Expansion in SL_d(Z/qZ), q arbitrary

Let S be a fixed finite symmetric subset of SL_d(Z), and assume that it generates a Zariski-dense subgroup G. We show that the Cayley graphs of pi_q(G) with respect to the generating set pi_q(S) form a family of expanders, where pi_q is the projection map Z->Z/qZ

Expansion in perfect groups

Let Ga be a subgroup of GL_d(Q) generated by a finite symmetric set S. For an integer q, denote by Ga_q the subgroup of Ga consisting of the elements that project to the unit element mod q. We prove that the Cayley graphs of Ga/Ga_q with respect to the generating set S form a family of expanders when q ranges over square-free integers with large prime divisors if and only if the connected component of the Zariski-closure of Ga is perfect.Comment: 62 pages, no figures, revision based on referee's comments: new ideas are explained in more details in the introduction, typos corrected, results and proofs unchange

Mochras borehole revisited: a new global standard for Early Jurassic earth history

The Early Jurassic epoch was a time of extreme environmental change: there are well-documented examples of rapid transitions from cold, or even glacial, climates to super greenhouse events, the latter characterized worldwide by hugely enhanced organic carbon burial, multiple large isotopic anomalies, global sea-level change, and mass extinction (Price, 1999; Hesselbo et al., 2000; Jenkyns, 2010; Korte and Hesselbo, 2011). These icehouse–greenhouse events not only reflect changes in the global climate system but are also thought to have had significant influence on the evolution of Jurassic marine biota (e.g. van de Schootbrugge et al., 2005; Fraguas et al., 2012). Furthermore, the events may serve as analogues for present-day and future environmental transitions

Long-period orbital climate forcing. Cyclostratigraphic studies of Cenozoic continental and marine successions in Europe. Geologica Ultraiectina (297)

Orbital climate forcing is well-known for its strong impact on Earth’s climate as for example the switching from glacial to inter-glacial states in the Late Pleistocene. Typical ‘Milankovitch’ cycles are climatic precession (21.000 years or 21 kyrs), obliquity (41 kyrs), and short and long eccentricity (circa 100 and 405 kyrs). Million-year scale astronomical cycles are as well present in eccentricity modulation of precession amplitude (0.97 and 2.4 Myrs) and in obliquity amplitude modulation (1.2 Myrs). These long-period cycles have been suggested to imprint Earth’s climate in the past, although to date direct prove is scarce in marine and absent in non-marine settings. Here, cyclostratigraphic studies have been performed to Cenozoic marine and continental successions in Europe all aimed at examination the impact of long-period orbital climate forcing. The results substantiate that long-period obliquity climate forcing played a major role in the timing of the middle Miocene global cooling as recorded in marine sediments on Malta. Our results of the Madrid in Spain reject the imprint of this long-period obliquity forcing on the Miocene infill. Nevertheless, the formation-scale genetic red bed – limestone sequences in the continental basin might be related to low frequency eccentricity cyclicity. The data from the Late Miocene infill of the continental Teruel Basin remarkably reveal a similar orbital configuration for the transition from red bed to limestone successions as in the Madrid Basin. However, enhanced sediment supply related to the entrance of an axial fluvial system time-equivalent at the other basin margin might rather suggest a local geomorphic or tectonic origin of the transition instead of climate change. The sediment record of the shallow marine succession of the Boom Clay Formation is too short and poorly dated to investigate the presence of low frequency astronomically forced variability, although high frequency glacio-eustatic sea level variations driven orbital forcing are now well depicted

Rijke riviersedimenten in het Bighorn Bekken, Wyoming

De eerste stap die ik in mijn nieuwe veldwerkgebied ga zetten, is in de sneeuw. Het is begin juni 2009. Ik had bedacht vroeg in ‘t jaar het veld in te gaan zodat ik deze eerste keer ruim de tijd zou hebben. Het propellervliegtuigje waar ik in zit daalt langzaam uit de wolken om zo meteen te landen op Yellowstone National Airport in Cody, Wyoming, in het noordwesten van de Verenigde Staten. Terwijl de laatste slierten wolk langs mijn raampje wegglijden, zie ik voor het eerst de ontsluitingen waar ik zo veel over gelezen heb. Er zouden zandige rivierafzettingen en rode bodems te zien moeten zijn. Die witte lithologie echter, daar heb ik niets over gelezen. Het blijkt sneeuw. Dat kan hier op 44 graden noorderbreedte, ter hoogte van Bordeaux, begin juni nog prima

Precession-scale cyclicity in the lower Eocene fluvial Willwood Formation of the Bighorn Basin, Wyoming (USA)

Little is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well-defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch-sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super-elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large-scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change

Modelling astronomical climate signals in fluvial stratigraphy

Orbital climate forcing is demonstrated to result in cyclic changes as reflected in the catchment, including precipitation, temperature, vegetation, sediment supply and water discharge. All of these are known to largely impact alluvial architecture. Climate change related to the 21-kyr precession cycle was proposed as driver of regularly-alternating river avulsion and overbank phases in the Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA (Abels et al. 2013; 2016). This study aims to explore the conditions that are favourable for these climate cyclic signals to be preserved in the fluvial stratigraphy

Depositional and Diagenetic Heterogeneity Control on Aquifer Quality: a Case Study of the Lower Triassic Sandstones in the Southeastern Part of Netherlands

A combined study of depositional facies and diagenesis variation was carried out to understand the main controls on aquifer quality of the Middle Buntsandstein in the southeastern part of the Netherlands. Heterogeneities in continental sandstone bodies occur at different spatial scales, ranging from micrometers to hundreds of meters. Commonly, such heterogeneities result from the interaction of depositional processes at various spatial and time scales. These processes partially also influence subsequent diagenetic evolution, hence present-day aquifer properties. Understanding the role of the resulting architectural heterogeneities in controlling the dynamic reservoir behavior is key in determining aquifer properties and improving pre-drilling prediction. The sandstones of the Main Buntsandstein subgroup in the southeastern part of the Netherlands provide an excellent example where different detrital compositions, internal sedimentary architectures, and diverse burial histories have resulted in a wide range of present-day aquifer properties. In the study area, the aquifers are composed of stacked heterogenous alluvial sandstones bodies intercalated with mud-prone intervals deposited in arid to semi-arid conditions. Differences in sediment sources, transport mechanisms, and intrabasinal conditions resulted in a wide distribution of composition and texture. Additionally, the effect of post-depositional burial diagenesis in a basin with complex tectonic history created diverse burial histories across the basin. The study aims to investigate the variation of present-day aquifer hydraulic parameters about changes in aquifer facies and architecture, detrital composition, as well as compaction and cementation during burial. Core sample analysis unfolded a diverse spectrum of sedimentary facies and lithic fragments, which differ between formations. Thin section analysis provides insights about mechanical compaction, cementations, and authigenic phases. By combining these results with petrophysical data on permeability and porosity of core samples, the major controls on present-day aquifer quality can be assessed