A Magnetic Geothermometer in Moderately Buried Shales

Shales contain magnetic minerals generally at very low concentrations. In the early stages of diagenesis, the inherited magnetic minerals are altered, while magnetic nanominerals are formed. In this study, we proposed a study of shales over a stratigraphic thickness of 1.3 km from a borehole in the Paris basin (Borehole EST 433, France), and shales from the same formation (Opalinus Clay) collected in seven boreholes in the Jura molasse basin (Swiss). Magnetic measurements at experimental temperatures <30 K allowed the formation of a proxy of magnetite nanograins named PM. We showed that some of these nanograins formed around the pyrite grains, probably under the action of temperature and organic matter. PM was then compared to the maturity values of the organic matter. We found a correlation between PM and the percentage of reflectance of vitrinite. The shales from both Paris and molassic Swiss basins showed very comparable magnetic characteristics for a given maturity level. The magnetic study therefore provided constraints on the maturity level of the shales in the oil window area. Our study showed that PM can be used as a geothermometer in shales in which CaCO3 is lower than 60%.

Supplemental Material: Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography, regional tectonics, and development of strike-slip basins

(1) Descriptions of spatial and temporal stratigraphic thickness variations in the Chumstick basin and methods for sediment accumulation rate calculations, (2) Detailed descriptions and photographs of each lithofacies association of the Chumstick Formation defined in the text of the manuscript, (3) Tables of raw and summary conglomerate clast count data for each member of the Chumstick Formation, (4) Summary tables of conglomerate detrital modes for each member of the Chumstick Formation, (5) Summary tables and age probability plots of detrital zircon ages from each sandstone sample collected within the Chumstick Formation, (6) Conglomerate clast raw data from LaCasse (2013) and (7) Tables of detrital zircon raw data from each individual sandstone sample within the Chumstick Formation (Donaghy, 2015).

Supplemental Material: Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography, regional tectonics, and development of strike-slip basins

(1) Descriptions of spatial and temporal stratigraphic thickness variations in the Chumstick basin and methods for sediment accumulation rate calculations, (2) Detailed descriptions and photographs of each lithofacies association of the Chumstick Formation defined in the text of the manuscript, (3) Tables of raw and summary conglomerate clast count data for each member of the Chumstick Formation, (4) Summary tables of conglomerate detrital modes for each member of the Chumstick Formation, (5) Summary tables and age probability plots of detrital zircon ages from each sandstone sample collected within the Chumstick Formation, (6) Conglomerate clast raw data from LaCasse (2013) and (7) Tables of detrital zircon raw data from each individual sandstone sample within the Chumstick Formation (Donaghy, 2015).

An improved high-precision Jacob's staff with laser sighting and topographic capabilities for the high-resolution stratigrapher

ABSTRACT Accurate and quick high-resolution measurements of stratigraphic thicknesses and geologic profiles exposed in surface outcrops are critically important for interpreting depositional environments, basin development, and tectonic evolution. Therefore, a high-precision instrument is required for high-resolution stratigraphic measurements. Herein, a new version of the Jacob's staff incorporating an Abney level or a geological compass, a laser pointer, and a handheld GPS is presented. Both the Abney level and the laser pointer are oppositely positioned side by side at the 1.5 m position on the 1.6-m-long fiberglass rod, and they can synchronously slide vertically and rotate horizontally. Because accurate sighting of bedding surfaces is facilitated with both the sighting tube of the Abney level and straight-line beam of the laser pointer, this novel design greatly improves the precision and speed of stratigraphic thickness measurements. More importantly, when the section requires a lateral shift, the straight-line beam can easily and precisely identify the new start point without rotating the rod, which greatly reduces potential changes in the angle of the rod. The GPS unit installed at the top of the rod synchronously records the track of the measured section and coordinates of each measurement, which enables acquisition of a topographic profile along the stratigraphic section. Uncertainties in stratigraphic thickness measurements are minimized when observations are made through the sight tube of the Abney level when the rod is oriented perpendicular to the bedding surface.

Arquitectura estratigráfica, paleogeografía y proveniencia sedimentaria de las rocas cenozoicas del sur de Perú (Tacna, 18° S)

The Cenozoic rocks lying in the Province of Tacna (18° S), southern Perú, represent approximately 600 m of stratigraphic thickness. This stacking groups the Sotillo (Paleocene), Moquegua Inferior (Eocene), Moquegua Superior (Oligocene), Huaylillas (Miocene) and Millo formations (Pliocene), and these are the sedimentary fill of the Moquegua Basin. The sediments of the three latter formations are organized into nine sedimentary facies and five architectural elements. Their facies associations suggest the existence of an ancient highly channelized multi-lateral fluvial braided system, with upward increase of pyroclastic and conglomeratic depositions. The heavy mineral spectra make each lithostratigraphic unit unique and distinguishable, being the sediments of the Moquegua Superior Formation rich in garnets, titanites and zircons; while the sediments of the Huaylillas and Millo formations in clinopyroxenes. This mineral arrangement becomes an excellent tool for stratigraphic correlations between outcrops and subsurface stratigraphy (by means of well cores studies) and allow to sketch out a new stratigraphic framework and a complex of rocky blocks bounded by normal faults, often tilted. The sediment mineralogy also suggests that the rocks conforming the Western Cordillera were the main source of sediments for the Moquegua Basin in Tacna. In this context, the detritus of the Moquegua Superior Formation derives mainly from the erosion of the rocks forming the Coastal Basal Complex (Proterozoic), the Ambo Group (Carboniferous) and the Junerata/Chocolate Formation (Early Jurassic). The Huaylillas Formation is a pyroclastic and sedimentary unit which components derived mainly from the Huaylillas volcanism (Miocene) and partly from the denudation of the Toquepala Group (Late Cretaceous). The Huaylillas Formation widely contrasts to the underlying Moquegua Superior Formation due its mineralogy and facies. Finally, the detritus of the Millo Formation derived mostly from the rocks forming the Barroso Formation (Pliocene), and their facies represent a higher contrast in relation to the underlying units due its notorious conglomerate facies.

Solidification timescale for the Dufek Intrusion, Antarctica determined by U-Pb zircon ages

&lt;p&gt;The 8-9 km thick Dufek layered mafic intrusion of Antarctica was emplaced at approximately 182 Ma associated with the Ferrar dolerites and the breakup of the supercontinent Gondwana.&amp;#160; It is rivaled in thickness only by the Bushveld Complex of South Africa and shows a similar progression in mineral compositions all the way to the uppermost contact with an overlying granophyre layer.&amp;#160; This progression in mineral composition suggests that it crystallized from the bottom to the top and did not form an upper solidification front (a.k.a., Upper Border Series) typical of smaller intrusions such as the Skaergaard Intrusion.&amp;#160; Unlike the Bushveld Complex, however, the Dufek Intrusion is exposed in only two ~1.8 km thick sections: the lowermost Dufek Massif, and the uppermost Forrestal Range, which are separated from one another by a ~50km wide snowfield.&amp;#160; The remainder of the stratigraphy is inferred from geophysics, evolution of mineral compositions, and projection of the dip of the layering through the snowfield.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; We obtained precise CA-ID-TIMS U-Pb zircon ages from samples from the Dufek Massif and Forrestal Range in order to determine the timescale of solidification of a large layered mafic intrusion.&amp;#160; What we found is surprising - zircons from the bottom of the intrusion record younger ages than those from the top of the intrusion.&amp;#160; Two samples from the Dufek Massif have zircon U-Pb ages of 182.441&amp;#177;0.048 Ma and 182.496&amp;#177;0.057 Ma; whereas three samples from the Forrestal Range have zircon U-Pb ages of 182.601&amp;#177;0.064 Ma, 182.660&amp;#177;0.10 Ma, 182.78&amp;#177;0.21 Ma.&amp;#160; Thus, the lower section of the Dufek Intrusion solidified approximately 160,000 years after the upper.&amp;#160; We explore two possibilities for this reverse-age stratigraphy, (1) that the ages reflect the solidification of interstitial melt in a single magma chamber cooling from the top down, or (2) that the Dufek Massif and Forrestal Range are two separate magma chambers that are not connected at depth.&amp;#160; Our results have implications for the stratigraphic thickness estimates of the Dufek Intrusion as well as the duration of magmatism associated with continental breakup.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

The 3D seismic characteristics and significance of the strike-slip faults in the Tazhong area (Tarim Basin, China)

The eastern part of Tazhong area in the Tarim Basin consists of three sets of vertical strike-slip faults oriented in north–northeast (36°azimuth), east–northeast (68° azimuth), and west–northwest (126°azimuth) directions that cut the strata from Cambrian to Carboniferous. The fault belts indicate significant horizon upwarp and downwarp deformations and variations in their stratigraphic thickness on seismic profiles. Through detailed interpretation of the 3D seismic data, we consider that these phenomena reflect the different stress properties and active stages of the faults. The horizon upwarp and downwarp within the fault belts correlated respectively to the decrease and increase in stratigraphic thickness within the fault belts in comparison to the coeval counterpart of the bilateral fault blocks. For the same fault, different stratigraphic intervals express different types of horizon deformation and thickness changes. The horizon downwarp and the contemporaneous stratigraphic thickening inside the fault belts suggest the transtensional actions of the fault. The horizon upwarp and the contemporaneous thinning within the fault belts suggest transpressional actions of the fault. Based on this, we inferred the active periods of the three sets of strike-slip faults. The north–northeast-striking faults were formed in the late Ordovician Sangtamu Formation. This set of faults experienced four stages, i.e., sinistral transpression, sinistral transtension, static, and transtension. The east–northeast and west–northwest-striking faults initiated in the mid-Cambrian period as coupled transtension. Activity ceased in the west–northwest faults after the mid-Cambrian and in the east–northeast faults during the late Ordovician. The three sets of strike-slip faults all affect the formation of the hydrothermal dissolution reservoirs that are distributed in the Ordovician carbonate rocks.