From Sediment To Stone

Sediment

Sediment is loose material deposited at or near Earth’s surface. It may consist of sand, silt, clay, shell fragments, mineral grains, biological debris or mixtures of these materials. A pile of sand is sediment. Sandstone is rock.

Domain 9 begins with the physical question between those two states: what actually changes the loose material into stone?

Lithification

Lithification literally means making into stone. It is the collection of physical and chemical processes through which loose sediment becomes consolidated sedimentary rock.

The familiar transformations include loose sand becoming sandstone, mud becoming mudstone or shale and carbonate sediment becoming limestone. These transformations require physical mechanisms. Elapsed time alone does not rearrange grains, remove pore space, expel fluid, dissolve minerals, transport dissolved material or precipitate cement.

Diagenesis

Diagenesis is the broader term for physical, chemical and biological changes that affect sediment after deposition and during burial, generally before conditions reach those conventionally classified as metamorphism.

Lithification therefore occurs largely within the broader process of diagenesis. Diagenesis can include compaction, cementation, dissolution, mineral replacement, recrystallization, biological activity and reactions involving pore fluids. Different sediments and environments can reach a coherent rock state through different combinations of these processes.

Compaction

Compaction is the reduction of sediment volume and pore space as grains rearrange, deform, or become more tightly packed under loading and burial. Pore space is the open volume between grains and may contain water, gas or other fluids.

As burial increases, overlying material can increase stress carried through the grain framework. Grains move into tighter arrangements, pore volume decreases and fluids may be expelled. Compaction performs genuine mechanical work on sediment, but it does not by itself explain every sedimentary rock.

Cementation

Cementation occurs when minerals precipitate from fluids within pore spaces and bind sediment grains together. Common cements include silica, calcite, dolomite and iron-bearing minerals.

Water moving through sediment can contain dissolved mineral constituents. As temperature, pressure, chemistry, acidity, saturation or fluid composition changes, dissolved material can precipitate between grains. The new mineral growth can bridge grain contacts and progressively convert loose sediment into coherent rock. The cement is actual mineral material occupying pore space and binding the sedimentary framework.

A Coupled Physical System

Lithification is best understood as a coupled system rather than a contest between pressure and chemistry. Burial and confinement alter grain contacts and pore space. Pressure and stress influence compaction and dissolution. Fluids occupy and move through the pore network. Chemistry determines what can dissolve and precipitate. Temperature affects reaction rates and mineral stability.

These variables interact. A change in one can alter the behavior of the others.

Pressure is of the essence without being the only variable.

Laboratory Two

The Natural Burial Sequence

Natural sedimentary basins allow the transformation to be followed through increasing burial. Gulf Coast sandstone sequences provide a useful example. In the Wilcox system, sandstones have been examined from shallow burial to depths approaching 6.7 kilometers.

Across that sequence, average porosity falls dramatically. Near the shallow end, porosity can be approximately one-third of the rock volume. At greater burial and higher temperature, average porosity falls toward approximately one-eighth. Mechanical compaction removes substantial early pore space, while mineral cementation increasingly occupies the remaining pore system.

Other Gulf Coast sandstone systems preserve the same broad progression. Original depositional porosity near 40 percent can fall rapidly during relatively shallow burial as grains rearrange, followed by additional loss as quartz, calcite and other cements precipitate.

The natural sequence identifies physical changes rather than merely an age progression: grains become more tightly packed, pore space decreases, fluids and minerals interact and cement develops.

What the Rock Records

A finished sandstone records at least two distinct things. Its grains record the material that was deposited. Its contacts, porosity, deformation, cement, replacement minerals and other diagenetic features record what happened after deposition.

These observations allow formative mechanisms to be investigated independently from the age of the sediment. A sandstone may possess a long geological history, but its age is not automatically the amount of time required for its grains to become cemented into rock.

Age and Lithification Time

The age of sediment, the duration of burial, and the duration of lithification are related questions, but they are not automatically identical measurements.

A dating method may establish when a mineral crystallized, when volcanic material was deposited, when a biological organism lived or when another datable event occurred. Those measurements can constrain sedimentary history. They do not automatically measure how long compaction or cementation itself required.

The lithification clock must therefore be investigated through the processes that actually convert sediment into rock.

Domain 9A Finding

Lithification is the physical and chemical conversion of loose sediment into coherent sedimentary rock. Diagenesis is the broader system of post-depositional changes within which much of that transformation occurs.

The principal mechanisms include compaction, reduction of pore space, fluid movement, dissolution, mineral precipitation, cementation, replacement and related chemical and mechanical changes. Natural burial sequences demonstrate these processes operating together as sediment becomes increasingly consolidated.

Time records the interval during which those processes operate. It does not perform the compaction, move the fluids or precipitate the cement.

The first question is therefore not how old the sandstone is.

The first question is what physically turned the sand into stone.

Research Record

Key comparisons used in this investigation include natural burial and diagenetic sequences in Gulf Coast Wilcox and Frio sandstones; measured decreases in porosity with burial; mechanical grain rearrangement and compaction; progressive quartz and calcite cementation; and the established distinction between lithification as conversion to coherent rock and diagenesis as the broader system of post-depositional physical, chemical and biological alteration.

Produced by The Lilborn Equation Team:

Michael Lilborn-Williams

Thomas Jackson Barnard

Audrey Williams