4 research outputs found
Reassessing evidence of MoonâEarth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)
Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22âGa) of the Barberton Greenstone Belt, South Africa. From the time-series analysis of tidal bundles from a well-exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moonâs anomalistic month at 3.2âGa was closer to 20âdays than the present 27.5âdays. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this studyâs detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel-margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower-intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5âm wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with EarthâMoon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster-spinning Earth
Reassessing evidence of MoonâEarth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)
Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22âGa) of the Barberton Greenstone Belt, South Africa. From the time-series analysis of tidal bundles from a well-exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moonâs anomalistic month at 3.2âGa was closer to 20âdays than the present 27.5âdays. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this studyâs detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel-margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower-intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5âm wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with EarthâMoon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster-spinning Earth
New types of boulder accumulations in the hyper-arid Atacama Desert
The accumulation of thousands of boulder-sized clasts into boulder fields in the Atacama Desert has been linked to seismic-driven downslope transport, a rare sedimentary process corroborated by this study. We surveyed boulder arrangements occurring in the Atacama Desert and identified three accumulation types for further investigation: a small circular boulder cluster (BC), a long channelized boulder stream (BS), and a wide convex-shaped boulder field (BF). Drone-based photogrammetric techniques and field observations were used to generate high-quality digital elevation models and orthophotos to determine boulder count, size, coverage, orientation, lithology and local topography. Our data shows that the arrangement of boulder accumulations corresponds with the shape of the accommodation space and the boulder input, where BCs occur at the center of completely confined topographic depressions, BSs occur along laterally confined and incised hill slopes with boulders stacked above each other, and BFs occur on largely unconfined shallow and low-relief slopes with a distinct boulder front. A general downslope increase of average boulder size and coverage was measured in all boulder accumulations and a long-axis orientation of boulders parallel to the transport direction was observed for the BS. Based on these results and the lack of fluvial transport indicators, we conclude that transport and arrangement of boulder accumulations are largely controlled by the interplay of topography and seismic-driven boulder transport, resulting in unique landscape features present in the hyper-arid Atacama Desert. Such sedimentary transport processes are rare on Earth but potentially play a greater role on other arid planetary surfaces that are covered by boulders and subject to sufficient seismic activity
Reassessing evidence of MoonâEarth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)
Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22âGa) of the Barberton Greenstone Belt, South Africa. From the timeâseries analysis of tidal bundles from a wellâexposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moonâs anomalistic month at 3.2âGa was closer to 20âdays than the present 27.5âdays. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this studyâs detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channelâmargin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lowerâintertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5âm wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with EarthâMoon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a fasterâspinning Earth.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/50110000165