Ancestral Basin Research Articles

Stratigraphy and mineralogy of Candor Mensa, West Candor Chasma, Mars: Insights into the geologic history of Valles Marineris

Candor Mensa, an interior layered deposit (ILD) in Valles Marineris, Mars, consists of two stratigraphically distinct units, the lower of which comprises the bulk of the mensa. This lower unit is approximately 5 km thick and composed of parallel layers, 4 to 14 m in thickness and associated with monohydrated sulfates. The lower unit is disconformably overlain by an upper unit composed of thinner (< 3 m) layers with diagnostic polyhydrated sulfate signatures. The original extent of proto-Candor Mensa and its lower unit included neighboring Baetis Mensa. We suggest that the source material for both units is airborne dust or ash but that the depositional environment for the units differs. First, the lower unit was deposited during the subsidence of an enclosed water-filled basin. This basin/lake could have been frozen periodically, with freeze-thaw episodes possibly linked to Martian obliquity cycles. Erosion, including the potential action of glaciers, was able to remove large volumes of material out of the basin during the tectonism that produced the current geometry of Valles Marineris. Deposition of the upper unit postdates this event and took place in the absence of standing water at high elevation. Groundwater or snowmelt may have provided the water required for sulfate formation and deposit induration. We conclude that the major break in sedimentation recorded by this ILD deposit coincides with linking of ancestral basins into the current geometry of Valles Marineris chasmata and that it was possible to form hydrated minerals after this event.

Interior layered deposits within a perched basin, southern Coprates Chasma, Mars: Evidence for their formation, alteration, and erosion

A basinA¢Â�Â�like area containing three interior layer deposits (ILDs) on the southern margin of Coprates Chasma was studied. We interpret the area as an ancestral basin and demonstrate that ILD deposition postdates the formation of the current wall rock slopes. The geometry of the ILD and the wall rock spurs form a catchment area between each ILD and the plateau to the south. Erosional remnants of extensive ash or dust layers deposited on the plateau south of Valles Marineris also crop out on the southern plateau of Coprates Chasma. A mass balance calculation suggests that the volume of each ILD is compatible with the volume of the ash or dust that would have been deposited within each catchment area. We propose that the ILDs likely formed by episodically washing such aerially deposited material down from chasma walls. Rifting of the IusA¢Â�Â�MelasA¢Â�Â�Coprates graben opened the enclosed basin and removed any standing water. Faults within the ILDs are compatible with this chasm opening. Sulfates are associated with the ILDs and lightA¢Â�Â�toned material on the basin floor. We suggest that they result from water alteration of preexisting deposits, though the timing of that alteration may predate or postdate the breaching of the basin. Scours within one ILD are similar to terrestrial glacial scours. During a period of high obliquity ice would accumulate in this region; hence we argue the scours are Martian glacial scours. A late deposited layer marks the end of the active local geological history between 100 My and 1 Gy.

Structural analysis of interior layered deposits in Northern Coprates Chasma, Mars

Interior layered deposits within an embayment on the northern wall of Coprates Chasma in the Valles Marineris, Mars, are studied using HRSC, CTX, HiRISE and CRISM data. The layered material outcrops in three separate locations. The largest layered deposit within the embayment, a free standing central mound, has an approximate stratigraphic thickness of 2 km. Dip directions change along the central axis of this mound, which is also a zone of deformation. The surface texture of layers within this mound displays polygonal structures at HiRISE scale. By contrast, the western layered deposit abuts directly against the chasm wall and appears to have a relatively uneroded depositional surface approximately 600 m below the current top elevation of the central mound. A basement ridge, exposed by a landslide scar near the eastern portion of the area, is covered with layered material and shows downward displacement. We suggest that the entire embayment originated as a small ancestral basin. The displaced basement ridge is evidence of the early basin collapse. The central mound was most likely deposited on a wall rock spur. Deposition did not fill the basin evenly. The detection of hydrated sulfates attests to alteration or deposition by liquid water. Following an erosion event, which coincided with or postdated Valles Marineris formation, thin mesa-forming materials covered most of the area.

Multiple-process origin of Valles Marineris basins and troughs, Mars

An improved sequence of formation for Valles Marineris depressions is proposed. It involves three key stages; (a) dike emplacement radial to Syria Planum during Late Noachian to Early Hesperian time; (b) localized subsidence of crustal rocks during post-Early Hesperian time, forming ancestral basins such as Hebes Chasma; and (c) regional normal faulting that overprints the ancestral basins and forms the structural troughs, such as Coprates Chasma, principally during Amazonian time. Attainment of high topography at Syria Planum, completed by the Early Hesperian, was accompanied and/or followed by extrusion of ridged plains lavas, emplacement of subsurface dikes, and wrinkle-ridge deformation. The high topography contributed to a substantial hydrostatic head, leading to subsurface volume reduction, vertical displacement of crustal strata to from chaotic terrain and ancestral basins, and localized outbreaks of fluid as outflow channels. The dikes contributed a pervasive structural influence on the orientations of later nearsurface processes. Sometime after infilling of ancestral basins with their interior layered deposits, normal faults related to Tharsis centered stresses were superimposed on the relict high topography, ancestral basins, and dike network, forming large grabens. Recognition of the polygenetic origin of troughs such as Melas Chasma, and the disparate origins of irregular and rectangular troughs, provides the key to unraveling the conflicting accounts of trough origin and timing.

Unraveling the boundary between turbidites of the Kisseynew belt and volcano-plutonic rocks of the Flin Flon belt, Trans-Hudson Orogen, Canada

The Flin Flon – Kisseynew boundary in the eastern Trans-Hudson Orogen is interpreted here as an early thrust fault that places 1.86–1.84 Ga Kisseynew belt turbidites over previously deformed 1.91–1.88 Ga arc and ocean-floor assemblages of the Flin Flon belt. The basin in which sedimentary rocks of the Kisseynew belt were deposited has been interpreted to have formed partly within the Flin Flon belt. The fault that juxtaposes the two belts is interpreted to have been localized near the ancestral basin margin, resulting in development of a major ramp zone during basin closure. This interpreted ramp zone provides an explanation for the steep to shallow structural transition that corresponds to increasing metamorphic grade. Collapse of the Kisseynew sedimentary basin and juxtaposition of the two belts are attributed to southwest-verging folding and thrusting that initiated prior to emplacement of 1.83 Ga plutons. This magmatism was followed by regional greenschist- to upper-amphibolite-grade metamorphism (1.82–1.805 Ga) and renewed southwest-directed folding and thrusting. Late backfolds developed at the leading edge of the fold-thrust belt. Postpeak metamorphic deformation resulted in large-scale, upright folding of the fold–thrust stack (including the Flin Flon – Kisseynew boundary). This stage of deformation is interpreted to record a transition from southwest-directed transport to northwest-southeast-directed shortening at ~1.8 Ga.

Topography of Valles Marineris: Implications for erosional and structural history

Compilation of a simplified geologic/geomorphic map onto digital terrain models of the Valles Marineris permitted an evaluation of elevations in the vicinity of the troughs and the calculation of depth of troughs below surrounding plateaus, thickness of deposits inside the troughs, volumes of void spaces above geologic/geomorphic units, and volumes of deposits. The central troughs north Ophir, north and central Candor, and north Melas Chasmata lie as much as 11 km below the adjacent plateaus. In Ophir and Candor Chasmata, interior layered deposits reach 8 km in elevation. If the deposits are lacustrine and if all troughs were interconnected, lake waters standing 8 km high would have spilled out of Coprates Chasma onto the surrounding plateaus having surface elevations of only 4–5 km. In this case, interior deposits above about 4 km in the central troughs would not be lacustrine. They could be volcanic. On the other hand, the troughs may not have been interconnected at the time of interior‐deposit emplacement; they may have formed isolated ancestral basins. The existence of such basins is supported by independent structural and stratigraphic evidence. The ancestral basins may have eventually merged, perhaps through renewed faulting, to form northern subsidiary troughs in Ophir and Candor Chasmata and the Coprates/north Melas/Ius graben system. The peripheral troughs are only 2–5 km deep, shallower than the central troughs. They may have formed from a combination of erosional collapse and structural activity. Chaotic terrain is seen in the peripheral troughs near a common contour level of about 4 km on the adjacent plateaus, which supports the idea of release of water under artesian pressure from confined aquifers. The layered deposits in the peripheral troughs may have formed in isolated depressions that harbored lakes and predated the formation of the deep outflow channels. If these layered deposits are of volcanic origin, they may have been emplaced beneath ice in the manner of table mountains. Areal and volumetric computations show that erosion widened the troughs by about one‐third and that deposits occupy one‐sixth of the interior space. Even though the volume eroded is larger than the volume deposited, topographic and geologic considerations imply that material eroded from trough walls was probably part of the interior layered deposits but not their sole source. Additional material may have come from subterranean piping, from reworking of local disintegration products on the floors, such as chaotic materials, or from eolian influx. But overall it is likely that the additional material is volcanic and that it forms mostly the upper, more diversely bedded layers of the interior deposits.

Santa Monica and Santa Ana Mountains--Relation to Oligocene Santa Barbara Basin

An analysis of the stratigraphy and structure of the Santa Monica and Santa Ana Mountains demonstrates a close affinity between the two, one that can be explained only by a much closer original geographic relation of the two ranges. The Santa Monica Mountain block, bounded by the Malibu Coast fault on the south and the Las Posas fault on the north, began a westward movement across the ancestral basin during the Luisian stage of the middle Miocene. Approximately 33 mi (53 km) of left-lateral displacement took place prior to the Mohnian stage of the upper Miocene, breaking the original basin into the separate Ventura and Los Angeles basins joined by a San Fernando subbasin. This displacement took place in no more than 2.5 m.y. at a minimum rate of movement of 0.8 in. (2 cm) year. Four mi (6.4 km) of post-Mohnian left-lateral displacement subsequently occurred. The magnitude of displacement is determined by projecting the Santa Ana Mountains parallel with the San Andreas direction, to a point of intersection with the Raymond fault, and then by matching similar stratigraphy on opposite sides of the fault. Restoration of the Santa Monica Mountains block to its pre-Luisian location, combined with the addition of known thicknesses of sections of the Sespe Formation, permits the construction of original Sespe isopachs and the reconstruction of the Oligocene basin, herein called the Santa Barbara basin. This basin was bounded, south of the present Ventura basin and west of the present Los Angeles basin, by a positive Franciscan area, the Anacapia of Reed and Hollister. The Luisian orogeny, responsible for the westward rifting of the Santa Monica Mountains, also produced the breakup of Anacapia, and exerted the most profound influence on southern California geology of any Cenozoic event.

Forest City Basin of Missouri, Kansas, Nebraska, and Iowa

The Forest City basin of Missouri, Kansas, Nebraska, and Iowa is the area in which the first oil and gas production west of the Mississippi River was found. Production near Paola, Kansas, followed within a few years of the birth of the oil industry at Titusville, Pennsylvania, in 1859 Initial movements of the Ozark uplift and the Chautauqua arch began in Late Ordovician time. Attendant subsidence in the north Kansas area formed an ancestral basin which was bisected later by the Nemaha anticline forming the Salina basin and an unnamed basin on the east. Post-Mississippian, pre-Atokan peneplanation of the entire area took place before renewed activity along the Nemaha anticline uplifted the area west of the anticline while downwarping east of the Nemaha scarp formed the Forest City basin. Thus it is defined as a Pennsylvanian basin. Movement along the Nemaha anticline may have begun during the late Early Ordovician but certainly it was mobile during Early Mississippian time and continued to be active intermittently until at least the early Permian. The Cherokee and Forest City basins were separated by a low arch until middle Cherokee time when the Forest City basin filled with sediments and the two basins linked across the Bourbon arch. Northeast-trending folds developed after Mississippian deposition whereas previous structural orientation was northwestward.

Forest City Basin of Missouri, Kansas, Nebraska, and Iowa: ABSTRACT

The Forest City basin of Missouri, Kansas, Nebraska, and Iowa is the area of the first oil and gas production west of the Mississippi River. Production was found near Paola, Kansas, within a few years of the birth of the American oil industry at Titusville, Pennsylvania. Initial movements of the Ozark uplift and the Chautauqua arch began in Late Ordovician time. Attendant subsidence in the northern Kansas area formed an ancestral basin which was later bisected by the Nemaha anticline, forming the Salina basin on the west and an unnamed basin on the east. Post-Mississippian, Pre-Atokan peneplanation of the entire region took place before renewed activity along the Nemaha anticline uplifted the area west of the anticline while downwarping east of the Nemaha scarp formed the Forest City basin. Thus, it is defined as a Pennsylvanian-age basin. Movement along the Nemaha structure may have begun as early as pre-St. Peter time but certainly during Early Mississippian time and continued intermittently until at least the Early Permian. The Cherokee and Forest City basins were separated by a low arch until middle Cherokee time when the Forest City basin filled with sediments and the two basins joined across the Bourbon arch. Northeast-trending folds developed after Mississippian deposition, whereas previous structural orientation had been toward the northwest. End_of_Article - Last_Page 1564------------