Non-Marine Carbonate Rocks Environments

Non-Marine Carbonate Environments

Non-marine carbonate rocks form in environments that are not associated with the open ocean. Non-marine carbonates originate in terrestrial and aquatic environments without marine influence. These carbonates, formed by abiotic and/or biotic processes differ in many aspects from marine carbonates, which are much better known to geologists.

These carbonates are formed in terrestrial subaerially exposed settings and in submerged aquatic settings.

Pedogenic (Soil) Environments

Pedogenic Carbonates: These are formed in soils rich in calcium carbonate through processes involving chemical and physical weathering of host carbonate rock, soil development, and the accumulation of calcium carbonate from evaporating porewaters.

Paleosols: A paleosol is defined as 'a buried soil horizon of the geologic past', often indicating past subaerial exposure conditions.

Caliche and Calcrete:

Caliche refers to a carbonate horizon in the soil where secondary carbonate accumulation occurs, often less indurated.

Calcrete typically denotes the more hardened, cemented carbonate layer, although in many contexts, the terms are used interchangeably. The term "caliche" stems from Latin calx, meaning lime.

Formation of Caliche/Calcrete: Critical factors include:

Climate: A significant net annual moisture deficit.
Topography.
Vegetation.
Presence of carbonate and related minerals.
Properties of the host substrate: Carbonate content, texture, porosity, and permeability.
Microbial activity.
Time: Exposure can range from tens of thousands of years to millions, influencing the degree of development.

Palustrine Environments (Wetlands)

Palustrine limestones are deposits formed in shallow freshwater environments that show extensive pedogenic modification. The term derives from 'paludal', indicating marshy or swampy conditions.

Characteristics: Originally considered as near-shore deposits of shallow lakes with fluctuating levels, they exhibit an interbedding of lacustrine facies (with specific fossils indicating aquatic conditions) and pedogenic facies (showing signs of soil formation like root traces, desiccation cracks, etc.).

Modern Analogue: The freshwater carbonate marshes of the Florida Everglades serve as a good example, where seasonal water depth variations and minor topographic changes lead to a mix of soil formation, carbonate precipitation, and occasional submergence.

Cave Environments

Speleothems: These are secondary mineral deposits formed in caves, serving as valuable archives for paleoclimate research due to their ability to record climatic fluctuations over various timescales.

Cave Deposits:

Calcareous Tufa and Moonmilk: Porous and often soft, these are formed from waters in the vadose zone (above the water table) or shallow phreatic zone (just below the water table).

Speleothems: These include:

Flowstones: Deposited from flowing water films on cave walls or floors.

Cave Pearls: Small, spherical formations created in pools by the agitation of water around a nucleus.

Dripstones Types:

Stalactites: These are the formations that hang from the ceiling of caves. They start as a hollow tube (often called a "soda straw" stalactite when very young) where water flows through the center and deposits calcium carbonate at the tip. As the outer ring thickens, the stalactite can grow in diameter, becoming conical or carrot-shaped over time.
Stalagmites: Corresponding formations that grow upward from the cave floor where the drips from stalactites (or directly from the ceiling) land. Stalagmites do not start as hollow tubes; instead, they grow from the accumulation of minerals in a more solid form. They tend to have a broader base and can become quite large and rounded or even columnar if they eventually meet with a stalactite.
Columns or Pillars: When a stalactite and stalagmite meet, they can form a column, which continues to grow as long as water flows along it.
Flowstones: These are sheet-like formations that form when water flows over a surface in a cave.
Curtains: These are thin, sheet-like formations that hang from the cave ceiling.

Eolian Carbonates

Eolian Carbonate Environments refer to settings where wind (eolian processes) plays a significant role in the deposition, erosion, and formation of carbonate rocks or sediments.

Formation: Eolian carbonates typically form in arid to semi-arid regions where carbonate-rich material can be eroded, transported, and deposited by wind. Eolian carbonates typically form in coastal or near-coastal environments where carbonate-rich material, such as shell fragments, ooids (small, spherical grains), or carbonate mud, is available for wind transport.

Eolianite: This is the rock formed from the consolidation of carbonate dunes. Eolianites can preserve the original dune structure, including large-scale cross-bedding, which indicates the paleowind direction.

Modern Examples:

The Bahamas and Bermuda have extensive eolianite formations, where carbonate sands from beaches have been blown inland to form dunes that later lithified.
In Australia, the Nullarbor Plain has regions where carbonate eolian processes have been significant.

Spring Deposits

Travertine and Tufa: These are formed by the precipitation of carbonate minerals from spring water. Travertine forms in hot springs or limestone caves, while tufa is associated with cooler, ambient-temperature springs where algae and mosses can aid in carbonate precipitation.

Lacustrine Environments (Lakes)

Lake Deposits: Carbonates like limestone and dolomite can form in lakes where the water is supersaturated with calcium carbonate. These environments can produce varved carbonates, which are finely layered deposits often reflecting seasonal changes. Marl, a calcium carbonate-rich mud, is also common in lake settings.

Glacial Carbonates

Glacial carbonates refer to carbonate deposits that are directly or indirectly associated with glacial environments. These can include:

Tillites: Lithified glacial till which might contain carbonate clasts or matrix derived from the erosion of carbonate bedrock by glaciers.

Glaciomarine Sediments: Carbonates that precipitate in marine settings influenced by glaciers, where cold, meltwater-laden currents can lead to unique depositional environments. These sediments often mix with terrigenous material from glacial meltwater.

Glendonites are calcite pseudomorphs after ikaite (CaCO₃·6H₂O, which is a hydrated form of calcium carbonate that forms at near-freezing temperatures. Ikaite is metastable and typically transforms into calcite when temperatures rise or when removed from its formation environment.