The new study, led by Bennet Schuster of the University of Freiburg together with colleagues from the University of Bern, the Leibniz Institute for Applied Geophysics, and the Baden-Württemberg State Office for Geology, was published in the journal Scientific Drilling. It presents a 166.25-meter-long sediment core from drill site ICDP DOVE 5068_1_C, retrieved in April 2021 on the western flank of the basin. The core, recovered with an overall rate of 95 percent and stored in opaque PVC liners under refrigeration, captures a layered history of glacial erosion, lake sedimentation, and tectonic-style deformation frozen into unconsolidated sediments.

Overdeepened valleys are one of the strangest legacies of glaciation. Glaciers do not simply follow the landscape; they dig below it, eroding elongated troughs beneath the fluvial base level through a combination of abrasion, plucking, and, crucially, the erosive power of pressurized subglacial meltwater. In the low-relief Alpine foreland, where steep valley gradients are absent, an increased availability of basal water is believed to have amplified this erosional potential far out onto the plains. When the ice retreated, these deep depressions became enormous sediment traps, capable of hosting stacked and nested sequences of glacial, proglacial, and post-glacial deposits that preserve evidence of advances otherwise erased from the surface record.

The Tannwald Basin first attracted scientific attention when a research drilling in 1993 and 1994, the Schneidermartin core, recovered fine-grained lacustrine sediments containing pollen indicative of the Holsteinian Interglacial, equivalent to Marine Isotope Stage 11. That discovery pushed the basin’s history back before the Holsteinian, leading researchers to assign its origins to the pre-Holsteinian Hosskirch glaciation. Seismic reflection surveys conducted in 2014 and 2015 subsequently revealed a depression up to 50 meters deep and several hundred meters wide within the basin, along with faults offsetting the upper fill by more than 2 meters and evidence of shallow glaciotectonic folding. The stage was set for a modern, high-resolution core to tie these threads together.

The DOVE team subjected the new core to a rigorous analytical workflow. Non-destructive whole-core scanning measured wet bulk density and magnetic susceptibility at 5-millimeter resolution, while high-resolution line-scan images supported the initial core description. A vane-shear tester systematically estimated undrained shear strength, and selected sections, particularly those suspected of containing ice-contact sediments, were examined with X-ray computed tomography at the University of Bern. Geochemical samples taken at 1-meter resolution provided carbon content, including total organic and inorganic carbon, which helped distinguish sediment sources and depositional settings throughout the sequence.

From these data the researchers defined 17 distinct lithotypes, 6 of them unique to the Tannwald core, and grouped them into seven lithofacies associations. At the base, the core encountered moderately consolidated sandstones, siltstones, and marls of the Upper Marine Molasse bedrock, whose sharp, planar contact at 155.90 meters depth marks the erosional floor of the overdeepening. Directly above lies a striking assemblage of deformed Molasse bedrock rafts, both consolidated and unconsolidated, intercalated with stratified diamicts containing glacially striated clasts of Alpine lithologies. Low density and shear-strength values below 50 kilopascals, combined with in-situ crushing and brecciation, identify this interval as a Type B glaciotectonite: bedrock and sediment sheared in place beneath moving ice. The team interprets it as evidence of subglacial till shearing and the mobilization of bedrock rafts, a process mirrored by the much larger, more than 10-meter bedrock slab documented in the older Schneidermartin core.

Above this basal shear zone, the record tells a story of ice proximity and retreat. Massive, matrix-supported diamicts with high densities of 2.4 grams per cubic centimeter and shear strengths of 200 to 270 kilopascals point to subglacial lodgement till deposited under temporary ice loading. These are interbedded with well-sorted, fining-upward fines bearing frequent dropstones, indicators of floating ice and deposition during episodes when the glacier bed decoupled from the ice, filling subglacial cavities or accommodation space beneath a floating tongue. Higher still, fine-grained rhythmmites and laminated clays with dropstones record an ice-contact proglacial lake, with soft-sediment deformation ranging from centimeter-thick slumps to meter-thick folded and faulted sections, evidence of gravitational slumping on water-saturated depositional slopes. The sequence culminates in glaciodeltaic sands and gravels formed by underflows from the retreating glacier front.

By integrating the core data with seismic profiles, morphostratigraphy, and previously published drill records, the team organized the basin fill into three lithostratigraphic units corresponding to at least three distinct glacial advances. Unit A records the initial erosion of the basin during the Hosskirch glaciation, subglacial traction processes, ice-proximal lake sedimentation, and finally delta formation as the glacier retreated; it is assigned to the Dietmanns Formation. Unit B documents a later glacier re-advance, likely accompanied by shallow secondary erosion, followed once again by delta development, and corresponds to the Illmensee Formation and the Riss glaciation of Marine Isotope Stage 6. Unit C, at the top, represents a proglacial outwash plain of the Würm glaciation, the Last Glacial Maximum, whose terminal moraine lies only about 100 meters southwest of the drill site. Notably, the organic matter content of the fill never reaches interglacial levels, suggesting the basin filled under persistently cold, periglacial conditions rather than during warm periods.

Perhaps the most dramatic finding concerns deformation deep within the subsurface. The Würm ice advance, which pushed push-moraines across the Lake Constance region, left cuspate-lobate folds documented by 3D seismic imaging at 60 meters depth near the drill site. Seismic reflections appear displaced and inclined at even greater depths of around 100 meters, which the authors correlate with faulting, folding, and oversteepening of strata in the older deltaic units. In other words, the Last Glacial Maximum ice, advancing across outwash beyond its moraine, physically squeezed and deformed sediments buried tens of meters below the surface, an alternative explanation to syn-sedimentary slumping for structures previously attributed to gravity alone.

The Tannwald record thus captures a multi-phase, stacked basin infill shaped by at least three glacial invasions, yet important questions remain. Without independent age control, the researchers cannot yet determine whether the recorded advances represent fluctuations within single glaciations or separate glacial cycles. Ongoing work, including a high-resolution cross-hole seismic survey between three boreholes drilled in a triangle with 28-meter sides, luminescence dating, and cosmogenic nuclide dating, promises to pin down the timing of each advance. Together with computed tomography scanning to characterize ice-contact sediments systematically, these efforts will place the Tannwald Basin within the broader glacial evolution of the Lake Constance amphitheater, and help decipher how climate teleconnections synchronized ice age glaciations across the Northern Hemisphere.

Subject of Research: Glacial overdeepening and sedimentary basin infill of the Tannwald Basin by repeated Rhine Glacier advances

Article Title: Shaped and filled by the Rhine Glacier: the overdeepened Tannwald Basin in southwestern Germany

Article References: Schuster, B., Gegg, L., Schaller, S., Buechi, M. W., Tanner, D. C., Wielandt-Schuster, U., Anselmetti, F. S., & Preusser, F. (2024). Shaped and filled by the Rhine Glacier: the overdeepened Tannwald Basin in southwestern Germany. Scientific Drilling, 33(2), 191-206. https://doi.org/10.5194/sd-33-191-2024

Image Credits: AI Generated

DOI: 10.5194/sd-33-191-2024

Keywords: Tannwald Basin, Rhine Glacier, overdeepened valleys, ICDP DOVE, glacial stratigraphy, subglacial till, glaciotectonics, Lake Constance, Quaternary glaciations, Scientific Drilling, drill core, Holsteinian Interglacial

The drilling itself was an epic of endurance. Equipment was mobilized roughly 4,400 kilometers from Mariana in southeastern Brazil to the municipality of Rodrigues Alves, arriving in May 2023. Using a wireline diamond coring rig, the team began coring on 16 June 2023 with PQ tools producing cores 8.3 centimeters in diameter, progressively narrowing to HQ tools as depth increased. The original target was 2,000 meters, deep enough to reach the base of the Cenozoic fill, but the operation was halted at 923 meters after a cascade of setbacks. Alternating layers of unconsolidated sand and strongly cemented mudstone repeatedly defeated the engineers: casing had to be extended far deeper than planned at an added cost of about 150,000 US dollars, a casing string was dropped into the hole and had to be fished out, and at 750 meters the drill string became stuck by differential pressure and was ultimately cemented in place. When the borehole blocked at roughly 752 meters and two sidetrack attempts failed, the project’s executive committee chose to abandon the hole in December 2023 to preserve funds for a second site in the Marajó Basin. Even so, the team achieved 93 percent core recovery, an exceptional figure under such conditions.

What the core revealed surprised the sedimentologists. Apart from the uppermost 12 meters, the entire sequence belongs to a single lithostratigraphic unit, the Solimões Formation, dominated by feldspar-rich sands of Andean origin. The sediments record four facies associations: fining-upward channel and bar deposits up to 25 meters thick, cross-laminated fine sandstones, inclined heterolithic successions of sand and mud, and laminated claystones laid down on floodplains. Together they paint a picture of a vast, dynamic river system with migrating channels, exposed floodplains, and recurring soil formation, punctuated by paleosols bearing carbonate concretions and, in the deepest levels, gypsum crystals. Notably, the core contains no marine or brackish palynomorphs, weighing against long-standing hypotheses of marine transgressions or mega-lakes in the western Amazon during this interval and supporting instead a fluvial world akin to today’s great rivers and floodplain lakes.

The provenance story is unambiguous. Detrital zircon uranium-lead geochronology on sandstones, analyzed at rates of 300 grains per hour at the University of Arizona LaserChron Center, shows multi-modal age populations dominated by Permo-Triassic grains, with maximum depositional ages increasing steadily down-core: 5.8 plus or minus 0.21 million years at 178 meters, 7.8 at 300 meters, 8.6 at 497 meters, and 11.8 plus or minus 0.4 million years near the base at 863 meters. Heavy mineral assemblages rich in augite, hypersthene, and hornblende mirror those of the modern Amazon River channels, pointing to basic igneous rocks in the Andes as the dominant sediment source. Neodymium and strontium isotopes plot squarely within the field of eastern tropical Andean sediments, matching the suspended loads of the Solimões and Madeira rivers, the Amazon’s two great Andean tributaries. Feldspar-rich sands with low luminescence sensitivity quartz confirm immature, rapidly eroding Andean sources throughout most of the drilled section.

The pollen record may prove the core’s greatest gift to science. Of more than 1,800 palynomorph species identified so far, the assemblage differs markedly from Early to Middle Miocene floras recovered elsewhere in the western Amazon, such as at Contamana in Peru and in the Solimões Basin. Key biostratigraphic markers, including the first appearance datum of Cyatheacidites annulatus at 512 meters and the last appearance of Grimsdalea magnaclavata at 154 meters, indicate the upper 512 meters is no older than about 6 million years. Many of the dominant floral elements of the famous Pebas wetland system are rare or absent, suggesting the core samples a post-Pebas ecosystem that has rarely been studied because its sediments are seldom preserved. The flora is dominated by angiosperms and is highly diverse, with samples yielding up to 183 taxa in a single count of 745 grains, many represented by only one or two individuals, a pattern strikingly reminiscent of modern rainforest. Grasses, cycads, and conifers remain consistently low, offering no evidence of extensive savannas or climates significantly drier than today’s.

The uppermost 12 meters tell a different tale. Resting on a major erosional unconformity, these quartz-rich, mature sands contrast sharply with the feldspar-dominated sediments below. Luminescence dating, with quartz optically stimulated luminescence and feldspar post-infrared infrared stimulated luminescence protocols, indicates the sediments are older than roughly 200,000 years, likely older than 250,000 years, and the shift in composition suggests a fundamental reorganization of drainage: possibly the moment the Acre region was uplifted and disconnected from Andean sediment supply, giving rise to the modern watersheds of the Juruá and Purus rivers with headwaters in the elevated Fitzcarrald Arch.

The project also probed the deep biosphere and the carbon cycle. Real-time monitoring of drilling mud gas using the ICDP Online Gas Analysis system detected methane-rich zones at 250 to 380 and 420 to 588 meters, with stable carbon isotope values of methane ranging from minus 35 to minus 25 per mil, hinting at a mix of microbial and thermogenic sources. Pore-water isotopes, extruded from whole-round core samples with a hydraulic press, show a lower deuterium excess than local rainwater, consistent with water-rock interaction over long residence times. Laboratory incubations of sediment from 140 and 502 meters produced carbon dioxide but no methane after 22 days, leaving open whether the methane measured during drilling derives from an active microbial system, a dormant ancient one, or gases migrating from depth. DNA extractions from 66 depths, despite nine different protocols, consistently fell below detection limits, underscoring the extremely low biomass of this deep subsurface habitat.

Chemical weathering proxies add another layer. X-ray fluorescence of 213 samples reveals that the iron-to-potassium ratio, a tracer of Andean versus lowland sources and of weathering intensity, averages 0.56, close to modern suspended sediments of the Solimões River. X-ray diffraction shows sandstones averaging 55 percent quartz and 22 percent feldspar, confirming their arkosic, mineralogically immature character. Natural gamma spikes at seven depths, most dramatically a sharp peak at 346 meters coinciding with a magnetic susceptibility maximum, are suspected to be altered volcanic ashes, which could eventually provide precise tephrochronological anchors for the sequence.

Why does this matter beyond Amazonia? The Late Miocene and Pliocene were pivotal on multiple fronts: most living Amazonian species emerged then, Andean uplift accelerated and reorganized drainages, and atmospheric carbon dioxide levels were similar to those of 2025 and the coming decades. A high-resolution record from this interval, in the heart of the world’s largest rainforest, offers a natural experiment in how tropical biodiversity responds to tectonic landscape change and greenhouse climates. The companion core from the Marajó Basin in eastern Amazonia, drilled in 2024, likely covers the same interval, promising a basin-wide comparison from the Andean foreland to the Atlantic margin.

The team also invested in the region’s people. Twelve educational activities, from workshops to hands-on sessions with cores, rocks, and river sands, reached 240 residents of Rodrigues Alves and surrounding communities, while an inventory of 11 geosites along the Juruá River, including the thrust-faulted landscapes of Serra do Divisor National Park, laid groundwork for geoconservation. A virtual tour built from 21 panoramic 360-degree images now traces a geocultural itinerary from the Divisor Range to the Croa River. For the scientists, however, the real treasure remains locked in the 860 meters of core now archived at the University of Minnesota, where 34 researchers from seven countries logged and sampled roughly 5,000 specimens for more than 30 types of analyses. As those analyses mature, the Acre core is poised to fill the single most conspicuous gap in the fossil record of the Amazon’s making.

Subject of Research: Sedimentology, geochronology, and paleoecology of a Trans-Amazon Drilling Project core from the Acre Basin, western Brazil

Article Title: Initial results from a Trans-Amazon Drilling Project core from the Acre Basin of Brazil

Article References: Fritz, S. C., Sawakuchi, A. O., Noren, A., Baker, P. A., Silva, C., Jaramillo, C., de Almeida, R. P., Janikian, L., Bezerra, I. S., Barbosa, M., Bertassoli, D., Blankenship, R., Chiessi, C. M., Feakins, S. J., Garcia, M. D. G., Gautheron, C., Grivna, B., Hartmann, G., Kunkel, C., … Zamudio, B. (2026). Initial results from a Trans-Amazon Drilling Project core from the Acre Basin of Brazil. Scientific Drilling, 35(1), 127-157. https://doi.org/10.5194/sd-35-127-2026

Image Credits: AI Generated

DOI: 10.5194/sd-35-127-2026

Keywords: Amazon rainforest, Trans-Amazon Drilling Project, Acre Basin, Solimões Formation, drill core, Miocene, Pliocene, paleoclimate, biodiversity, detrital zircon, palynology, Andean uplift

The setting alone makes the work compelling. Bangladesh sits atop the Bengal Basin, one of the world’s great sedimentary repositories, where strata eroded from the rising Himalaya and carried by the Ganges, Brahmaputra and Meghna rivers have buried the ancient crust under hundreds to thousands of metres of Late Permian to Quaternary sediment. In the northwest, however, the Precambrian basement rises unusually close to the surface. The shallowest known occurrence lies at just 128 metres depth at Maddhapara, where diorite-tonalite-granodiorite rocks have been dated at 1.64 to 1.73 billion years using U-Pb SHRIMP analysis of zircon. That age places the Dinajpur Block firmly within the Paleoproterozoic, an era when the planet’s landmasses were assembling into the supercontinent Columbia, also known as Nuna.

Until now, drill hole GDH-62 had never been examined petrographically. The team collected nine core samples from the Geological Survey of Bangladesh repository and prepared twenty-seven thin sections, two or three per sample to capture mineralogical variation along and across the core axis. Under a standard polarizing microscope, they identified minerals and counted them along twenty-five to thirty-five grid-based traverses per section, estimating modal compositions visually with an uncertainty of about plus or minus five percent for major minerals. The approach is deliberately classical, but it is exactly the kind of careful, quantitative petrography that turns a stack of grey core boxes into a readable geological history.

The results split cleanly into two rock families. The felsic rocks, which occupy about 20.2 metres of the core or roughly 73 percent of the studied interval, are granodiorite: massive, grey to greenish grey, leucocratic to mesocratic, and fine to medium grained. Their modal composition ranges from 33.4 to 49.2 percent plagioclase, 11.8 to 27.0 percent quartz, 6.6 to 27.2 percent biotite and 6.7 to 11.7 percent K-feldspar, with secondary chlorite and epidote and accessory muscovite, zircon, hematite and opaque minerals. Plotted on the standard Quartz-Alkali Feldspar-Plagioclase discrimination diagram, every felsic sample falls within or adjacent to the granodiorite field, some near the granodiorite-tonalite boundary. The textures are hypidiomorphic and inequigranular, with fine- to medium-grained interlocking fabrics that speak of slow crystallization in a deep plutonic environment.

The microscopic detail is where the rocks begin to tell their deformation story. Quartz grains show wavy, undulose extinction, a classic signature of strain accumulated in the crystal lattice, and locally the quartz has recrystallized into finer equant aggregates. Biotite occurs as blade-like, euhedral to subhedral crystals with light brown to yellowish brown pleochroism, and many grains are twisted, bent and partially foliated. Aligned biotite and chlorite along recrystallized quartz grain boundaries define a weak foliation whose intensity, notably, decreases with depth in the section, hinting at a gradient in deformation intensity that the authors caution cannot be quantified from petrology alone. K-feldspar displays textbook Carlsbad twinning and hosts inclusions of acicular muscovite, biotite and rare epidote, the latter possibly pointing to hydrothermal alteration. Plagioclase shows weak to moderate sericitization, and much of the biotite is altered to chlorite, evidence that fluids, possibly of hydrothermal origin, percolated through these rocks long after they crystallized.

Cutting through the granodiorite near the base of the hole is the study’s most eye-catching discovery: an appinite dyke, massive, greenish grey to very dark green, and dominated by hornblende at 67 to 73 percent of its mode, accompanied by biotite, plagioclase, chlorite, hematite and accessory opaque minerals, with calcite forming micro-veins and veinlets. Appinites are an unusual and petrogenetically significant class of rock, typically hornblende-rich mafic intrusions associated with hydrous, mantle-derived magmas in convergent settings. In thin section the appinite shows hypidiomorphic, inequigranular and locally poikilitic textures, with hornblende enclosing plagioclase crystals. Hornblende exhibits the characteristic amphibole cleavage angles of 56 and 124 degrees with extinction angles between 12 and 34 degrees, while reddish brown biotite suggests a titanium-rich composition that the authors flag as potentially significant for petrogenesis. A tentative identification of orthopyroxene, based on pleochroism and parallel extinction, remains unconfirmed without optical or chemical constraint on its 2V angle.

The contact between the dyke and its host is sharp and non-gradational, with no chilled margin, and the authors interpret it as evidence that the appinite was emplaced after the felsic host had consolidated, most likely as a hypabyssal dyke within the main acid to intermediate association. From granodiorite to appinite, quartz and plagioclase decrease while biotite increases, tracking the shift from felsic to mafic chemistry. What petrography cannot constrain, the researchers emphasize, is the emplacement depth, a limitation they state plainly rather than paper over. That honesty matters in a field where over-interpretation of thin-section evidence is a chronic hazard.

Perhaps the most technically rich part of the study is its analysis of vein microstructures, which function as a kind of strain gauge for events long after the magmas cooled. In the granodiorite, lensoidal quartz veins with smooth vein-wall interfaces and lensoid wall-rock fragments point to a fracture-controlled vein system formed by syntaxial growth, in which crystals grow from both walls toward the vein centre. The small angle between the growth phase and the crack wall suggests a low growth rate, and the absence of fluid inclusions indicates that the vein crystals were deformed under non-hydrostatic stress during or after formation. In the appinite, by contrast, monomineralic calcite veins show slightly curved growth and selvages at their margins, hallmarks of antitaxial growth in which crystals expand outward from the vein centre, recording progressive, stress-related deformation over time. Minor calcite veins oriented obliquely and sub-parallel to the major vein hint at changing stress directions and possible fracture reactivation across the life of the intrusive complex.

The comparison the authors draw with the West African Craton gives the work its global resonance. Paleoproterozoic granitoids of the Bas Draa inlier in Morocco’s Anti-Atlas, dated around 2.1 billion years, share calc-alkaline, subduction-related characteristics with the Dinajpur Block, and both regions feature sodic TTG-style suites derived from partial melting of hydrated mafic sources. Mafic and appinitic dykes in GDH-62 resemble the lamprophyric dyke swarms of the West African Craton, though the Bangladeshi examples point more strongly to metasomatic and mantle-wedge influences than to plume-related processes. The similarities support the interpretation of GDH-62 as a fragment of Paleoproterozoic arc crust, a piece of a convergent-to-collisional margin that helped stitch together Columbia. For a country whose geology is usually described in terms of deltas, floods and gas fields, the message is striking: beneath the youngest sediments lies a two-billion-year-old arc, complete with dykes, veins and deformation histories, waiting in core boxes for anyone patient enough to grind the thin sections.

Subject of Research: Petrographic characterization of Precambrian crystalline basement rocks in drill hole GDH-62, Dighipara, northwest Bangladesh

Article Title: Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh

Article References: Zahan, H., Ameen, S. M. M., Hossain, M. S., & Hossain, M. S. (2026). Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh. Discover Geoscience, 4(1), Article 332. https://doi.org/10.1007/s44288-026-00703-2

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00703-2

Keywords: petrography, crystalline basement, granodiorite, appinite, Bangladesh, Dinajpur Block, Bengal Basin, Precambrian, vein microstructures, Columbia supercontinent, drill core, hornblende