Components of Geologic Components of Geologic MapsMaps

ByMark A. Jirsa and Terrence J.

Boerboom2003

Objective of a Geologic Map: To interpretively portray the spatial and temporal relationships of rocks, unconsolidated earth materials, and landforms at the earth’s surface.

Types of Geologic Maps (on Earth)• Complete Geologic MapsComplete Geologic Maps – portrays both bedrock and unconsolidated material in areas where both are prevalent• Bedrock MapsBedrock Maps – portrays only the bedrock geology as best as can be inferred from available outcrop• Surficial (or Quaternary) MapsSurficial (or Quaternary) Maps – portrays the distribution of unconsolidated earth materials and landforms in areas affected by Pleistocene glaciation• Specialty MapsSpecialty Maps – tectonic, geohydrologic, isopach, geophysical, mineral potential, groundwater contamination susceptibility

Scales of Geologic MapsScales of Geologic MapsThe scale of a map controls the amount of detail The scale of a map controls the amount of detail that can be portrayed. that can be portrayed. AndAnd the amount of the amount of geologic detail controls the appropriate scale of a geologic detail controls the appropriate scale of a geologic map. geologic map.

Project/Deposit Scales (≤1:10,000)Project/Deposit Scales (≤1:10,000)

Quadrangle (7.5’-15’)/Quadrangle (7.5’-15’)/Township*Township* Scales Scales (1:12,00/(1:12,00/20,00020,000/24,000//24,000/50,00050,000/48,000/63,3/48,000/63,360)60)

Regional/County/1°x2°/60’x30’/Scales Regional/County/1°x2°/60’x30’/Scales (1:100,000/200,000/250,000)(1:100,000/200,000/250,000)

State/Provincial/National Scales State/Provincial/National Scales (≥1:500,000)(≥1:500,000) * Canadian

Parts of a Geologic MapParts of a Geologic Map The MapThe Map

- - Base MapBase Map (type is scale-dependent) (type is scale-dependent)- - Map UnitsMap Units (w/ abbreviated labels) (w/ abbreviated labels)- - Geological LinesGeological Lines: Contacts, Faults, Fold axes, Structural fabric, : Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Geophysical lineaments, Landform outlines...Landform outlines...- - OutcropsOutcrops (at scales <1:50,000) (at scales <1:50,000)- - Structural Feature MeasurementsStructural Feature Measurements

Planar:Planar: Bedding, Contacts, Foliation, Cleavage, Layering, Joints, Bedding, Contacts, Foliation, Cleavage, Layering, Joints, Dikes, Veins, Shear Dikes, Veins, Shear zones, Fault planes...zones, Fault planes...

Linear:Linear: Mineral lineations, Cleavage-Bedding intersections, Mineral lineations, Cleavage-Bedding intersections, Slickensides, Fold axes, Slickensides, Fold axes, Glacial striations...Glacial striations...- - Other Features of Note:Other Features of Note: Drill hole/handsample locations, Mineral Drill hole/handsample locations, Mineral deposits...deposits...

The LegendThe Legend- - Title/Title/Authors/DateAuthors/Date (again the most important part!!)(again the most important part!!)- - Scale/North Arrow Scale/North Arrow (w/ magnetic declination)(w/ magnetic declination)/Location Map/Location Map- - Map Unit DescriptionsMap Unit Descriptions - - Correlation of Map Units Correlation of Map Units (Younging up is the only constraint)(Younging up is the only constraint)- - Map Symbol Key Map Symbol Key (USGS recommendations:(USGS recommendations: USGS OFR 99-430))- - ReferencesReferences (previous mapping, citations from Map Unit Descriptions) (previous mapping, citations from Map Unit Descriptions)

Optional Map ComponentsOptional Map Components- Cross-sections- Cross-sections - Geophysical images - Airphotos - Geophysical images - Airphotos - Source of - Source of data mapdata map- Inset Maps showing detailed geology- Inset Maps showing detailed geology - Structural/Tectonic elements map- Structural/Tectonic elements map

The Map – Base MapsThe Map – Base Maps Objective – Displays surface features (topography, Objective – Displays surface features (topography,

roads, lakes & river, cultural features) and a grid roads, lakes & river, cultural features) and a grid system (TRS, UTM, Lat/Lon) that allow one to system (TRS, UTM, Lat/Lon) that allow one to readily locate the geologic features portrayedreadily locate the geologic features portrayed

Best choice of base map is largely dependent on Best choice of base map is largely dependent on the map scale the map scale

1:24,000 scale (7.5’ USGS quadrangle)1:100,000 scale (30’x60’ USGS sheet)

Global Grid – Latitude-Global Grid – Latitude-LongitudeLongitude

Map ProjectionsMap ProjectionsMercator

Transverse Mercator

Conical

Polyconic

Universal Transverse Universal Transverse Mercator (UTM) GridMercator (UTM) Grid

Two Versions of North American Datum (NAD) grid – 1927 and 1983

Surveyed Grid Surveyed Grid Township, Range, SectionTownship, Range, Section

Section ~1 sq. mile

Township ~36 sq. miles

Potential Misfit of Surveyed Potential Misfit of Surveyed SectionsSections

Grids on Topographic Grids on Topographic MapsMaps

Latitude Longitude

TownshipBoundary

Section Number

The Map – OutcropsThe Map – Outcropsthe “facts”the “facts”

The Map – Structural The Map – Structural MeasurementsMeasurementsmore “facts”more “facts”

Planar Features - Planar Features - Strike and DipStrike and DipSedimentary Bedding Sedimentary Bedding Lava flow contactsLava flow contactsIntrusive contacts Intrusive contacts Foliation (metamorphic and igneous)Foliation (metamorphic and igneous)CleavageCleavageGneissic bandingGneissic bandingLayering (modal, textural, phase)Layering (modal, textural, phase)LaminationsLaminationsDikesDikesVeinsVeinsJointsJointsShear zonesShear zonesFault planes...Fault planes...

Linear Features – Linear Features – Trend and PlungeTrend and PlungeMineral lineationsMineral lineationsCleavage-Bedding intersectionsCleavage-Bedding intersectionsSlickensidesSlickensidesFold axesFold axesGlacial striations...Glacial striations...

The Map – Map UnitsThe Map – Map Units

Definition: includes rocks or Definition: includes rocks or unconsolidated materials at the earth’s unconsolidated materials at the earth’s surface that generally share a common surface that generally share a common mode of origin and age. Rarely mode of origin and age. Rarely composed of a single rock type.composed of a single rock type.

““Lumping & Splitting” – the iterative Lumping & Splitting” – the iterative process of deciding what attributes and process of deciding what attributes and variables to include in a particular map variables to include in a particular map unit unit

The Map - Geologic The Map - Geologic LinesLines

Gradations in confidence: • Certain• Approximately located• Inferred• Speculative• Concealed

Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Landform outlines...Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Landform outlines...

The Legend- The Legend- Title/Authors/Date/Scale/North Title/Authors/Date/Scale/North

Arrow/Location MapArrow/Location Map

The Legend – Description of The Legend – Description of Map UnitsMap Units

•Heirarchical; arranged from youngest to oldest

•Variable in detail

•Minimum information will include major rock types

•Other information may include mineralogy, texture, structural elements, relations to adjacent map units, absolute age, fossil content, etc. Legend from the Babbitt SE quad, MN (Miller, 2005)

The Legend – Correlation of The Legend – Correlation of Map UnitsMap Units

TIM

E

The Only Requirement

Diagram from the Little Marais quad, MN (Miller,Boerboom, & Jerde, 2006)

The Legend – Map Symbol The Legend – Map Symbol Legend Legend

US Geological Survey Recommendations for Symbology

USGS Open-file Report 99-430

http://pubs.usgs.gov/of/1999/of99-430/

Legend from the Duluth quad (Green & Miller, 2008)

Optional Map Optional Map ComponentsComponents

Mapping Responsibility DiagramMapping Responsibility Diagram

Aeromagnetic ImageAeromagnetic Image

Geologic Cross Section