Shenk & Muller rock units & two subdivisions based on position in geologic time (time units & time stratigraphic units)
North American Code of Stratigraphic Nomenclature 1983 AAPG Bull p. 841-845
formal & informal strat unit terms
stratum: a layer with certain unifying features, boundaries; can be visible or subtle
stratigraphic unit: a stratum or assemblage of strata recognized as a distinct entity when classifying the earth's lithologic sequence based on different characters - units may not coincide
stratotype: original or designated representative of a specific strata,
zone: a common minor strat. interval, especially important in biostrat
interval: the body of strata between markers or time between events
horizon: an interface indicative of a particular position in the strat sequence, often a distinctive thin bed
correlation: show correspondence in character and position, different types of correlation
composite stratotype: combination of several specified type intervals of strata that collectively form a unit stratotype
boundary stratotype: specific point in a specific sequence of rock strata that serves as the definition of the boundary of a unit
type locality: specific geographic location of the stratotype
type area or region: geographic territory surrounding the stratotype
variety of strato types
holo-: original designated by author at time of establishment; "the definition"
para-: supplementary type used in original definition by original author as an aid
neo-: new type, replaces older type that was destroyed or nullified
lecto-: selected later in absence of of adequate designated holotype
hypo-: reference or auxiliary section, designated to extend knowledge of a unit or boundary already established by a type into a new geographic area or facies
How to describe/designate a type section
I. describe location - map, air photo location, make it accessible; II. keep it simple structurally - complications cause errors; III. geologic description: 1. thickness, 2. lithology, 3. paleontology, 4. mineralogy, 5. structure, 6. geomorphic expression; IV. describe boundaries in detail with reason; V. include a strat column, photograph, XS etc.; VI. publish the work so others know about it
lithozone: informal unit
lithohorizon: a surface of change, usually on a boundary or marker bed, very thin
formation: the basic lithostrat unit, intermediate in size (rank), has a type section, the lithostrat column should be divisible into formations; formal unit, variable thickness (<m to 100's m), amount of lithologic change is unspecified - complexity of geology is variable; prime consideration are mappability & putting into XS
Fm. Name: includes geographic term & dominant lithology
Group: formal name for 2 or more fms. with something in common, type section for group is type sections of constituent fms., it is not required to group fms into groups, group name should include a geographic feature near the type sections; generally can't characterize lithology of a group - exception is CRB's, wedging out can can reduce a group to a fm
member: formal name for parts of a fm; fms can have 0,1, or more mbrs, no formal fixed standard for thickness or extent, mbrs are lithologic distinct parts of a fm
bed: the smallest of the formal units, typically a few cm to few meters thick, must be distinguishable from layer above & below it
complex: lithostrat unit of diverse lithology & structurally complex; now covered by lithodemic stratigraphy - primarily intrusive , highly deformed/metamorphosed, most lack stratification, usually doesn't conform to superposition
contact: is what lithostrat boundaries are termed, unconformities are good contacts if visible, angular unconformities & nonconformities are easy to see in field, disconformities are often difficult to see in the field - may need paleo to find