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Geology topics

Keith A. Howard

Publications and source records attributed to Keith A. Howard.

106 records · Page 6Linked to original sources
USGS1974

Fresh lunar impact craters: Review of variations in size

Thirty-three morphologic characteristics are reviewed for fresh lunar impact craters wide than1 km. Bar graphs express the way each characteristic varies with crater size. The features are grouped as crater structure, ejecta, and downhill flow features. Major structural transition occur at diameters of about 15 and 200 km. Details of the ejecta blanket, which include several kinds of lineations, dunelike ridges, troughs, and lobes, reflect different transport regimes in the ejecta. Some materials at larger craters flowed downhill in lavalike fashion after the ejecta was deposited; the lavalike materials are probably impact melt.

Geochimica et Cosmochimica Acta↗
USGS1973

Geologic Exploration of Taurus-Littrow: Apollo 17 Landing Site

Apollo 17 landed in a deep graben valley embaying the mountainous highlands southeast of the Serenitatis basin. Impact-generated breccias underlie the massifs adjacent to the valley, and basalt has flooded and leveled the valley floor. The dark mantle inferred from orbital photographs was not recognized as a discrete unit; the unusually thick regolith of the valley floor contains a unique high concentration of dark glass beads that may cause the low albedo of much of the surface.

Science↗
USGS1972

Geologic setting of the Apollo 15 samples

The samples and photographs returned from the Apollo 15 site show that Hadley Delta is largely underlain by breccias whose clasts are mainly fragments of coarse-grained feldspathic rocks and nonmare-type basalt. Conspicuous sets of lineaments, visible in surface and orbital photographs of Mount Hadley and Hadley Delta, may represent systematic layering or fracture sets. The mare surface, with regolith about 5 meters thick, is underlain by two major basalt types, at least one of which has extensive lateral continuity and is exposed in the upper wall of Hadley Rille. Gradual erosional recession of the edges and filing of the interior of the rille by talus have contributed to the present cross sectional profile.

Science↗
USGS1972

Structure of Sierra Madera, Texas, as a guide to central peaks of lunar craters

Like hundreds of other lunar craters of probable impact origin, Copernicus contains central peaks presumed to expose rocks uplifted from beneath the crater floor. A possible analog of these peaks on Earth is the central uplift of the Sierra Madera cryptoexplosion structure, a probable impact scar (astrobleme) in stratified Permian and Cretaceous rocks of west Texas. The most conspicuous part of this 12-km-wide structure is its central uplift, 6 to 8 km across, in which the oldest rocks have been raised 1,200 m above their normal position. The uplift is surrounded by a structural depression, beyond which is a concentric rim containing some folds and circumferential normal faults with downthrow toward the center. The intensity of folding and faulting increases inward from the flanks of the central uplift toward a central zone about 2 km across where dips and fold plunges are near vertical or overturned. As in salt domes, the beds moved inward as well as upward to occupy their present positions, causing faulting and radial folding due to crowding in the center. Repetition by folding due to inward movement of upper beds over lower appears to have been followed by upward movement of the core of the uplift pulling away from the flanks by faulting. Minor outward-directed thrusting may have been caused by gravitational spreading of the rising dome. The structural depression surrounding the uplift resulted from tectonic thinning accompanying the inward movement of rocks in the uplift. Analogy with experimental craters and with other cryptoexplosion structures indicates that the uplift at Sierra Madera protruded into a crater (since destroyed by erosion) that was about 12 km across. Inasmuch as the rocks forming such uplifts are derived from below the crater floors, analogous peaks in lunar craters such as Copernicus may offer samples of lunar crust uplifted from distances below the crater floor on the order of one-tenth of the crater diameter.

Texas↗
USGS1972

Geology of Hadley Rille preliminary report

Hadley Rille generally trends in directions that are controlled by pre-mare structures. The southern half of the rille, south of the Apollo 15 site, follows a mare-filled graben that is concentric to the Imbrium basin; however, the zigs and zags in the course are mainly north and east-northeast, directions that do not reflect pre-mare structures in adjacent highlands. The northern half of the rille follows a general direction that is parallel to pre-mare Imbrium-radial faults, is less sinuous than farther south, and intersects several fault-like features, all of which suggest some structural control along this part of the course. At the Apollo 15 site, the east rim is 30-40 m higher than the west rim as if the rille were a normal fault. The rille cannot be a simple fault, however, because the two sides do not match; instead, at bends, the outside has less curvature than the inside, which is partly attributed to recession of the rims by mass-wasting.

Open-File Report↗
USGS1972

Orbital-science investigation: Part F: regional geology of Hadley Rille

Study of the sinuous Hadley Rille (fig. 25-45) was a primary goal of the Apollo 15 mission. Local geology of the rille near the landing site is described in section 5 of this report. Preliminary study of orbital photography from Hasselblad, metric, and panoramic cameras makes possible a description of some regional relationships of the rille. Considerable use is also made of a preliminary topographic map (10-m contour interval) of part of the rille (part C of this section). Contours in the mare area generalized from the map (fig. 25-35(a)) are shown in figure 25-46.

Book chapter↗
USGS1972

Orbital-science investigation: Part G: lineaments that are artifacts of lighting

Many Apollo 15 orbital photographs, particularly those taken at low Sun-elevation angles, reveal grid patterns of lineaments. In some circumstances, the grid pattern is present in areas where structural control seems unlikely. For example, in an oblique view (fig. 25-52), the ejecta blankets of two fresh impact craters seem to have two intersecting sets of lineaments. Because previous studies of impact craters indicate that concentric and radial trends are commonly present, that this pattern is unexpected. A crater-saturated surface on which a faint grid of linear markings can be discerned is shown in figure 25-53. Again, this pattern is unexpected for a surface that has been exposed to random impacts. In both situations, the azimuths of the main lineaments are approximately symmetrical to the direction of the Sun.

Book chapter↗
USGS1972

Photogeology: Part N: ejecta blankets of large craters exemplified by King Crater

Details of the ejecta blankets of large, fresh craters provide insight into the mechanics of deposition and the sequence of emplacement of impact debris. King Crater is the freshest of the three large, rayed craters photographed from Apollo 16; the others are Theophilus and Langrenus Craters. King Crater is comparable in youth to Tycho Crater, and the details of its ejecta blanket help to interpret degraded equivalents at older craters. The clarity of detail and the occurrence of new types of lunar landforms rank King Crater among the most significant targets photographed from orbit in the Apollo Program (part M of this section).

Book chapter↗
USGS1972

Photogeology: Part S: mare ridges and arches in southern Oceanus Procellarum

Low-relief mare features such as ridges and arches are best studied by using stereoscopic photographs taken at low Sun angles. Apollo 16 metric camera photography of the southern Oceanus Procellarum east of Letronne Crater reveals a diversity of subtle features (fig. 29-125) and adds significantly to an understanding of the forms of mare ridges and arches their relative ages, and their association with fractures and sinuous rilles.

Book chapter↗
USGS1971

Impact breccias in carbonate rocks, Sierra Madera, Texas

Two main types of deformational breccia occur in the Sierra Madera cryptoexplosion structure: monolithologic breccias composed of shattered rock of a single lithology and mixed breccias composed of rocks of several lithologies. Monolithologic breccias generally show no mineralogic signs of shock deformation, but a few samples are shatter-coned in a manner suggesting simultaneous formation of breccias and shatter cones. Mixed breccias, forming irregular, cross-cutting bodies, consistently contain moderately to highly shocked material, with mineralogic evidence of shock pressures of 50 kb to more than 200 kb, which, with evidence from the structural geometry of Sierra Madera and orientation of shatter cones, indicate an impact origin of the breccias. The mode of occurrence of the breccias, petrographic characteristics, and association with shock features are shared by breccias in many other cryptoexplosion structures in both carbonate and crystalline rock terranes, suggesting that such breccias have a common origin.

Texas↗
USGS1971

Paleozoic metasediments in the northern Ruby Mountains, Nevada

New evidence indicates that high-grade regionally metamorphosed marble and quartzite in the northern Ruby Mountains are Paleozoic strata. Correlation is based on a match of the lithologic sequence to strata of Cambrian to Devonian age in nearby areas; particularly diagnostic is a brown dolomite at the base of the Cambrian carbonate sequence. The metamorphic complex contrasts with unmetamorphosed terranes exposed in nearby areas, and apparently formed in the Mesozoic as part of a metamorphic infrastructure.

Nevada↗
USGS1970

Mascons, mare rock and isostasy

The nature of the large positive gravity anomalies discovered on the Moon (mascons) has been the subject of lively discussion. The high density (3.1 to 3.5 g cm −3 ) of mare rocks at Tranquillity Base supports one hypothesis—that mascons are surface sheets of high density volcanic rock. The non-isostatic conditions of the mascon areas shown by the free-air anomalies may have terrestrial analogues. in some regions of recent volcanism where breaks in the crust have allowed dense basalts to pile up on the surface faster than isostatic balance can be maintained.

Nature↗
USGS1968

Shatter cones at Sierra Madera, Texas

Shatter cones abound in the central uplift of Sierra Madera and they occur as far as 6.5 kilometers from the center. Apical angles average near 90 degrees. Whole cones and full cones represented by diversely oriented cone segments in any structural block show relatively uniform orientations of axes and a dominant direction of point. The cones predate faulting and folding in the central uplift, and, when beds are restored to horizontal, most cones point inward and upward, a pattern that supports the hypothesis of an impact origin.

Texas↗