Basin Analysis is the three dimensional reconstruction of regional depositional dynamics in a sedimentary basin. It produces a 3-D geometric depositional history of the basin, including information on:
- Sediment source
- Direction of transport
- Architecture and thickness of units
- Relative depositional chronology
- Regional controlling tectonic factors
- Deeper and more detailed understanding of regional evolution
- Enhanced ability to assess hydrocarbon generation and storage potential.
The first task in characterizing an ancient basin is the identification of source regions. This requires that their weathering products be identified in the sediment, as in the Cretaceous Laramide basin at right.
Garnets as framework clasts in point bar from Mineral Resources and Geofluids
Direct identification of lithic fragments:These directly indicate source rocks and provenance. Specifics:
- Conglomerates and breccias especially useful because provenance can be assessed in the field.
- In finer sediments, useful polycrystalline lithic grains are typically revealed in thin-section. If these are absent, the identity of source regions can be approached through the frequency distribution of physically robust heavy minerals:
- Relative proportions of quartz, feldspars, and lithic fragments can, in themselves, be indicative of source regions.
- Garnet (right), epidote, staurolite indicate metamorphic source rocks.
- Tourmaline, beryl, and topaz usually pegmatitic origin.
These are indicated by the distribution of sediment textures:
- Grain roundness increases with distance from source as grains are physically abraded. Thus, mapping regional trends in roundness in a rock unit indicates relative proximity to the source.
- Grain size also usually decreases with distance from source. Map of regional grain size changes indicates relative proximity to the source.
- Grain imbrication indicates direction of current flow
- Sedimentary structures including:
- Cross beds
- current ripples
- tool marks
- flute casts
Even in a modern environment, characterizing general current direction is problematic if all we have access to are single small points of observation. For example, in the flood plain of a meandering stream, current, at any given point, may be flowing in any direction. It is only by sampling many localities that we develop a general sense of the current direction. Reliable reconstruction of ancient current directions, likewise, requires hundreds of measurements of cross beds, current ripples, etc. Unlike modern observations:
- Measurements must be corrected for secondary dip imparted by postdepositional tectonic motion (often accomplished with stereonets or computer programs)
Rose diagram: Commonly, paleocurrent data are graphically represented on circular graphs that summarize current vector data.
The diagram takes the form of a radial histogram divided into convenient angular increments (5º, 20º, etc.) Number or percent of measurements that fall in each section are graphed (like a histogram).
Dominant current direction will be the largest "pie slice." Secondary signal that might not be readily apparent can often be seen.
Stratigraphic cross sections
geologic cross section in which surface topography and structure are shown, a stratigraphic cross section (right) omits surface topography and suppresses structural information or displays it only schematically. All data are aligned along a shared datum or elevation. Intended to emphasize:
- Rock units
- thickness changes
- facies changes
- biostratigraphic information
Stratigraphic cross sections are two dimensional by definition. To illuminate three dimensional patterns, a common convention is the fence diagram where a series of cross sections is arranged like fences on a map. To be intelligible, these usually must be simplified from standard 2D sections and scaled appropriately.
- Structure contour map: Contours the surface of an underground lithologic unit in the same way as a topographic contour map contours the surface. Utility includes the identification of structural traps for petroleum or water.
- Isochore maps
Rather than contouring a surface, these contour the thickness of a rock unit. The basic unit of thickness is the isopach: points of equal thickness of a rock unit. (analogous to an isotherm on a weather map). Isochore maps display isopachs, adjusted to factor out surface topography and structure, just like stratigraphic cross sections do. This corrects for apparent thickness resulting from dip. Note, maps that don't make this compensation are properly called isopach maps. Utility:
- Sediments are thickest in basins and thinnest in uplifts and shelves
- Basin outlines can be located using isochore maps
- Tectonic hinges boundary between a shelf and basin can be identified
- Zero isopachs indicate pinch-outs, fault interuptions, erosion
- Facies maps Depict variation in facies units in map view.
Lithofacies map: Show variations in lithology, including:
- Ratios of 2 different grain sizes or lithologies
- Triangle facies map: relative abundances of three or more components.
- And others
- Biofacies maps: Variations in faunal aspects
- Paleogeologic map: shows distribution of rock outcrops as they would have appeared before burial. In fact, this is a typical geologic map that strips away units younger than the time slice of interest. The effect is to depict an ancient landscape.