The Venus Geologic Mappers' Handbook

Geologic Mapping of Venus

The basic objective of the coordination efforts of The Venus Geologic Mapping Program is to ensure that the geologic maps have reasonable consistency (such as in usage of map-unit names and map symbols) and agreement, which will provide a useful basis for geologic interpretation. Mappers should be guided not only by fundamental geologic principles, but also by many precedents and approaches (some more applicable to Venus than others) that have been established through previous planetary mapping programs and various early mapping studies of Venus. (See Recommended Reading; in particular, the book chapter by Wilhelms (1990) should be regarded as essential reading.) In addition, mappers will face the new challenges of the distinct geologic character of Venus, broadly and in detail, and of the nature of the radar datasets. Mappers should follow guidelines of the USGS; even though adherence to basic mapping principles is a must, mappers have considerable latitude in their application and are encouraged to investigate new approaches that may result in a more instructive and useful geologic map. For example, structurally complex terrains generally cannot be mapped as conventional rock (material) units; inclusion of terrain units may necessitate altering their depiction on correlation charts and cross sections. Also, additional small-scale maps depicting major tectonic structures or surficial features may be necessary to depict the geology of complex regions clearly and comprehensively.

Rationale and Methods

Defining map units. Map units will be defined on the basis of various morphologic, textural, and structural characteristics observable in Magellan images. Geologic (or rock or stratigraphic) units are made up of bodies of rock that are thought to have formed by a particular process or set of related processes over a discrete time span. Even though the interpretation of a map unit or its relative age may not be clear, the unit must have distinctive characteristics. Some surface rocks, however, are so modified by processes postdating their emplacement that their original key characteristics no longer are decipherable. In many of these situations, it is impossible to define geologic units with confidence. Rather than leaving the map blank, it is appropriate to map geomorphologic units that are based on the same types of characteristics, even though the characteristics developed much later than the emplacement of the modified rocks (see Milton, 1975). Where particular morphologic structures or other associated features are rare, the mapper should choose simply to discuss them in the unit description and perhaps map them as symbols rather than delineate a new unit based on them.

Some mappers may find that a different approach to defining map units is more suitable for their Venusian quadrangle. In particular, tectonic units should be considered. Until now, planetary maps have generally excluded tectonic mapping as has been practiced for the Earth. Tectonic mapping has been varied and highly subjective because of changes in paradigms used to interpret and understand terrestrial tectonics. Actually, mapping styles once used for the terrestrial continents--styles that predated our understanding of plate tectonics--appear most applicable to Venus, because they distinguished rocks and terrains associated with foldbelts and cratons (see King, 1969a, b; King and Edmonston, 1972). King promoted a relatively conservative style (although much of the terminology of his day is losing favor among geologists). He stated (1969b, p. 87) that a tectonic map "portrays the architecture of the upper part of the earth's crust, or the features produced by deformation and other earth forces, and represents them by means of symbols, patterns, and colors." Recently, because plate tectonics has gained wide acceptance because it explains much of terrestrial tectonics, the mapping of tectonostratigraphic terranes or elements has become popular (for example, see volume edited by Howell, 1985). Such units are understood to be fault bounded and defined according to stratigraphy, tectonic disruption, or metamorphic overprint. Small-scale mapping of these units caters to accretionary plate tectonics (see Howell, 1985, map insert; Dickinson and others, 1986).

Units should be mappable on Magellan SAR backscatter image mosaics, which form the primary dataset that permits identification of morphology and structure. Unit descriptions should be augmented by radar characteristics such as radar "brightness," backscatter coefficients, emissivities, reflectivities, rms slopes, and topography from altimetry (for example, see Arvidson and others, 1992, fig. 7 and table 1; Moore and others, 1992, table 3). However, mappers should avoid defining units solely by such characteristics, which may be related to weathering or deposition of thin eolian or impact material and have little or nothing to do with the emplacement or structural modification of the affected map unit (Arvidson and others, 1992; Greeley and others, 1992). Also, the nonbackscatter radar data seldom clearly define mappable areas because of their common variation with respect to surficial rock properties and relatively low resolution. Where stereopairs are available, stereoscopy adds the important dimension of local relief for characterization of geologic units at a scale that is not possible with Magellan altimetry or synthetic stereopairs; stereoscopy enhances geologic mapping and interpretation to a degree that cannot be overestimated.

Units may have distinct contacts, perhaps expressed topographically or by cross-cutting and overlap relations. Where contacts are indistinct, mappers may make them long-dashed or queried (which signifies, respectively, gradation or uncertainty). Alternatively, mappers may redefine the observational basis that distinguishes the units. As a last resort, units can be lumped.

Correlating map units. Contacts between map units are critical in defining emplacement relations and relative ages and should be clearly presented on the map. Contact geometry may suggest overlap, embayment, crosscutting, or abutment relations that may be used to infer relative age. Structural relations may also be useful in determining relative age.

On Venus, relatively late resurfacing and a thick atmosphere have resulted in crater densities that are too low for detailed stratigraphic work; thus far, only 921 impact craters have been identified on about 98 percent of the surface. Over broad areas of Venus, crater densities are spatially random (Schaber and others, 1992), although lower than average densities of some areas are interpreted to be related to extensive volcanic resurfacing and tectonism (Phillips and others, 1992).

The global chronology of Venus will be determined through the collective efforts of the geologic mappers, who will establish the local geologic history within their individual map areas. Time markers may include widespread geologic and geomorphologic units, structures, and surficial signatures related to impact events or weathering (Izenberg, 1992; Tanaka and Schaber, 1992).

Mapping approach. (This section is largely distilled from Wilhelms, 1990, section 7.4.) Initial familiarity with the map area is achieved by reconnaissance mapping. This first step reveals the overall geology and identifies major map units, their stratigraphy, and structures. To explain the geologic evolution of the area, working hypotheses are formulated that can be tested and revised as more detailed mapping proceeds. The reconnaissance also assists in identifying the most significant and challenging problems in the area, whose resolution will be the major objective of the mapping.

Detailed mapping is best started where units and contacts are most clearly mappable. Commonly the location is determined by the availability of the highest resolution data. Thus, where FMIDRs are available, they can be individually used for mapping in greater detail than can be shown on the quadrangle, which will provide the mapper with a more complete understanding of the local geology. Another approach is to map younger units first--these units are generally better exposed, and their relative age relations are easily portrayed. A working set of map units and symbols is generated and then modified as mapping progresses. Where key features (such as small volcanoes) are too small to map as units, they may be portrayed as map symbols. Ultimately, the level of detail shown on the maps will be dictated by scale and perhaps ancillary data such as stereopairs and FMIDRs.

Other datasets can be incorporated in the mapping as appropriate, including SAR backscatter cross section, topography, emissivity, reflectivity, and rms slope. However, radar characteristics in most cases constitute a poor basis for unit definition because of their dependence on surficial properties. Some units may even be difficult to map consistently on the basis of SAR backscatter, because backscatter intensity varies according to incidence angle and, in some cases, look direction. If mapping of surficial characteristics is desired, it should be regarded as secondary in importance to mapping of geologic materials and structure and shown in a separate map figure.

As in traditional field mapping, note-taking is vital in documenting the mapping procedures and approach used and in highlighting key geologic observations and relations. Such observations include morphologic characteristics, overlap and cross-cutting relations, and evidence for style and sense of structural deformations. A notebook dedicated solely to the map is very valuable; in it, extended notes can be located by annotations, perhaps on a map overlay. Such documentation contributes to the production of a thorough, well-balanced, consistent, and insightful interpretation of the geology of the map area.

After an initial set of associations and age relations among map units has been derived, a correlation chart consisting of boxes for each map unit can be developed. Map units generally lend themselves to grouping according to terrain type or geologic or geographic associations. Hierarchic names include many possibilities that reflect what the units have in common, such as terrain type (for example, plains, plateaus, mountain belts, shields, and canyons), perhaps subdivided according to local individual geographic features. When the association is based on a geographic feature, the name of the feature coupled with descriptive terms such as "assemblage" or "sequence" will form the hierarchic name. Relative ages of the map units are represented on the correlation chart by vertical position. Thus units that are clearly younger should be shown above older units; those that overlap in age will have boxes that overlap vertically. Because some units developed over a considerable timespan, their boxes may be much longer than those of other units. For a poorly defined age limit, a sawtooth box edge should be used. Boxes for closely related units share a box edge that is horizontal or vertical as appropriate; where their ages overlap, they may share a diagonal box edge. (See examples of planetary geologic maps cited below.) Where geologic and structural relations are complex, informal cross sections can be attempted as tests of possible scenarios of development. Preliminary correlation charts and cross sections are excellent tools to identify areas and relations that require more careful examination.

Coloring the preliminary map as parts are completed is the best way to identify incomplete contacts, incorrect symbols, and inconsistencies in mapping style. Other special maps, perhaps of selected areas at larger or smaller scales, may be used to show tectonic structures or surficial materials and features. Such maps not only highlight specific aspects of the geology, but they also may reduce the clutter on the primary geologic map. In a few instances, particularly in areas of high relief, a schematic cross section can be added to interpret structure; these sections generally have a vertical exaggeration, which is stated.

When the mapping is complete, the description of map units (DOMU) and text can be written. The DOMU describes the map units shown on the correlation chart according to groupings in the hierarchy, from youngest to oldest (in reverse chronologic order--opposite the oldest-to-youngest order used in the text's discussion of stratigraphy). Descriptions and interpretations of units are always clearly separated. The description should include the unit's physical characteristics, occurrence, and relations with other units. The interpretation may include the inferred rock type and mode of origin of the unit; multiple interpretations may be included. Map symbols are explained after the DOMU. The map text should include an introduction that describes the basic geologic setting and physiography of the map area, relevant previous work, objectives of the mapping, and constraints of image resolution that may have affected it. The body of the text should reconstruct in detail the geologic history (from oldest unit to youngest) on the basis of map relations and interpretations. However, the discussion should not include directed, refined geologic analyses typical of those found in research articles.

Types of Mapped Units and Features

Prior to Magellan, small-scale (1:15,000,000) geomorphologic mapping of the northern quarter of Venus was based on kilometer-resolution radar mosaics imaged by the Venera 15 and 16 SARs (Sukhanov and others, 1989; Schaber, 1990; Schaber and Kozak, 1990). At that resolution, many important geologic (especially stratigraphic) relations were not discernible. However, the higher resolution Magellan data permit more detailed and "classic" stratigraphic and structural mapping and interpretation, as can be seen from examples in recent journal articles. These various examples are useful in visualizing how map units can be defined and how contacts and various structures can be mapped. (Keep in mind that the published examples use various names and conventions that may or may not be appropriate for VMAP). This section is divided into discussions of the major types of terrains and structures to be mapped on Venus.

Plains materials. Plains units generally are characterized by relatively smooth-appearing (at image resolution) surfaces at low to intermediate elevations. Relative ages are determined by embayment and cross-cutting relations. Units can be subdivided according to morphology (for example, smooth (at pixel scale), ridged, hummocky, fractured, complex); geographic and terrain associations; relative stratigraphic position (such as lower, middle, and upper); and, locally, radar brightness (for example, bright, dark, mottled; see Solomon and others, 1992, fig. 32);

Lava flows and volcanoes. In some areas, individual or multiple lava flows can be subdivided according to radar brightness, superposition, morphology, and surface texture (for example, Arvidson and others, 1992, fig. 6; Head and others, 1992, fig. 9a, d; Moore and others, 1992, fig. 4; Senske and others, 1992, plate 1). Large volcanoes may have distinctive and mappable summit or central caldera areas and associated structures (Head and others, 1992, figs. 3a, b, 4a-c, 5b; Senske and others, 1992, fig. 6). Small volcanic shields or domes may be outlined individually (Head and others, 1992, figs. 2b-d) or shown by point symbols. In some areas, flow directions can be indicated.

Structural terrains and features. The greatest challenge faced for many of the VMAP quadrangles is the mapping of structural terrains and features. A common mistake is to draw in as many structures as possible; this approach results in clutter and is not helpful to the reader. (Remember that the quadrangle base will portray much of the character of highly deformed map units.) Instead, map highly deformed terrains as units and trace only particularly significant or representative structures on the geologic map (compare King and Edmonston, 1972; King, 1990a). Structural terrains include ridge and fracture belts, tesserae (or complex ridged terrains), and other highly deformed areas. These terrains may be delineated on the basis of elevation, relief, dominant structural type(s), structural patterns, size of individual structures, and structure density (Solomon and others, 1991, figs. 7D and 8B; Bindschadler and others, 1992a, figs. 3-7; Fienen and others, 1992). Care will be needed in areas where structural characteristics change gradually; if units are not sufficiently distinct to map separately, it may be better to lump units and show gradational trends through the mapping of representative individual features.

Significant individual features can be mapped by symbols (see Appendix B). Many mappable Venusian features are tectonic structures; however, topographic features (ridges, troughs, depressions, and scarps) and erosional features (channels) also are common. Mapping of faults or folds requires some supporting evidence for the deformation (such as offset surfaces). If such evidence is absent, a dashed or queried symbol, a less interpretative structural feature (for example, fracture instead of fault or graben), or a topographic feature (ridge instead of fold) can be mapped instead. Regional structures may be distinguished from local structures on the map by a heavier line weight.

Many geologic mappers will find it advantageous to represent both detailed structure and large tectonic features on a separate base at a similar or smaller scale. For example, coronae and coronalike features differ greatly in structural detail (Stofan and others, 1992, figs. 2-5, 13). Even so, detailed structural mapping may be tedious (Head and others, 1991, fig. 7D; Squyres and others, 1992a, fig. 4c). Instead, representative features may be mapped and perhaps summarized by rose diagrams where appropriate (Head and others, 1992, figs. 5-8; Senske and others, 1992, figs. 9-12, 16, 17, 20; Squyres and others, 1992a, fig. 8d; 1992b, figs. 5 and 7). Simplified, smaller scale tectonic maps may also be drawn (see Senske and others, 1992, figs. 4, 19, 21). (All graphs and line work will be redrawn by the USGS Office of Scientific Publications, but mappers should follow guidelines in Hansen, 1991.)

Impact craters. A special type of structure shown on most planetary maps is the impact crater. On other bodies, generally only larger craters are mapped, consistent with map scale. But, because only about 900 craters are recognized on Venus, they are all significant and should all be mapped. However, craters smaller than about 20 km in diameter may have to be indicated by a map symbol. In addition to the crater rim crest, the outer boundary and facies of impact ejecta, floor material, secondary craters, ring structures, central peaks, and outflows can be mapped in many cases (Schaber and others, 1992, figs. 23, 25-27). Also, mappers may choose to subdivide craters into various morphologic classes (for example, Schaber and others, 1992, fig. 4). Extensive surficial features associated with craters (such as dark splotches) should not be mapped as geologic units, but they may be shown by stipple patterns or in a separate, reduced-scale figure.

Mapping Conventions

Unit names, letter symbols, and colors. (See also Wilhelms, 1990; Hansen, 1991, p. 43-52; Reynolds and others, in press.) The general practice in planetary mapping is to use descriptive informal names (such as crater material, ridged plains material, or fractured highlands material). Informal names should include a term that shows that the unit is either a material (geologic) or surface (geomorphologic) unit. Formal names are occasionally applied to stratigraphically distinctive or complex rock units in planetary geology (for example, "Medusae Fossae Formation" on Mars); a formal name requires a formal definition following as closely as possible the established guidelines of The North American Commission on Stratigraphic Nomenclature (1983) (see Hansen, 1991, p. 44-49). We discourage the use of formal names in early stages of mapping a planet, however, because experience shows that many unit definitions and portrayals change substantially through the course of several years of research. We also discourage use of jargon (for example, "tick"), because of the resulting confusion to general readers.

As in terrestrial mapping, the unit-letter symbol is an abbreviation of the unit name. Because a formal stratigraphic system has not yet been established for Venus, no capital letter representing a time-stratigraphic system will be included. The symbol should have as few characters as possible. Avoid ambiguous usage (for example, use p for plains and pl for plateau). The letters should be arranged such that the basic formational name is followed by modifiers for members and submembers (for example, "lower ridged plains material of the Artemis assemblage" would be "unit aprl"). Some mappers identify a sequence of unit members by subscripts, the stratigraphically lowest unit being designated 1. (Thus member 1 of the example given above would be "unit aprl_1.") In the map text, a unit's name is always used, with or without its letter symbol; the symbol never stands alone. A symbol may be queried on the map (for example, "aprl?") if the unit assignment is in doubt; the reason for the doubt should be given in the DOMU.

The Venus Geologic Map Series will follow a consistent color scheme to the extent possible. Colors on the published maps are limited by the USGS color palette and will be selected by the map coordinator. However, authors should adopt the following general guidelines on their author-colored ("mill") copies--browns for older or heavily deformed terrains and units, purples for less heavily deformed terrains, greens and blues for plains units, reds and oranges for volcanic materials, yellows for craters, and grays for other materials. In addition, stippled overlays can show surficial units, broad tectonic zones, etc., that are superposed on the other map units. Some variation from this scheme is occasionally warranted where many subdivisions of units are made and a wide selection of color shades is not available. In general, maps are more visually pleasing and easier to read if the areally large map units are represented by light shades (pastels) and the small, patchy units are darker or more intense. Also, the colors of adjacent units should display sufficient visual contrast so that they do not become confused. Areas of missing data will be left uncolored.

Line and point symbols. A host of line and point symbols is available to the mapper; where possible, symbols should be those standardized by the USGS for terrestrial maps (Reynolds and others, in press) or those used on published USGS planetary maps (such as those used for crater rims). Many of the symbols used on planetary geologic maps are shown in Appendix B. (If a new symbol is needed, the VMAP Coordinator should be consulted.) Symbols need to be used judiciously to reduce map clutter. Thus, mapping of most secondary morphologic features such as fractures, wind streaks, yardangs, or channels should be avoided or done sparingly; for example, one large arrow (rather than several small ones) can often be used to show flow direction. In many places topography will be visible on the SAR backscatter base of the published geologic map. If the mapper wants to highlight specific features, they may be shown on a supplementary map at reduced scale.

Although structural symbols are desirable and informative, their application in some cases may be highly conjectural and uncertain. Thus fault symbols should be avoided except where offset is evident or probable. Normal faults, grabens, and some strike-slip faults may be acceptable. However, suspected thrust-fault scarps and folds should generally be mapped as queried, dashed, or as topographic symbols (scarps and ridges); their structural interpretation can be discussed in the text or shown in cross sections.

Geographic names. The mapper will be provided with an index map showing all named features in the map area. Only geographic names officially approved or provisionally accepted by the International Astronomical Union (IAU) can be shown on the map or mentioned in the text; a name's provisional status must be indicated, usually by an asterisk after the name on the map. Reference to unnamed features should be made by latitude and longitude (all features mentioned in text or DOMU, if not shown on map or in figures, must be located by map coordinates). Note that many craters, coronae, and other geographic features do not have names (see Schaber and others, 1992, table A1; Stofan and others, 1992, table 1). If you feel that a feature needs a name, please consult with the USGS representative to the IAU; guidelines for naming features are given in Appendix C.