Geologic Map and Cross-Section Interpretation

CHAPTER 3: MAP AND CROSS-SECTION INTERPRETATION

KEY TERMS 1

  • Angular Unconformity: An unconformity between two groups of rocks that are dipping at different angles. The older rocks below are often dipping more steeply than the rocks above.

  • Disconformity: A type of paraconformity where the rocks above and below are essentially parallel, but the unconformity surface is not parallel to the bedding.

  • Eon: Four subdivisions of time on Earth: three eons—Hadean, Archean, and Proterozoic—cover almost half the time on Earth; the fourth eon incorporates the Paleozoic, Mesozoic, and Cenozoic eras.

  • Era: Subdivisions of the Phanerozoic: Paleozoic, Mesozoic, and Cenozoic.

  • Epoch: Subdivisions of the Tertiary and Quaternary periods.

  • Ka: Abbreviation for Kilo-annum; one thousand years.

  • Ma: Abbreviation for Mega-annum; one million years.

  • Ga: Abbreviation for Giga-annum; one billion years.

  • GIS: Abbreviation for Geographic Information System, a computer-based tool used for mapping, analyzing, and visualizing geographically referenced data.

  • GPS: Abbreviation for Global Positioning System, a radio navigation system that determines the exact location, time, and velocity (from triangulation).

  • Key Beds: A well-defined, easily identified strata that is distinctive enough to be useful in correlation in mapping.

  • Magnetic Declination: The angle between true North and magnetic North.

  • Nonconformity: An unconformity formed by older igneous rocks in contact with younger sediments, indicating a missing time after the igneous intrusion.

  • Paraconformity: An unconformity in which the beds are parallel above and below the break.

MAP INTERPRETATION

Key Concepts

  • Chapter 3 synthesizes basic stratigraphic, structural, and lithologic relationships and principles used in map interpretation.

  • Geologic maps can yield various interpretations based on the user's analysis of the data presented.

The Role of Interpretations

  • It is necessary to know the succession of stratigraphic units to infer structures on the map.

  • In some cases, only the relative order of deposition of units can be established using fundamental geologic laws applied to the map's outcrop pattern.

  • Features of a bed may help ascertain the relative sequence of deposition based on the position of the top of the bed.

Structural Aspects of Map Interpretation

  • Understanding the dip of units is crucial for interpreting geologic maps.

  • Identify folds and their direction of plunge, classifying fault movement and amount of slip.

  • Knowledge of rock types and outcrop patterns is essential for realistic map interpretation.

Features That Define Top of Beds

  1. Graded beds: Finer grained at the top.

  2. Truncated cross-bedding: Slightly concave up.

  3. Ripple marks: Concave up.

  4. Sole marks: Natural casts form on the bottom of siltstone or sandstone bed.

  5. Basal conglomerate: Located at base of formation.

  6. Rip-up clasts: Shale clasts ripped up from deposits below and transported by currents.

  7. Scouring or channeling: Concave.

  8. Mud cracks: V points down.

  9. Solution surfaces: Irregular surface at the top of soluble rock.

AGE RELATIONSHIPS

Identification of Stratigraphic Succession

  • Establishing the sequence of events from the ages of the units is crucial for interpreting geologic maps.

  • Ages can be available as absolute dates or relative dates.

Geologic Time Scale

  • As shown in Table 3-1, it includes various eons, eras, and periods with specific time boundaries expressed in millions of years before present (Ma).

  • The Precambrian is not a formal time unit; it is divided into three eons: Proterozoic, Archean, and Hadean.

Fundamental Geologic Principles for Age Determination

  1. Law of Initial Horizontality: Assumes layers were deposited horizontally; the oldest layer is therefore on the bottom.

  2. Law of Superposition: The oldest layer is at the bottom, and the youngest layer is at the top unless overturned.

  3. Cross-cutting relationships: If a feature cuts across another, the feature that has been cut is older.

  4. Lateral Continuity: Layers of sediment initially extend laterally in all directions.

  5. Faunal and floral succession: Fossils can be used to establish relative ages based upon their evolutionary sequence.

  6. Law of Inclusions: Inclusions in a rock are older than the rock itself.

  7. Metamorphic relationships: Metamorphic beds are younger than the rock prior to metamorphism.

CORRELATION OF UNITS

  • After establishing the sequence of events, correlation helps ascertain whether rocks at different locations were once continuous or deposited simultaneously.

Evidence for Correlation

  1. Physical continuity: Strata are generally continuous unless eroded or faulted.

  2. Lithology: Distinctive units can be correlated with high confidence.

  3. Sequence of strata: Orderly rock units allow correlation of entire sequences.

  4. Rock properties: Correlation can be done via electrical/radioactive properties as seen on well logs.

  5. Key beds: Layers deposited simultaneously across large areas.

  6. Index fossils: The appearance and disappearance of certain fossils can aid correlation.

  7. Fossil assemblages: Grouping of several fossil species enhances correlation.

UNCONFORMITIES

  • Unconformities represent breaks in the geological record and can often be identified through map observations.

Types of Unconformities

  1. Angular unconformity: Strata below the unconformity are cut off and overlain at an angle by overlying strata.

  2. Nonconformity: Sedimentary deposits rest upon older igneous or metamorphic rocks.

  3. Paraconformity: The unconformity is parallel to the strata above and below it.

  4. Disconformity: Beds above and below the unconformity are parallel, but the unconformity surface is not parallel to the bedding.

RULE OF V's

  • The Rule of V's is a critical component in interpreting geologic maps.

Guidelines for Interpretation

  1. V's made by a geologic unit crossing a stream valley point in the direction the formation underlies the valley.

  2. Six cases arise from applying the Rule of V's in valleys and ridges:

    • Horizontal bedding: V points upstream parallel to topography.

    • Dip upstream: V points upstream outside topography.

    • Vertical bedding: Straight lines cutting across valley topography.

    • Dip downstream greater than valley gradient: V points downstream.

    • Dip downstream equals valley gradient: Parallel lines along valley sides.

    • Dip downstream less than valley gradient: V points upstream inside topography.

FOLDS

  • Folded units display distinctive curved or U-shaped outcrop patterns.

Recognition of Folds

  • Determine potential folds using the Rule of V's and look for reversals in V patterns along stream valleys.

  • A U-shaped fold indicates a plunging fold, with the direction of plunge determined from the wrapping of beds. Anticlines plunge towards closure; synclines in the opposite direction.

FAULTS

  • Understanding faults is crucial as they complicate the interpretation of geologic maps.

Types of Faults

  1. Normal faults: The hanging wall moves down relative to the footwall.

  2. Reverse faults: The hanging wall moves up relative to the footwall.

  3. Strike-slip faults: Lateral motion occurs between two segments of the crust.

Recognizing Faults

  1. Look for repetitions or omissions of units due to fault movement.

  2. Observe discontinuity in structures; offsets may reveal the movement direction.

  3. Geomorphic features such as fault scarps can indicate faulting.

  4. Presence of slickensides, drag, or breccia signifies faulting.

LITHOLOGIC UNITS

  • Lithological patterns are apparent from geologic maps; knowledge of these patterns aids in interpretation.

General Characteristics of Rock Types

  • Sedimentary Rocks: Generally parallel-sided and laterally extensive.

  • Igneous Rocks: Varied in map patterns; extrusive volcanics and intrusive sills are often parallel-sided.

  • Large Intrusions: Often circular in shape with foliation parallel to margins.

  • Metamorphic Rocks: Display specific banding or changes around intrusive contacts.

DRAINAGE PATTERNS

  • Drainage patterns form based on slope and precipitation rates, reflecting surface and subsurface geology.

Analysis of Drainage Patterns

  • Identifying drainage patterns can assist in estimating topography and geologic structures.

OTHER ASPECTS OF GEOLOGIC MAPS

  • Important features include a north arrow, magnetic declination, map scale, mapping datums, and map symbols.

North Arrow

  • Must indicate true North direction; if absent, assume North is up parallel to the side of the map.

Magnetic Declination

  • The difference between true North and magnetic North materials change over time.

Map Scales

  • Various scales are utilized, showing representation in inches to real-world distances.

TOWNSHIP AND RANGE SYSTEM

  • This system divides land into 40-acre parcels and is primarily used in land-sorting west of the Mississippi River.

MAP USES

  • Different scales of maps are used for varying purposes, from local detailed surveys to broader regional planning.