Principle of StratigraphicLecture 8#

STRATIGRAPHICStudy about:

Rock bedding (strata)-description- cluster – classification

Stratigraphic unit, Age relative relation (bedding sequence), The process and the forming environment. The process sequences: relation between the

process.

Rock strata fundamental principal:

SuperpositionOriginal horizontalityLateral continuity

SuperpositionSUPERPOSITION (Steno, 1669)

Original statement :

At the time when any given stratum was being formed, all the matter resting upon it was fluid, and therefore, at the time when the lowest stratum was being formed, none of the upper strata existed.

Modified statement :

Law of Superposition : the younger strata at the top in an undisturbed sequence of sedimentary rocks (Anthony, 1955).

….At the time when any given stratum was being formed, all the matter resting upon it was fluid…. (Steno, 1669)

Original HorizontalityOriginal statement (Steno):

As regards form, it is certain that at the time when any given stratum was being produced, its lower surface, as also its lateral surfaces, correspond to the surfaces of the lower substance and lateral substances, but that the upper surface was parallel to the horizon so far as possible; and that all strata therefore, except the lowest were bounded by two planes parallel to the horizon. Hence it follows that strata either perpendicular to the horizon or inclined toward it were art one time parallel to the horizon.

Modified statement : Law of Horizontality : Sedimentary strata are laid down nearly horizontally and are essentially parallel to the surface upon which they accumulate (Anthony, 1955).

Lateral ContinuityOriginal statement (Steno) :

At the time when any given stratum was being formed it was either encompassed on its sides by another solid substance, or it covered the entire spherical surface to the earth. Hence it follows that in whatever place the bared sides of the strata are seen, either a continuation of the same strata must be sought, or another solid substance must be found which kept the matter of the strata from dispersion.

Modified statement : Law of Original Continuity : The original continuity of water-laid sedimentary strata is terminated only by pinching out against the basin of deposition, at the time of their deposition (Anthony, 1955).

1. Sediment being deposit laterally until the end of the sedimentation boundary.

2. Sediment cannot be suddenly cut out laterally.

Principle of sedimentation

Classical unconformity; Hutton, 1785

1. Time gap exist2. Depositional stop

Process sequences: Uplift Folding and/or faulting Erosion Subsidence Re-Depositional

Arizona Grand Canyon

Tilted sedimentary rocks of Precambrian age (lower half of photo) are overlain by younger sedimentary rocks of Phanerozoic age (Cambrian and younger, upper half of photo).

Types of classical unconformity; Hutton, 1785

• Angular unconformity• Disconformity• Non-conformity• Paraconformity

An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers

Angular UnconformityDisconformityA disconformity is an unconformity between parallel layers of sedimentary which represents a period of erosion or non-deposition

A nonconformity exists when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock

Non-conformityParaconformityA paraconformity is in which strata are parallel; there is little apparent erosion and the unconformity surface resembles a simple bedding plane

Rock Strata Relationship:1. Continue

2. Unconformity

3. Crosscutting : depend on the relative age and occurrence time

Stratigraphic Unit

Geological Time

Sequence Stratigraphy

Definition of Sequence Stratigraphy:

“It is a conceptual framework and methodology to get an integrated interpretation of strata units and their patterns from available data (seismic, well logs, cores, outcrops)…”

Vail & Sangree, 1988

Eustasy, Relative Sea Lavel, and Water Depth

Accommodation “ The space available for sediments to

accumulate at any point in time” (Jervey, 1988)

Fundamental Concepts of Sequence Stratigraphy

The stratigraphic signatures result from the interaction of the following factors:

1. Tectonics2. Eustasy (changes in the amount of water in the oceans,

usually due to global climate change)3. Sedimentation

Tectonic and eustasy control the amount of the space available for sediments to accumulate accommodation

Tectonic and Climate control the amount and types of sediments available to be accumulated Sediment supply

Tectonic, Eustasy, and Climate control how much of the accommodation is filled by the sediments

Sedimentary process control the detailed facies that fill the accommodation.

Vall et al., 1990Elmery & Myers., 1996

PARASEQUENCEParasequence is a relative conformable succession of genetically related beds or bedsets bounded by marine flooding surface or their correlative surfaces

Marine flooding surface is a surface separating younger from older strata across which there is evidence of an abrupt increase in water depth

Parasequence have been identified in coastal plain, deltaic, beach, tidal, estuarine, and shelf environments

It is difficult to identify parasequences in fluvial sections

Parasequences are absent in the marine or marginal marine rocks which are deposited too far below sea level to be influenced by an increase in water depth

PARASEQUENCE SET

Parasequence set is a relative conformable succession of genetically related parasequences forming a distinctive stacking pattern bounded by major marine flooding surface or their correlative surfaces.

Stacking patterns of parasequences within parasequence sets are progradational, aggradational or retrogradational depending on the ratio of depositional rates to accommodation rates.

SEQUENCESequence is a relatively conformable succession of genetically related strata bounded by surfaces of unconformity or their correlative surfaces

Surface of unconformity is a surface separating younger from older strata along which there is evidence of subaerial erosion, or subaerial exposure, with a significant hiatus indicated

Surface of conformity is a surface separating younger from older strata along which there is no evidence of erosion (neither sub-aerial nor submarine) or non-deposition, and along which no significant hiatus is indicated.

SYSTEMS TRACTSSequence can be subdivided into systems tracts based on objective criteria including types of bounding surfaces, parasequence set distribution, and position within the sequence

System tract is a linkage of contemporaneous depositional systems

Depositional system is a three dimensional assemblage of lithofacies, genetically linked by active (modern) or inferred (ancient) processes and environments

There are several systems tract identified in sequences:• Lowstand systems tract• Transgressive systems tract• Highstand systems tract• Shelf-margin system tract

Questions…