Edward L. Simpson

Muddy Roll-up Structures in Siliciclastic Interdune Beds of the c. 1.8 Ga Waterberg Group, South Africa

Abstract Concentrically rolled-up silty mudrock laminae, 1–2 mm thick, are found in the uppermost of four, 40–110 cm thick interdune beds within a thick aeolian succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa. These curved laminae are analogous to previously described “roll-ups,” biogenic structures generally ascribed to soft-sediment deformation or desiccation of microbial mats overlying either carbonate or siliciclastic sediments, within shallow- to deep-marine paleoenvironments. The Makgabeng roll-ups are thought to reflect desiccation of a microbial mat, followed by resedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The significance of the South African example of roll-ups is that they appear to have formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for microbial colonisation of a terrestrial setting.

Quantifying the oldest tidal record: The 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

The 3.2 Ga Moodies Group in the Barberton Greenstone Belt, South Africa, contains the oldest preserved record of tides. The tidal record is preserved in a tidal sand-wave deposit in the lower Moodies Group as bundles of sandstone foresets separated by mudstone drapes. Detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal, fortnightly, and monthly tidal periodicities. Thick-thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood-tidal currents, respectively. Cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap-spring-neap tidal cyclicity. Alternating thicker and thinner neap-spring-neap cycles are comparable to anomalistic, perigean-apogean tidal signatures. This quantitative record of tides in the middle Archean Moodies Group represents, by 2.2 b.y., the oldest such documentation. Tidal cyclicity recognized in the Moodies sand-wave deposit is comparable to that recorded in modern tidal settings and identified in the Carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today.

Controls on spatial and temporal distribution of Precambrian eolianites

Abstract Inversely graded stratification, generated by the migration of wind ripples, and adhesion structures permit unequivocal identification of Precambrian eolianites. These criteria, in combination with scale of cross-beds, angle of inclination of foresets, geometry of depositional units, and associated non-eolian facies, are used to discriminate between Precambrian dune/draa, dune-plinth, sand-sheet, and interdune deposits that formed in inland and coastal settings. Based on an analysis of published literature, fundamental conclusions can be drawn on the spatial and temporal distribution of Precambrian eolianites. The oldest reported eolianites are from the ca. 2.1 Ga Deweras Group in Zimbabwe and Hurwitz Group in Canada and numerous examples of eolianites are reported from the 1.8 Ga and younger rock record. Lack of Archean and early Paleoproterozoic eolianites and their widespread development after 1.8 Ga are examined with respect to: absence of vegetation, crustal growth and tectonic setting, relative sea-level fluctuations, unfavorable atmospheric and/or climatic change, and non-recognition. The lack of pre-2.2 Ga eolianites may be related to reworking by braided rivers combing across non-vegetated floodplains, reworking of coastal eolianites during transgression or their non-recognition in the Early Precambrian record. The temporal concentration of eolianites at 1.8 Ga may best be related to the early stages of breakup and the assembly phases of supercontinents.

Redoximorphic Paleosols in alluvial and lacustrine deposits, 1.8 Ga Lochness Formation, Mount Isa, Australia; pedogenic processes and implications for paleoclimate

ABSTRACT Paleosols in the Lochness Formation (1.8 Ga, Australia) include both rare, non-red and abundant, strongly reddened varieties that formed at subaerial exposure surfaces in both ephemeral-river and lacustrine settings. Physical processes dominated non-red paleosols, which were characterized by repeated episodes of desiccation, shrinking, and cracking alternating with wetting and introduction of sand, silt, clay, and iron oxyhydroxides into planar voids. Redoximorphic (oxidation-reduction) processes were especially intense for the red paleosols; redox depletions of Fe and Mn (hypoalbans) occur immediately adjacent to desiccation-related macropores and peds, whereas redox concentrations of Fe and Mn (quasi-coatings) occur within paleosol matrix adjacent to redox depletions. Illuviated cla and Fe-oxide coatings and hypocoatings are also common in planar macropores and on ped faces. Redoximorphic features indicate periodic water infiltration and saturation, accompanied by development of reducing conditions along planar macropores and ped surfaces in Lochness Formation paleosols. Variations in soil saturation were caused by seasonal fluctuations of lake level in lacustrine deposits, and by formation of perched saturation zones within floodplain deposits, respectively. Occurrences of red, hematitic paleosols in the Lochness Formation are compatible with previous interpretations of a weakly oxygenated 1.8 Ga paleoatmosphere. Redoximorphic features in the paleosols suggest a warm to cool temperate paleoclimate (mean annual soil temperature 5-20°C) characterized by seasonal saturation, by analogy with Quaternary redoximorphic soils. A minimal concentration of organic C (at least 1 wt %), possibly of microbial or bacterial origin, must have been present in these Proterozoic soils to allow for Fe reduction.

Eolian Dune Degradation and Generation of Massive Sandstone Bodies in the Paleoproterozoic Makgabeng Formation, Waterberg Group, South Africa

ABSTRACT Massive sandstone bodies in the 1.85 Ga Makgabeng Formation, South Africa, are located within the middle and upper part of the formation and are associated with the oldest reported deposits of barchan and barchanoid-ridge dunes. Bases of massive sandstone bodies are channelized or planar. Channel-based bodies typically overlie low-angle or up to 26° inclined wind-ripple strata. Channel margins vary from low angle to vertical. Rarely, massive sandstones at channel margins display vague horizontal stratification and/or contain ripped-up fragments of wind-ripple strata. Planar-based massive sandstone bodies are lenticular and characteristically are interbedded with low-angle to horizontal dune toesets. These lobate massive sandstone bodies vary from 5 cm to 6 m in thickness and from 3 m to possibly over 50 m in width; some of the sandstone bodies onlap dune reactivation surfaces. Rare dewatering structures, parting lineations, and adhesion structures occur at tops of the massive sandstone bodies. Lower boundaries of massive sandstone bodies change from channelized to planar-based down the dune foresets. Massive sandstones were generated by two different mechanisms, which are inferred to have been triggered by significant precipitation events. The presence of steep margins, channeling, rip-up fragments, and dewatering features, and a lack of tractional structures, indicate that most massive sandstones were deposited from hyperconcentrated flows down the dune lee face. Flows were initially turbulent, analogous to hyperconcentrated flows, resulting in scouring of wind-ripple strata. As hyperconcentrated flows migrated onto the dune plinth, rapid deposition produced lobate massive sandstone deposits with planar bases. This sedimentation is linked to development of a hydraulic jump at the slipface-plinth intersection, which increased flow depth and reduced flow velocity. Some flows maintained their turbulence onto the dune plinth, resulting in erosion of the plinth and scouring into the underlying preserved dune deposits. Massive sandstone bodies linked to reactivation surfaces are more likely a result of partial lee-face collapse, which generated translational slides.

Amalgamated Interdune Deposits, White Sands, New Mexico

ABSTRACT Cores taken by McKee and Moiola (1975) from White Sands dune field indicate that interdune areas are underlain by 7.0-10.4 m of clastic gypsum deposits. Clean sand bodies as much as 4.5 m thick are separated by thin beds of silly sand. McKee and Moiola interpreted the sequence to be the product of climbing bed forms; interdune silty sands were interpreted to be deposited upon truncated dune sands and to be soon buried by the next wave of advancing dunes, If the dunes are climbing in such a manner, a pinchout of interdune sediments downwind across interdune surfaces is required. The thickness of the silly sands should be related to the size of the interdune area and the rate of dune migration. We tested this autocyclic hypothesis by trenching the length of the interdune areas in both t e barchanoid and transverse dunes. Trenches revealed that interdune deposits form flat-lying sheets which extend uninterrupted to positions directly beneath the windward slopes of dunes at the downwind end of each interdune area. This relationship precludes the possibility that, under present conditions, thick dune sands are being preserved. Because there is little or no net accumulation of dune sand, interdune deposits are being amalgamated; thin lenses of dune sediment exposed on the wall of one trench show that a minimum of three interdune deposits have been fused to form the uppermost 30 cm of the interdune deposit. Clean sand in the cores may have accumulated during periods when sediment supply was greater than at present. Silly sediments accumulated during periods when, as in the p esent, sand supply was limited and dunes did not climb appreciably. Future studies of modern and ancient eolian sediments should attempt to document whether the interdune deposits represent deposition within single interdune areas or amalgamation of several discrete deposits.

The fluvial-to-marine transition within the post-rift Lower Cambrian Hardyston Formation, Eastern Pennsylvania, USA

Abstract Identification of fluvial-to-marine transitions is often problematic in Neoproterozoic and Cambrian strata due to the absence of diagnostic, physical, and/or biogenic sedimentary structures. This study highlights the identification of a fluvial-to-marine transition in a post-rift setting. Siliciclastic sediments of the Early Cambrian Hardyston Formation, in eastern Pennsylvania, are assigned into four facies associations (FA A–D), within which a fluvial-to-marine transition can be delimited. The lowermost subdivision, FA A, consists typically of thick-bedded, fining-upward beds of structureless, pebbly conglomerate that grade into structureless sandstone. Within the uppermost FA A, structureless sandstone is replaced by trough and planar, cross-stratified sandstone. FA A is best interpreted as deposits of a bedload-dominated, braided fluvial system. FA B consists of densely packed Skolithos burrows, thick-bedded, normally graded sandstone with rare poorly preserved tabular-planar and trough cross-beds near its upper boundary. Sedimentary structures in FA B are analogous to those found in a shoreface setting. Normally graded beds reflect deposition from waning storm-induced flow, with structured sandstone recording possible fair-weather or post-storm reworking. The fluvial–marine transition lies at the abrupt change from FA A to FA B and is identified by vertical changes from: (1) feldspathic to quartz sandstone, (2) fluvial bar to shoreface storm deposits, and (3) nonbioturbated to bioturbated strata. Additionally, heavy minerals are concentrated along the boundary. FA C is composed of trough cross-stratified conglomerate and sandstone with paleocurrents interpreted as shore-parallel and offshore-directed marine currents. FA C represents a marginal-marine setting reflecting a gradation from FA B to FA D, including characteristics of both Skolithos of FA B and a number of sedimentary structures indicative of FA D. FA D consists of offshore-directed, tabular-planar and trough cross-stratified quartz sandstone separated by thin siltstone–mudstone beds. Some tabular-planar cross-stratified beds contain obliquely oriented ripples on siltstone-draped pause planes. Preserved sinuous-crested dunes with superimposed current ripples cap some cross-bed cosets. The presence of tidal currents is inferred to be the mechanism responsible for the formation of FA D in a probable subtidal marine setting. The Hardyston Formation accumulated in response to thermal or jerky subsidence. Aggradational fluvial deposits developed as a result of the initial Sauk trangression. The fluvial–marine transition most probably records the shoreface retreat unconformity.

Early Cambrian Progradational and Transgressive Sedimentation Patterns in Virginia: An Example of the Early History of a Passive Margin

ABSTRACT Occurrence of both the Hampton and Erwin Formations within a number of thrust sheets provides an oblique cross section of the paleoslope permitting an across-strike reconstruction of the Early Cambrian passive margin. Five environmental settings are reflected in facies comprising the Hampton and Erwin Formations. Sedimentation on the outer-shelf was dominated by geostrophic flows and suspension settling of mud under fair-weather conditions. Distal inner-shelf sediments resulted from stormwave reworking below fair-weather wave base. Proximal inner-shelf facies consist of detritus transported from the nearshore and reworked in situ by the complex interaction of unidirectional and oscillatory currents generated by storms. Shoreface sediments display evidence of reworking by shore- arallel currents generated by shoaling fair-weather or waning storm waves. Tidal-flat sedimentation was dominated by flood-currents with late-stage ebb runoff. Shelf and shoreface facies are associated in five, 65 to 250 m-thick parasequences that coarsen and thicken upwards and contain thin fining- and thickening-upward intervals at their bases. Parasequences within northwestern thrust sheets consist of outer-shelf, inner-shelf and shoreface facies, whereas outer-shelf and distal inner-shelf facies with condensed sections and glauconitic horizons predominate within southeasterly thrust sheets. The five stacked parasequences represent an overall offlap package and comprise a highstand systems tract. The uppermost Erwin Formation is a fining- and thinning-upward parasequence that represen s part of a transgressive systems tract. Tidal-flat and shoreface facies within northwestern thrust sheets unconformably overlie the highstand systems tract, whereas shelf facies within southeastern thrust sheets are conformable on the highstand systems tract. Stratigraphic evolution of the Hampton and Erwin Formations is related to different orders of relative sea-level change. Initial transgression is reflected in alluvial and tidal facies in the underlying upper Unicoi Formation and records the late-Precambrian to early Cambrian, second-order sea-level rise; a condensed section at the base of the Hampton Formation reflects maximum flooding. The Hampton Formation and most of the Erwin Formation document outbuilding of the passive-margin prism associated with a decrease in rate of rise of relative sea level resulting from a third-order eustatic fall superimposed on the second-order rise. Parasequences in the Hampton and Erwin Formations record lower-order eustatic oscillations superimposed on the third-order sea-level fall.

Growth faults in the Kaiparowits Basin, Utah, pinpoint initial Laramide deformation in the western Colorado Plateau

Growth faults and synorogenic sedimentary strata preserved in Upper Cretaceous units on the margin of the Kaiparowits Basin in southern Utah pinpoint the timing of onset of the Laramide orogeny in this region between 80 and 76 Ma. The newly identified listric normal faults, exposed in the steep limb of the East Kaibab monocline, sole into shales and evaporites of the Jurassic Carmel Formation. Faults lose displacement up-section through the Cretaceous Wahweap Formation and are associated with numerous coseismic sedimentary features. Fault orientations and slip vectors yield strain directions consistent with fold-related extension parallel to the axis of the growing East Kaibab monocline, or with development of a pull-apart basin at a bend in the trend of the fold. The association of the faults with the steep limb of a major basement-cored structure links them to initial Laramide movement along the Kaibab Uplift. When combined with recent radiometric ages of rock units bracketing the fault-induced growth strata, these sedimentary and structural features narrowly define the onset of Laramide deformation in the western Colorado Plateau.

Distribution of Diopatra cuprea across modern tidal flats; implications for Skolithos

Skolithos is the dominant sedimentary structure in Cambrian foreshore/shoreface deposits. Diopatra cuprea, a predaceous, tube-building polychaete, has been cited as a possible modern analog for the Skolithos organism. This investigation examines the spatial distribution of D. cuprea across several tidal flats and evaluates how well D. cuprea population parameters parallel those of some Cambrian and Silurian Skolithos. A sampling network was construeted encompassing a tidal pool and channel on an ebb-tidal delta shoal (ETDS) in Chincoteague Inlet, VA, and traverses were surueyed from the high- to low-tide line in Toms Cove of Assateague Island, VA. The number and position of worm tubes within a square meter grid were recorded for each station