Editing Stratigraphy (section)

==Lithostratigraphy==
{{main|Lithostratigraphy}}
[[Image:Geology of Cyprus-Chalk.jpg|thumb|Chalk layers in [[Cyprus]], showing sedimentary layering]]

Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type ([[lithology]]). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in [[sedimentary depositional environment|environments of deposition]] (known as [[facies]] change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called the [[law of superposition]], states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.

[[Chemostratigraphy]] studies the changes in the relative proportions of trace elements and [[isotope]]s within and between lithologic units. [[Carbon]] and [[oxygen]] [[isotope ratio]]s vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy.

[[Cyclostratigraphy]] documents the often cyclic changes in the relative proportions of [[mineral]]s (particularly [[Carbonate mineral|carbonates]]), grain size, thickness of sediment layers ([[varve]]s) and fossil diversity with time, related to seasonal or longer term changes in [[paleoclimatology|palaeoclimate]]s.