CORRELATION OF CORRELATION OF LITHOSTRATIGRAPHIC LITHOSTRATIGRAPHIC

UNITSUNITS

I) IntroI) IntroA. Correlation--shows units are equivalent,

or match1. thought to represent similar geologic age in past2. equivalency by lithology, paleontology, chronology3. must identify nature of correlation

B. Correlation--important in strat reconstruction1. to understand facies relationship

C. Attributes for correlation:1. lithologic homogeneity & constancy2. heavy mineral, trace elements, seismic velocity3. do not have to correlate with time4. marker-defined--e.g. key beds in monotonous strata

i. could be l.s., bentonite, ss in shale

Simplified Lithostratigraphic Simplified Lithostratigraphic CorrelationCorrelation

Blue Ridge Mtn Geology

More Lithostratigraphic CorrelationsMore Lithostratigraphic Correlations

Correlation Across TexasCorrelation Across Texas

II) MethodsII) Methods

A. Many methods1. choice depends on nature of problem

B. measure several sections in loop1.called closed correlation traverse

C. Tracing lateral continuity1. map strat units from outcrop to outcrop2. = walking out unit3. contacts well-defined

4. best where have good exposure or use aerial photographsD. lithologic identity

1. identify key characteristics in rocksi. color, weathering pattern, bedding, thickness, mineralogy, sed

structures2. finer detail--use IC5 attributes3. use anything observable to distinguish unit4. fossils

i. esp. rk units composed of fossil frag--independent of timeii. may be identified by assemblage zones

II Methods ContinuedII Methods Continued

E. Position in sequence1, must establish strat succession2. unit may not be litho distinct--strat succession may be only

satisfactory method3. Example-- 3 similar l.s

i. L.s. 1--shale below/ss aboveii. L.s. 2--dolomite below/red bed above

4. more difficult in cyclothemsi. e.g. Perm cyclothemii. cyclical sequence=similar lith successions

F. Structural relationships1. Sequential position relative to unconformity, folds, faults, etc2. not observable over broad area

Lateral Tracing of UnitsLateral Tracing of Units

Several Sections in Loop Several Sections in Loop

Tracing lateral continuity Tracing lateral continuity

Kansas limestones

lithologic identity lithologic identity

Position in sequencePosition in sequence

Position in SequencePosition in Sequence

Problem—cyclical sequenceProblem—cyclical sequence

• Figure 3: The Milroy Member of the Middle Ordovician Loysburg Formation (Mark Patzkowsky for scale, lower right). Four of the six measured carbonate cycles are visible and their tops are delineated with dashed lines. The darker rock is the subtidal facies; the lighter rock is the intertidal facies. Cycle thicknesses are greater at the base and thinner in the middle. Located at intersection of Rt. 322 and Rt. 26, State College, Pa.. hydro.geosc.psu.edu

Structural RelationshipsStructural Relationships

Rim of the Grand Canyon. Horizontal Tapeats Sandstone overlie tilted Grand Canyon Supergroup

Lower portion of Canyon Sin Nombre, Carrizo Plains www.geoepicenter.com

How do we establish correlation or How do we establish correlation or equivalency of units?equivalency of units?

Dealing With Removal of SectionDealing With Removal of Section

Dealing With Changes in ThicknessDealing With Changes in Thickness

• Procedures for Extending Lithostratigraphic Units-Lithostratigraphic Correlation

• A lithostratigraphic unit and its boundaries are extended away from the type section or type locality only as far as the diagnostic lithologic properties on which the unit is based may be identified.

• 1. Use of indirect evidence for identification of units and their boundaries. Where lithologic identity is difficult to determine because of poor or no outcrops, a lithostratigraphic unit and its boundaries may be identified and correlated on the basis of indirect evidence: geomorphic expression, wire-line logs, seismic reflections, distinctive vegetation, etc.

• 2. Marker beds used as boundaries. The top or the base of a marker bed may be used as a boundary for a formal lithostratigraphic unit where the marker bed occurs at or near a recognizable vertical change in lithology.