Chapter 12: Lithostratigraphy

Law of Superposition In any succession of strata, not disturbed or overturned since deposited, younger rocks lie above older rocks. 13 12 11 10 9 8 7 6 4 3 2 1 Folding 5

Stratotype: a designated type unit or section, consisting of readily accessible rocks, where possible, in natural outcrops, excavations, mines, or bore holes. Lithosome: masses of rock of essentially uniform character and having intertonguing relationships with adjacent masses of different lithology. Formation: a lithologically distinctive stratigraphic unit that is large enough in scale to be mappable at the surface or traceable in the subsurface. Member: smaller stratigraphic units within a formation. Bed: the smallest stratigraphic unit and found within a member. Group: composed of formations with some kind of stratigraphic unity. Supergroups: a combination of groups.

Stratigraphic relations

Contacts: planar or irregular surfaces that separate different lithologic units. Conformity: a surface that separates younger strata from older rocks but along which there is no physical evidence of nondeposition. Hiatus: a break or interruption in the continuity of of the geologic record Unconformity: a surface of erosion or nondeposition separating younger strata from older rocks, that represents a significant hiatus. Abrupt contact: directly separates beds of distinctively different lithologies Diastem: short hiatus in deposition with little or no erosion before deposition is resumed. Gradational contact: the change from one lithology to another reflects gradual changes in the depositional conditions over time.

Vertical and lateral contacts

Conformable contact types Progressive gradual contact: where one lithology grades into another by progressive, more or less uniform changes in grain size, mineral composition, or other physical characteristics. Intercalated contacts: gradational contacts that occur because of an increasing number of thin interbeds or another lithology that appear upward in the section. Pinch-out: a lateral change in lithology accompanied by progressive thinning of units to extinction. Intertonguing: lateral splitting of a lithologic unit into many thin units that pinch-out independently. Progressive lateral gradation: where one lithology grades onto another laterally by more or less uniform changes in grain size, mineral composition, or other physical characteristics.

Abrupt contact

Intercalated contact

Pinch-out

Unconformable contact types Angular unconformity: younger sediments rest upon the eroded surface of tilted or folded rocks. Disconformity: contact between younger and older beds is marked by a visible, irregular or uneven erosional surface. Paraconformity: beds above and below the unconformity contact are parallel with no discernable erosion, but whose ages are vastly different. Nonconformity: an unconformity developed between sedimentary rock and older igneous or massive metamorphic rock that has been eroded prior to being covered by sediments.

Angular unconformity in the Grand Canyon

Cyclic Successions Autocyclic successions: controlled by processes that take place within the basin itself, and their beds show only limited stratigraphic continuity. Allocyclic successions: caused mainly by variations external to the depositional basin. Allocyclic successions may extend over great distances and perhaps even from one basin to another.

Type (order) Other terms Duration m.y. Probable cause First ____ 200-400 Major eustatic cycles caused by formation and breakup of supercontinents Second Super cycle (Vail, Mitchum, and Thompson, 1977b); sequence (Sloss, 1963) 10-100 Eustatic cycles induced by volume changes in global mid-ocean spreading ridge system Third Mesothem (Ramsbottom, 1979); mega-cyclothem (Heckel, 1986) 1-10 Possibly produced by ridge changes and continental ice growth and decay Fourth Cyclothem (Wanless and Weller, 1932); major cycle (Heckel, 1986) 0.2-0.5 Milankovitch glacioeustatic cycles, astronomical forcing Fifth Minor cycle (Heckel, 1986) 0.01-0.2

5th order 3rd & 4th order 1st & 2nd order

1st and 2nd order global sea-level cycles

Estimated mean global temperature curve for Phanerzoic time and corresponding climate modes.

A Review of the Milankovitch cycles

Simplified, schematic representation of lithofacies Indicating both vertical and lateral changes

Walther’s Law: The various deposits of the same facies-area and similarly the sum of the rocks of different facies-area are formed beside each other in space, though in a cross-section we see them lying on top of each other…..huh? Simply stated….those facies seen in vertical section can be superimposed onto environment that today would have them side by side….huh? Transgression: the movement of a shoreline in a landward direction. Regression: the movement of a shoreline in a seaward direction.

Walther’s Law illustrated by the growth of a delta through time.

Transgression followed by regression tends to produce a wedge of sediments in which deeper water sediments are deposited on top of shallower water sediments in the basal part of the wedge, and shallower water sediments are deposited on top of deeper water sediments in the top part of the wedge.

Eustatic sea-level changes: changes in sea level that are worldwide and that affect sea level on all continents essentially simultaneously. Relative sea-level changes: changes of sea level that affect only local areas. Relative sea-level changes may involve some global eustatic change but are also affected by local tectonic uplift or downwarping of the basin floor and sediment aggradation.

Review of Lithostratigraphic terminology

Correlation of Lithostratigraphic Units Lithocorrelation: links units of similar lithology and stratigraphic position. Biocorrelation: expresses similarity of fossil content and biostratigraphic position Chronocorrelation: expresses correspondence in age and chronostratigraphic position. apparent actual

Chronocorrelation problem in the Grand Canyon

Grand Canyon Stratigraphy

Goosenecks of the San Juan River, Utah allow exposure of Permian-Pennsylvanian strata. The river flows for five miles while progressing linearly only a mile toward the Colorado River.

Lithologic similarity: established on the basis of a variety of rock properties including gross lithology (i.e., sandstone, shale, or limestone), color, heavy mineral assemblages or other distinctive mineral assemblages, primary sedimentary structures such as bedding and cross-lamination, and even thickness and weathering characteristics. Key bed or Marker bed: an easily recognized and relatively ubiquitous bed that cannot be confused with any other bed, (i.e., a thin ash bed among sedimentary beds.) Key/Marker beds are used to help correlate beds that lie immediately above and below the key bed.

Triassic and Jurassic formations of the Colorado Plateau

Correlation by Instrumental Well logs Well logs are simply curves sketched on paper charts that are produced from data obtained from measurements in well bores. These traces record variations in such rock properties as electrical resistivity, transmissibility of sound waves, or adsorption and emission of nuclear radiation in the rocks surrounding a borehole. Most of the rock properties measured by well logs are closely related to lithology and thus assist in lithocorrelation. Sonde: an instrument lowered to the bottom of a well bore that measures a specific rock property. The sonde is capable of measuring all of the beds it passes on the way to the bottom of the bore.

Tension measurement Control and recording equipment Depth measurement Winch Wirelines Sonde

Well log types Electric log: records the resistivity of the rock units as the sonde passes up the bore hole in contact with the wall of the hole. **A marine shale whose pore spaces are filled with saline formation water will have a much lower electrical resistivity than a porous sandstone filled with oil or gas. Gamma ray log: measures the natural gamma radiation in rock units. Sonic log: measures the velocity with which a sound signal passes through rock units—helps to determine porosity. Formation density log: provides information about porosity and lithology (including geochemical, formation microscanner and magnetic susceptibility logs.)

An example of a sonic and gamma ray log.

Correlation by use of well logs

Structural and Isopach map