About Carbonate Diagenesis: Meteoritic diagenesis is a transformation that occurs at or near the Earth’s surface in strata influenced or penetrated by water of recent atmospheric origin. The meteoritic environment is generally divided into unsaturated (vadose) and saturated (phreatic) zones separated by a water table (see top diagram, opposite page). The interfaces between meteoritic surface fluids and layers filled with other pore fluids (seawater or groundwater) are “mixing zones” that may have special diagenetic properties. Many, perhaps even most, shallow marine carbonate deposits are subject to meteoric diagenesis, either by the buildup of sediments above sea level or by a subsidence of sea level that exposes the platform carbonates. In addition, meteoric water can circulate far below the land surface and alter carbonate deposits that are far older than the exposure interval. Meteoric processes typically occur over periods of hundreds to millions of years.

1. September 2023

2. Carbonate Diagenesis: Eogenetic Meteoric Diagenesis
Abstract
• Meteoric diagenesis represents alteration that occurs at or near the earth’s surface in strata influenced or
pervaded by waters of recent atmospheric origin. The meteoric environment is typically divided into
unsaturated (vadose) and saturated (phreatic) zones divided by a water table (see top diagram, facing page).
The interfaces between surficial meteoric fluids and strata filled with other pore fluids (seawater or basinal
waters) are “mixing zones” that can have special diagenetic characteristics.
• Many, perhaps most, shallow marine carbonate deposits undergo meteoric diagenesis, either as a
consequence of the buildup of sediments above sea level or through drops in sea level that expose platform
carbonates. In addition, meteoric water can circulate well below the land surface to alter carbonate deposits
far older than the exposure interval. Meteoric processes commonly act over time periods of hundreds to
millions of years.

3. • Meteoric diagenetic patterns typically are complex and variable for the following reasons:
1. regional and temporal variations in starting material;
2. variations in rainfall and water throughput rates (in part, related to permeability variations);
3. variations in water chemistry (from locality to locality or vertically through the water column at any
one site, especially at mixing interfaces);
4. variations in the duration of exposure or alteration during multiple episodes of exposure; and
5. the effects of plants and plant-derived acids that vary regionally and also changed through geologic
time as a consequence of evolution of different plant groups.
• The vadose zone is characterized by extensive dissolution of unstable carbonate minerals (aragonite and
high-Mg calcite), often with reprecipitation of more stable carbonate (low-Mg calcite). As a consequence,
primary porosity commonly is filled during meteoric diagenesis, and secondary porosity is created.

4. • Unless there is a thinning or collapse of the rock section, meteoric diagenesis is relatively porosity neutral,
at least at the scale of grains, with dissolution at one site supplying solutes for reprecipitation elsewhere.
Meteoric diagenesis does, however, have a strong effect on permeability (e.g., permeability reductions
through cementation of interconnected primary pores or permeability increases through solution
enlargement of fractures).
• Many vadose cements have fabrics reflecting the selective distribution of water in that environment pendant
(microstalactitic or gravitational) cements hanging from the undersides of grains and meniscus cements
concentrated at grain contacts. Whisker crystals (also termed needle-fiber cement), calcified filaments,
blackened pebbles, root structures (rhizoliths), microspar, and Microcodium also are common features.
• Phreatic zone cements are typically isopachous rims or complete pore fillings of equant calcite.
• Freshwater meteoric calcites are depleted in Sr2+, Mg2+, δ18O, and δ13C, relative to their marine
precursors. Most, but not all, meteoric settings are oxidizing, resulting in typically low Fe2+ and Mn2+
contents in meteoric cements (reflected in non-ferroan staining and no cathodoluminescence response).

5. • Diagenesis is the process that describes physical and chemical changes in sediments first caused by
water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and
temperature only start to play a role as sediments become buried much deeper in the Earth's crust. In the
early stages, the transformation of poorly consolidated sediments into sedimentary rock (lithification) is
simply accompanied by a reduction in porosity and water expulsion (clay sediments), while their main
mineralogical assemblages remain unaltered. As the rock is carried deeper by further deposition above, its
organic content is progressively transformed into kerogens and bitumens.
• The process of diagenesis excludes surface alteration (weathering) and deep metamorphism. There is no
sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and
pressures.
• Hydrothermal solutions, meteoric groundwater, rock porosity, permeability, dissolution/precipitation reactions,
and time are all influential factors.

6. • After deposition, sediments are compacted as they are buried beneath successive layers of sediment and
cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be
replaced by other minerals (e.g. calcite, siderite, pyrite or marcasite) during diagenesis. Porosity usually
decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization.
• The study of diagenesis in rocks is used to understand the geologic history they have undergone and the
nature and type of fluids that have circulated through them. From a commercial standpoint, such studies aid
in assessing the likelihood of finding various economically viable mineral and hydrocarbon deposits.
• The course of diagenesis is dictated by sedimentary factors such as particle size and mineralogic
composition, fluid-rock ratio, fluid chemistry and flow rates, organic matter content, the presence of
microbial communities, as well as environmental conditions (temperature, fluid chemistry, pressure).

7. • Where multiple diagenetic events have influenced a sedimentary
rock, the paragenetic sequence of these events can be reconstructed
by integrating a microscope (thin sections to scanning electron
microscopy) and geochemical studies of the sedimentary rocks and
their fluid inclusion waters as well as associated organic matter. The
diagenetic transformation of sedimentary rocks can substantially
impact their porosity and permeability and in turn their fluid conduit
and aquifer or reservoir potential (Montañez 1997).

8. • Synonyms
• Diagenesis classified by setting and evolutionary stage of sedimentary basins:
1. Eogenetic or eodiagenesis (near surface and shallow burial),
2. mesogenetic or mesodiagenesis (deeper burial), and
3. telogenetic or telodiagenesis (uplifted succession) (Choquette and Pray 1970).
• Diagenesis classified by process:
1. Syndiagenesis (biogeochemical processes at the sediment-water interface through shallow burial),
2. anadiagenesis (dominantly physicochemical processes under deeper burial or orogenic conditions),
and
3. epidiagenesis (biogeochemical processes associated with fluid flow during uplift) (Fairbridge 1967).
4. Catagenesis (late, deep-burial diagenesis, referred by some as “burial metamorphism” as it
incorporates the earliest stage of metamorphism).

9. • Diagenesis is the sum total of physical, chemical, and biological processes that occur in sediments and
sedimentary rocks from immediately after deposition through to the metamorphic realm. No universal
definition exists for diagenesis and the term has evolved since it was defined nearly 150 years ago (de
Segonzac 1968). It is generally agreed that diagenetic processes occur under Earth surface conditions (~0–
30 °C and 1 bar of pressure) to temperatures of ≤250 °C and pressures of up to 2.5 kb (7 km) involving a
broad range of fluid compositions from fresh water to concentrated brines (Fig. 1).

10. • In geology, aspect ratio refers to the ratio of pore size to pore throat
size.
• The aspect ratio has small ranges in intergranular and intercrystalline
pore systems.
• The term "aspect ratio" is also used in photolithography to describe
the ratio of the height of a vertical sidewall to its width.

11. • Meteoric diagenesis is a type of diagenesis that occurs at or near the Earth’s surface in strata influenced or
pervaded by waters of recent atmospheric origin. The meteoric environment is typically divided into
unsaturated (vadose) and saturated (phreatic) zones divided by a water table. The meteoric diagenetic realm
is dynamic with fluctuations in base level, whether driven by eustasy or tectonics, that induce repeated large-
scale vertical migration of the marine, vadose, phreatic, and mixing zones through a sedimentary succession
over time. In a case study of the Jingxi Area in the Ordos Basin, China, three distinct phases of the meteoric
diagenetic process were observed in the Ordovician. The phreatic zone formed in the first phase is partially
preserved, whereas those formed in the second and third phases are completely retained. Strata in the
western (close to the Central Uplift Belt) and eastern parts underwent bedding-parallel meteoric dissolution
and cross-layer meteoric dissolution, respectively. The distributions of the dissolution formation thicknesses in
two successive phases of the meteoric diagenetic process exhibit complementarity at the same positions in
the plane. The favorable reservoirs at the top of the Ordovician are mainly distributed in the western parts. The
second phase of the meteoric diagenetic process is most conducive to the formation of carbonate reservoirs 3

12. Wetting front infiltrating into the vadose zone
• The occurrence and types of fingered flow are sensitive to many
factors such as
initial water content
size and distribution of media particles
rainfall intensity
water repellence
Reactive-infiltration instability
develops when a porous matrix is dissolved by a flowing reactive fluid
is a positive feedback between spatial variations in porosity in the initial
matrix and the local dissolution rate

13. Instability wavelength and the average distance
between the solution pipes
• The instability wavelength depends on
1. the flow rate
2. diffusion coefficient of the solute
3. reaction rate
4. the permeability contrast between the dissolved and undissolved phase
• Inversely, measuring the distance between the solution pipes will
allow us to estimate precipitation rates during their formation times.

14. • In fluid dynamics, a uniform flow is a type of flow where the velocity of
fluid remains constant with respect to space. On the other hand, a non-
uniform flow is a type of flow where the velocity of fluid changes with
respect to space.
• For example, in uniform flow, the fluid particles move parallel to each
other in straight lines, while in non-uniform flow, the fluid particles move
in curved paths and at different speeds.
• Would you like me to provide more information on this topic?

15. • In geomorphology, we argue that focused flow is one of the most fundamental and general
mechanisms of structure formation – it is not the erosion of a stable and evenly distributed
water flow itself that shapes the landscape, but the inhomogeneous erosion of a focused flow
that leads to the formation of river networks or cave systems and shapes valleys and
mountainsides.
• Understanding the initial trigger of a focused flow and its characteristics, parameters, and timing
provides knowledge essential for interpreting paleoenvironments and paleoclimates.
• For example, karst geomorphological features such as caves and pocket valleys have been used
to provide insight into landscape evolution that can span tens, thousands, or even millions of
years.
• They are indicators of the effects of climate on the landscape and thus provide information
about the response of the landscape to climate in the past, which is essential for making
predictions about the future response of the landscape.

16. • In physics, focused flow is one of the best examples of self-organizing
phenomena, where the interaction of different processes leads to the
formation of qualitatively new patterns of organization. focused flow occurs
in phenomena as diverse as viscous fingering, convective instabilities, and
reactive infiltration instabilities.
• The initial stages of these processes, with the formation of sinusoidal
perturbations in the initially uniform system, are now well understood.
However, much less is known about the nonlinear regime in which the
initial interfacial perturbations transform into finger-like structures that
enter the system. Interestingly, in the long-term limit, these emergent
structures often take a well-defined, invariant form/shape (such as the
Saffman-Taylor finger in viscous fingering or the Ivantsov paraboloid in
dendritic growth) that are independent of the initial conditions and
depend only on environmental parameters such as flow rate or system size.
It would be natural to expect that such invariant shapes also exist in
geological systems.

36. • Preferential flow pathways are conduits that allow water to flow with
little resistance and relatively fast speeds in the subsurface areas of
groundwater.
• They are considered to be among the most important sources of
contaminants in drinking water supplies for many reasons, including
their ability to transport contaminants rapidly and over long distances
1. Preferential flow is the uneven movement of water and solutes
through a relatively small portion of the soil volume at relatively high
flow rates allowing these substances to reach greater depth in shorter
time than would be possible in a uniform flow situation 2. The
differences in preferential flow paths under various land uses and
their relationships to hydraulic properties remain uncertain 3

37. https://doi.org/10.1017/9781009100717
• The convection-diffusion equation is a mathematical model that describes
the transfer of particles, energy, or other physical quantities within a
physical system due to two processes: diffusion and convection 1. The
equation is a combination of the diffusion and convection (advection)
equations 1. The general form of the equation is:∂t∂c​+∇⋅(vc−D∇c)=Rwhere
c is the variable of interest, such as species concentration for mass transfer
or temperature for heat transfer, D is the diffusivity (diffusion coefficient), v
is the velocity field that the quantity is moving with, and R describes
sources or sinks of the quantity c 1. The right-hand side of the equation is
the sum of three contributions. The first term describes diffusion, while the
second term describes convection (or advection) 1.The convection-
diffusion equation has many applications in various fields such as fluid
dynamics, heat transfer, mass transfer, and chemical engineering 1.