolation and/or at the water table. However, these dissolution processes together account for <60% of the average rock-derived calcium in groundwaters pumped from the freshwater lens. The additional calcium may derive from oxidation of organic carbon within the lens, accounting for the high P of the lens waters. Mixing between meteoric waters of differing chemistry is diagenetically insignificant, but evapotranspiration from the shallow water table is an important drive for subsurface cementation.Porosity generation in the shallow vadose zone averages 1·6–3·2% ka−1. Phreatic meteoric diagenesis is focused near the water table, where dissolution generates porosity at 1·4–2·8% ka−1. Maximum dissolution rates, however, are similar to those of evaporation-driven precipitation, which occludes porosity of 4·0 ± 0·6% ka−1. This drives porosity inversion, from primary interparticle to secondary mouldic, vug and channel porosity. In the deeper freshwater lens, oxidation of residual organic carbon and reoxidation of reduced sulphur species from deeper anaerobic oxidation of organic carbon may generate porosity up to 0·06% ka−1.Meteoric diagenesis relies critically on hydrological routing and vadose thickness (controlled by sea level), as well as the geochemical processes active. A thin vadose zone permits direct evaporation from the water table and drives precipitation of meteoric phreatic cements even where mineral stabilization is complete. Copyright © 2007 John Wiley & Sons, Ltd.

25-Norhopanes have been found for the first time in Jurassic organic-poor mudstones in the northern Qaidam Basin (northwestern China). The carbon number distribution in the m/z 177 mass chromatograms does not consist of the full suite from C28 to C34, although this is a well-known feature generally ascribed to 25-norhopanes in source rocks. The ranges of C29 25-norhopane/C29 30-norhopane and C29 25-norhopane/C30 hopane are 0.07–0.08 and 0.03–0.04, respectively. These values are within the recognized global background level for 25-norhopane concentrations in source rocks [Blanc, P.L., Connan, J., 1992. Origin and occurrence of 25-norhopanes: a statistical study. Organic Geochemistry 18, 813–828]. Here, we provide additional evidence for the existence of 25-norhopanes in mudstones/source rocks. They were not detected in organic-rich mudstones, implying that the accumulation or diagenesis of the organic-poor mudstones may have provided the right conditions for the formation of 25-norhopanes. The depositional setting is suggested to be anaerobic and distant from the lacustrine lakeshore, based on biomarker and petrological features and previous sedimentary environment studies. In such a nutrient limited environment with relatively low source organic matter, microbial reworking of biomass occurred – microbial consortia scavenged the existing hopanoids, contributing biomass and releasing nutrients to perform 25-norhopane-forming reactions.

ORIGINAL ARTICLE Multistage dolomitization in the cretaceous carbonates of the east Shahmirzad area, north Semnan, central Alborz, Iran Kamaladdin Bazargani-Guilani • MaryamFaramarzi • Mohammad Ali Nekouvaght Tak Accepted: 27 May 2010 Ó Springer-Verlag 2010 ...

Dolomitization of  relatively thick carbonate successions occurs via an effective fluid circulation mechanism, since the replacement process requires a large amount of Mg-rich fluid interacting with the CaCO3 precursor. In the western end of the Neotethys, fault-controlled extensional basins developed during the Late Triassic spreading stage. In the Buda Hills and Danube-East blocks, distinct parts of silica and organic matter-rich slope and basinal deposits are dolomitized. Petrographic, geochemical, and fluid inclusion data distinguished two dolomite types: (1) finely to medium crystalline and (2) medium to coarsely crystalline. They commonly co-occur and show a gradual transition. Both exhibit breccia fabric under microscope. Dolomite texture reveals that the breccia fabric is not inherited from the precursor carbonates but was formed during the dolomitization process and under the influence of repeated seismic shocks. Dolomitization within the slope and basinal succession as well as within the breccia zones of the underlying basement block is interpreted as being related to fluid originated from the detachment zone and channelled along synsedimentary normal faults. The proposed conceptual model of dolomitization suggests that pervasive dolomitization occurred not only within and near the fault zones. Permeable beds have channelled the fluid towards the basin centre where the fluid was capable of partial dolomitization. The fluid inclusion data, compared with vitrinite reflectance and maturation data of organic matter, suggest that the ascending fluid was likely hydrothermal which cooled down via mixing with marine-derived pore fluid. Thermal gradient is considered as a potential driving force for fluid flow.

The Middle to Late Pleistocene spring site of Florisbad, South Africa, is the name site of the Florisian Land Mammal Age (LMA), and it has produced a large collection of fossil bones from different sedimentary contexts. The largest of these faunal assemblages derives from vertically intrusive dormant spring vents and is taphonomically distinct from specimens associated with a Middle Stone Age occupational horizon. The Florisbad faunal assemblages include several extinct and extant animal species, mainly grazing ungulates that represent a highly productive open grassland environment. The Florisian LMA was characterized by substantially wetter conditions compared to the Holocene. Geological evidence at Florisbad and the characteristic occurrence of water-dependent species, such as lechwe, waterbuck, hippopotamus and a variety of water birds, reflect the presence of a vast intermittent paleo-lake forming during the wet phases of the Middle and Late Pleistocene. During dry phases, the area of spring activity was not inundated and provided a suitable habitat for both humans and animals. Bones collected from different sedimentary contexts exhibit a number of post-depositional alterations that reflect specific diagenetic processes, which led to fossilization. Two contrasting hypotheses regarding these processes, based on morphological studies of bones and groundwater chemistry, were proposed in the past. The first identified spring water as a major fossilization agent, whereas the second saw it as a medium conducive to bone dissolution. Using Fourier Transform infrared spectrometry (FTIR) and micro-spectrometry (µFTIR), we determined the mineralogical composition of these bones. We show here that specimens from inactive paleo-spring vents are characterized by the formation of carbonate fluorapatite on the outer surface of cortical bone, and locally exhibit authigenic calcite crystals, as opposed to bones collected from non-spring contexts at the site. The nucleation of carbonate fluorapatite is linked to the high fluoride content of the alkaline spring water, thus confirming its key role in the fossilization process of bones recovered from paleo-spring vents. Therefore, our results support the first of the two hypotheses advanced in the past.

Twenty nine sections measured, along 135km distance, in the Cyrenaican Miocene of Ar-Rajmah Group carbonate rocks, northeast Libya. Four sections were sampled every 0.5 m for sequence stratigraphy, stable isotope (δ18O, δ13C) chemostratigraphy and diagenesis. Petrographic study helped to identify facies based on texture, grain types, mud content, cements, porosity types and amount, dolomitization, dedolomitization, silicification, and recrystallization. Combined the sequence stratigraphy, chemostratigraphy, and petrography indicates two supersequences separated by a sequence boundary and two different diagenetic patterns.
The Miocene Ar-Rajmah Group carbonate rocks record two 2nd-order supersequences (97 m maximum thickness); six 3rd-order sequences, and 10 parasequences .The Ar-Rajmah Group depositional facies are grouped into peritidal facies, ramp crest facies and subtidal facies. The peritidal facies include: evaporites, microbialites, pelletal wackestone/packstone, porites reefs and bioclastic packstone, very fine to fine quartz sandstone, and green shale. The ramp crest facies includes: oolitic grainstones. The subtidal facies include: bioclastic carbonates, reworked bioclastic carbonates, red algae reefs, and reworked red algae.
The chemostratigraphic data of the Ar-Rajmah Group reconfirmed the preservation of the entire Miocene as indicated by previous biostratigraphic studies. The stable isotope data indicates the Early Miocene is generally enriched in both δ18O and δ13C, the Middle Miocene is enriched in δ13C but depleted in δ18O, and the Late Miocene is depleted in both δ18O and δ13C.
The petrographic analysis shows two distinct lithological, textural and paragenetic patterns corresponding roughly to the Middle Miocene Langhian-Serravallian boundary. The older Miocene interval (Langhian and older) is dominated by silicified dedolomitized bioclastic-rich, bryozoan, and red algal packstone. The younger Miocene interval (Serravallian and younger) is dominated by silicified and recrystallized oolitic grainstone, microbial-bioclastic-oolitic grainstone.
The main porosity types in the Miocene sequences are oomoldic, fenestral, dolomoldic, frame-growth, biomoldic, fracture, vuggy, cavernous, and intercrystalline. However, the dolomoldic and frame-growth porosity is restricted to the older dedolomitized Miocene interval, where the oomoldic and fenestral porosity is restricted to the younger recrystallized Miocene interval. The Late Miocene has the lowest average porosity values, where the Middle Miocene has the highest average porosity values, and the Early Miocene has medial average porosity values.
The paragenetic sequence in the ArRajmah Group is: 1) micritization, 2) rim cement, 3) pore filling cement, 4) fracturing, 5) initial dissolution, 6) recrystallization, 7) dolomitization, 8) dedolomitization, 9) secondary dissolution, 10) silicification, 11) a second period of fracturing, and 12) a third episode of dissolution.

Wood ash found at archaeological sites is the most direct evidence of the presence of combustion features. The preservation of wood ash within archaeological deposits is often poor due to diagenetic processes and therefore its identification requires the use of spectroscopic methods. FTIR spectrometry is able to detect the main calcite component of wood ash, unless this is mixed with other calcitic phases of geologic origin. Using FTIR spectrometry we identified non-biogenic aragonite in archaeological heat-altered sediments and lime plasters. The presence of aragonite in such conditions is unusual, as it is unlikely to crystallize at ambient temperatures and pressures or in absence of Mg. The experimental conditions which favor the nucleation of this aragonite phase were investigated with FTIR spectrometry, XRPD and SEM imaging of modern lime plaster and quicklime samples prepared in the laboratory using different calcium carbonate starting materials. We show that aragonite forms at ambient temperatures and pressures upon carbonation of calcium oxide and calcium hydroxide together with calcite and its nucleation and growth are influenced by environmental parameters, such as carbon dioxide partial pressure, relative humidity and temperature. This pyrogenic aragonite is a reliable indicator for calcareous materials exposed to temperatures above 600 °C, and therefore it can be used to determine the presence of heat-altered sediments and ash in the absence of structured combustion features. This mineral phase could have applications in radiocarbon dating as well.

Geochemical composition of carbonate sediments are in
equilibrium
with
the
contemporaneous
sea
-
water composition in absence of considerable
biotic
fractionation
. However, in carbonate sediments, diagenetic
im
prints
may
have considera
ble effects on primary sediments textures, mineralogy,
reservoir quality and finally geochemical characters. Depending on some
factors including primary (depositional) sediments characteristics,
governing climate and diagenetic history,
geochemical composi
tion of
carbonates has been altered by post
-
depositional overprints. Stable
isotopes and trace elements analyses can be used as a good tool to
measure
the extent
of these
alterations
1
,
2
. Coupled imprints of tropical climate and
recurring emersions ha
d
cons
iderable effects on Middle
-
Upper Cretaceous
carbonate reservoirs of the SW Iran (Dezful
E
mbayment) and Middle East
region
3
.
Petrographic studies (
from
core to thin section scales) and
geochemical analyses (stable isotopes and trace elements) were carried out
on 478 samples from
fi
v
e giant and supergiant oilfields in
these
e
mbayment
to reveal the main diagenetic alterations of these
karstified
carbonate
sequences
(Fig. 1)
.
Variations in
δ
18
O and
δ
13
C
compositions
and
trace
elements
(
Mn, Fe and Sr
)
concentrations
are useful diagenetic
indicators
that
resulted in
classification of
studied intervals
into
four
diagenetic
classes
according to their
meteoric diagenetic features,
intensities and developments.