Why is it difficult to correlate time horizons over wide areas using lithostratigraphic units and how can fossils help resolve this problem?
The best time-markers in stratigraphy are widespread, distinct, and geologically instantaneous. Rocks that form in moments, years, or decades are effectively instantaneously, like volcanic deposits, single catastrophic slides or flows, or calm, stable lakes with a uniform rain of suspended particles. Geochemistry of stable elements, radiometric dating, magnetic reversals, isotope shifts, and iridium anomalies may all be used to pin down dates.
Horizontal correlation is difficult in stratigraphy because sediments are not uniformly laid down in homogeneous, synchronous global beds of layer-cake geology. Instead, most lithographic units have limited lateral extent, pinching out or grading into other lithographies. The combination of limited outcrop exposure, rocks lost to melting or erosion, and difficulty with determining past facies relationships limits the utility of even suitable lithographies for large-scale temporal analysis. Instead, fossils can provide cross-lithographic clues to the time of formation.
Biostratigraphy is using fossil evidence to establish time of deposition for the surrounding strata. Evolution and paleoecology result in a change over time in a particular organism, with limited range to particular facies. The result is that evolving creatures enable determining if a fossil and the surrounding material is from a particular time period, while simultaneously limiting the locations where fossils are found and preserved. The combination of evolution, extinction, immigration, and emigration provide information, but are clouded by the noise of failure to preserve (lithographic limitations), destruction (through erosion or metamorphic destruction), be inaccessible or missed during evolution. A facies change can also lead to local extinction, another source of noise.
In order for fossils to be useful for dating, they need to be abundant, distinctive, facies-independent, and rapidly changing so that a species is only present for a short-ranging period of time. The best index fossils are rapidly evolving pelagic organisms, where distribution is unaffected by sea bottom facies. Looking at the first and last appearance of a creature, overlaps of unrelated taxa, or succession of related taxa all provide additional data. Some fauna also record particular climatic information, like plankton coiling right or left depending on the temperature.