Components of Geologic Maps By Mark A. Jirsa and Terrence J. Boerboom 2003

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

Scales of Geologic Maps The scale of a map controls the amount of detail that can be portrayed. And the amount of geologic detail controls the appropriate scale of a geologic map. Project/Deposit Scales (≤1:10,000) Quadrangle (7.5’-15’)/Township* Scales (1:12,00/20,000/24,000/50,000/48,000/63,360) Regional/County/1°x2°/60’x30’/Scales (1:100,000/200,000/250,000) State/Provincial/National Scales (≥1:500,000) * Canadian

Parts of a Geologic Map The Map The Map - Base Map (type is scale-dependent) - Map Units (w/ abbreviated labels) - Geological Lines: Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Landform outlines... - Outcrops (at scales <1:50,000) - Structural Feature Measurements Planar: Bedding, Contacts, Foliation, Cleavage, Layering, Joints, Dikes, Veins, Shear zones, Fault planes... Linear: Mineral lineations, Cleavage-Bedding intersections, Slickensides, Fold axes, Glacial striations... - Other Features of Note: Drill hole/handsample locations, Mineral deposits... The Legend The Legend - Title/Authors/Date (again the most important part!!) - Scale/North Arrow (w/ magnetic declination)/Location Map - Map Unit Descriptions - Correlation of Map Units (Younging up is the only constraint) - Map Symbol Key (USGS recommendations: USGS OFR ) USGS OFR USGS OFR References (previous mapping, citations from Map Unit Descriptions) Optional Map Components Optional Map Components - Cross-sections - Geophysical images - Airphotos- Source of data map - Inset Maps showing detailed geology- Structural/Tectonic elements map

The Map – Base Maps Objective – Displays surface features (topography, roads, lakes & river, cultural features) and a grid system (TRS, UTM, Lat/Lon) that allow one to readily locate the geologic features portrayed Objective – Displays surface features (topography, roads, lakes & river, cultural features) and a grid system (TRS, UTM, Lat/Lon) that allow one to readily locate the geologic features portrayed Best choice of base map is largely dependent on the map scale Best choice of base map is largely dependent on the map scale 1:24,000 scale (7.5’ USGS quadrangle)1:100,000 scale (30’x60’ USGS sheet)

Global Grid – Latitude-Longitude

Map Projections Mercator Transverse Mercator Conical Polyconic

Universal Transverse Mercator (UTM) Grid Two Versions of North American Datum (NAD) grid – 1927 and 1983

Surveyed Grid Township, Range, Section Section ~1 sq. mile Township ~36 sq. miles

Potential Misfit of Surveyed Sections

Grids on Topographic Maps Latitude Longitude Township Boundary Section Number

The Map – Outcrops the “facts”

The Map – Structural Measurements more “facts” Planar Features - Strike and Dip Sedimentary Bedding Lava flow contacts Intrusive contacts Foliation (metamorphic and igneous) Cleavage Gneissic banding Layering (modal, textural, phase) LaminationsDikesVeinsJoints Shear zones Fault planes... Linear Features – Trend and Plunge Mineral lineations Cleavage-Bedding intersections Slickensides Fold axes Glacial striations...

The Map – Map Units Definition: includes rocks or unconsolidated materials at the earth’s surface that generally share a common mode of origin and age. Rarely composed of a single rock type. “Lumping & Splitting” – the iterative process of deciding what attributes and variables to include in a particular map unit

The Map - Geologic Lines Gradations in confidence: Certain Approximately located Inferred Speculative Concealed Contacts, Faults, Fold axes, Structural fabric, Geophysical lineaments, Landform outlines...

The Legend- Title/Authors/Date/Scale/North Arrow/Location Map

The Legend – Description of Map 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 Map Units TIME The Only Requirement Diagram from the Little Marais quad, MN (Miller,Boerboom, & Jerde, 2006)

The Legend – Map Symbol Legend US Geological Survey Recommendations for Symbology USGS Open-file Report of99-430/ of99-430/ Legend from the Duluth quad (Green & Miller, 2008)

Optional Map Components Mapping Responsibility Diagram Aeromagnetic Image Geologic Cross Section