Geologic maps, cross-sections, block diagrams
The goal of geologic mapping is to determine the history of an area with the ultimate goal to
understand the processes that are responsible for unit/area formation--an understanding of
geological process(es) is the ultimate prize.
We ask many, many, many questions (the more questions the better), and try to answer those questions—
but there might be many possible answers—and that is all right, and in fact, perhaps best (the more
possible answers the better). Once we have these answers, or solutions, we try to sort through them; we
sort them by ‘proving’ them wrong—that is, discounting them. However, we don’t toss away possible
solutions, until we can ‘prove’ them wrong. In science in general, and certainly in geology, and most
certainly in geological mapping, we never prove anything, we only disprove things.
Expect and encourage: uncertainty, critical thinking, imagination, creativity, flexibility, and respect for
ideas and individuals.
There are various types of observations/relationships/data sets
Lithologic units (rock type); or material units
Clues: Mineralogy, grain size, textures, fabrics, primary structures, 3D pattern, map pattern, contact
relations, topographic expression, vegetation, radar characteristics, remote sensing characteristics (data
suite dependent), geomorphology/geomorphic features, etc.
Primary structures can be part of unit definition, but secondary structures cannot.
Questions to ask/To Do What?: spatial distribution (2D and 3D)? size/ shape of unit, temporal relations
with other units; clues come from contact relations with other units, as well as relationships with primary
and secondary structures that can be associated with other units. You’d like to understand the spatial
relationships with other units (and structures) as well as the relative temporal relations with other units
(4D).
Geomorphic or topology features (3D shapes on surface of a planet; can also be preserved as 3D
structure within lithologic unit)
Primary structures: Features related to, and formed during, the emplacement/ deposition/ formation of a
lithologic unit (e.g., volcanic: volcanic cinder cone, channels, lobate flows, pahoehoe or aa flow surfaces;
impact related (exogenic formed), ejecta deposits, crater basin, lobate flows; sedimentary: ripple forms,
cross-bedding, graded bedding, dunes, flame structures, mud cracks; glacial: lateral, medial, or end
moraines, eskers; igneous: flow foliation, cooling joints, columnar joints, xenoliths, oscillatory zoning,
etc.
Questions to ask/To Do What?: Help identify the type of lithology, and how the unit might have formed;
clues about details of unit emplacement: i.e., viscosity, high energy or low energy environment, depth
within body of water, environmental conditions, depth within crust, geothermal gradient…
Primary structures provide clues about the nature of unit emplacement/ deposition/ formation, or the
environment of unit emplacement/ deposition/ formation.
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Secondary structures: (also called tectonic structures): structures that form after a lithologic unit
emplacement/ deposition/ formation.
e.g., wrinkle ridges, fractures, faults, joints, folds, scarps, foliations, elongation lineation, boudins, shear
zones, shear fabrics, etc.
Questions to ask OR What do you want to determine/ understand: Orientation, density, wavelength,
length, location, continuity, spatial patterns, relationships to other units (e.g., do other units cover the
secondary structures, or are other units cut by secondary structures?)
Secondary structures provide clues about the nature of events that occurred after/between unit
emplacement/ deposition/ formation, as well as provide clues about the relative timing between the
emplacement/ deposition/ formation of different lithologic units. Secondary structures can lead to
interpretations/understanding of: kinematic evolution (movement picture) and possibly dynamic relations
(having to do with forces).
NOTE: Some structures can be both primary and secondary structures—depends on the nature of the
question you are asking, and/or your perspective; e.g. dikes, cooling joints, mud cracks.
Facies
Sedimentary facies, metamorphic facies, structural facies—transitions between different fabrics or
assemblages.
Relative time and absolute time:
commonly more robustly determined.
Relative temporal relations are more widely applicable, and
Because all geological features (lithologic bodies, primary and secondary structures) are at least 3D [and
commonly 4D], geologic mapping requires the construction of cross-sections and block diagrams.
We create: Maps, Cross-sections and Block diagrams, which can allow us to visualize spatial and
temporal relations between and among each of the geological features, and thus help to identify both
stated and unstated assumptions, or unrecognized problems, as well as unrecognized solutions.
Cross-sections and block diagrams (and even better, cross-sections and block diagrams through time—
cartoons) also encourage us to ask additional (new!) questions of our data. Many of these new questions
can be addressed either by the original data or through collection of additional data sets (but no question
can be addressed until it is asked—so asking questions is a first and very important step).
Once a geological hypothesis is formulated through geological mapping, including construction of crosssections and block diagrams, workers should successively add all available data sets/suites that might bear
on the testing of the proposed hypothesis or hypotheses.
Assumptions are a necessary part of the scientific process, and mapping is no exception!
Assumptions are critical— try and state each and every assumption, and question every day.
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