ABSTRACT The mineral and chemical composition of clastic deposits is commonly used as a proxy for reconstructing the climatic history of the Earth. A mineralogical and chemical study of clastic deposits from the Jurassic Tlaxiaco basin of southern Mexico illustrates that the entire sedimentary system in which detritus is generated, subaerially transported, and deposited exerts a major control on the composition of sedimentary rocks, placing some constraints on the use of mineral and chemical indices as paleoclimatic proxies. Since clastic deposits of the Tlaxiaco basin were formed under similar humid climatic conditions, but in sedimentary systems with different topography and transport histories, they represent a natural laboratory for testing the control that these parameters exert on the composition of clastic rocks. Our petrographic and chemical results show that, in sedimentary systems associated with low relief and in which detritus had the possibility of being temporarily stored in floodplains and meandering bars, parent rocks and the derivative detritus interacted with weathering fluids over a long time, producing clastic deposits with composition that is representative of the weathering conditions and climate of the region. On the other hand, in environments associated with steep and tectonically active relief, the rates of sediment transport and burial exceed the rate at which weathering can generate detritus, producing sedimentary deposits with composition that largely underestimates potential weathering and provide unreliable information on climate. Mineral and chemical indices should be considered as reliable paleoclimate proxies only when sedimentological data indicate that the sedimentary system allowed sediment interaction with weathering fluids over a long time.

ABSTRACT The deposits of flood- and earthquake-derived subaqueous sediment gravity flows represent a significant fraction of lacustrine and deep-sea sedimentary successions, thus providing a valuable record of such natural disasters. The magnitude of these events and the thickness of the associated deposits are considered to follow a lognormal or power-law frequency distribution, whilst that of time intervals between subsequent events appear to be best approximated by a Poisson model, indicative of a random, time-independent phenomenon. However, the debate on whether the sedimentary record of these natural disasters is governed by randomness alone or whether there is some underlying stratigraphic ordering is still unsettled and requires detailed time-series analysis. This study consists of a time-series analysis of mudstone- and sandstone-dominated turbidite successions offshore a fan-delta system in the Neogene Aoshima Formation that belongs to the sedimentary fill of the forearc basin of southwest Japan. The formation consists of a monotonous alternation of very fine- to medium-grained sandstones capped by hemipelagic mudstones and, more rarely, by turbidite mudstones. The results show that the autocorrelation function of the time series suggests quasi-periodic variability in the upper sandstone-dominated part, whereas the lower mudstone-dominated part shows a white-noise-like pattern. Rescaled range analysis shows that the number of events per unit time in the lower part is characterized by a random time series, such as Brownian noise with a Hurst exponent of 0.5. In contrast, the thickness of event beds of the lower part and the thickness and the number of events of the upper part are persistent time series with a Hurst exponent > 0.5. These results suggest that the number of turbidite depositional events in the mudstone-dominated part indicates random timing, whereas its thickness time series and the sandstone-dominant part are not governed by simple stochastic processes but are affected by sea-level changes, sediment transport dynamics, and other factors such as, for example, seafloor topography.

Braided rivers distribute sediment across landscapes, often forming wide channel belts that are preserved in stratigraphy as coarse-grained deposits. Theoretical work has established quantitative links between the depth distribution of formative channels in a braided river and the geometry of their preserved strata. However, testing these predictive relationships between geomorphic process and stratigraphic product requires examining how braided rivers and their deposits coevolve, with high resolution in both space and time. Here, using a series of four runs of a physical experiment, we examine the controls of water discharge and slope on the resulting geometry of preserved deposits. Specifically, we focus on how a twofold variation in water discharge and initial riverbed slope affects the spatiotemporal distribution of channel depths and the geometry of preserved deposits of a braided river. We find that the channel depths in the laboratory experiment are described by a two-parameter gamma distributio n and the deepest scours correspond to zones of erosion at channel-belt margins and channel-thread confluences within the channel belt. We use a reduced complexity flow model to reconstruct flow depths, which were shallower compared to channel thalweg depths. Synthetic stratigraphy built from timeseries of topographic surfaces shows that the distribution of cut-and-fill unit thickness is invariant across the experiments and is determined by the variability in scour depths. We show that the distribution of cut-and-fill unit thickness can be used to reconstruct formative channel-depth distributions and that the mean thickness of these units is 0.31 to 0.62 times the mean formative flow depth across all experiments. Our results suggest that variations in discharge and slope do not translate to measurable differences in preserved cut-and-fill unit thickness, suggesting that changes in external forcings are only likely to be preserved in braided river deposits when they exceed a certain threshold of change.

Carbonate rocks retain a well preserved record of biologically associated structures at the outcrop to millimeter scale, however microscale features such as cellular fossils are rarely represented. The lack of microscale textural information in ancient carbonates is commonly attributed to processes relating to carbonate diagenesis. However, there are relatively few examples of precisely how and when these destructive processes occur, particularly in active precipitating systems. To better understand the taphonomy of carbonate precipitating environments through early diagenesis, we investigated Crystal Geyser, an active cold water carbonate spring ( about 18 deg C) located in Grand County, Utah. Here we show that rapid precipitation is effective at initially capturing cell-like structures and forming associated microscale laminated stromatolites; however, these morphologies degrade immediately after their formation. We attribute destructive diagenetic effects to the recrystallization of metastable aragonite into the more stable polymorph calcite (i.e., inversion) and the associated textural coarsening that homogenizes and erases the original fabric (i.e., aggrading neomorphism). Despite the loss of microscale morphological information, chemical biosignatures in the form of macromolecular organics remain dispersed throughout the disrupted carbonate textures. These observations provide an example of penecontemporaneous diagenesis that obliterates primary microscale textures in carbonate rocks. Similar mechanisms and their rapid timing, as shown here, likely contributes to the observed lack of microscale morphological biosignatures in many ancient carbonates. This work further highlights that within such systems, permineralization by a more stable crystalline phase, such as chert, must occur rapidly after deposition to effectively retain these signatures over geological timescales.

The Eocene to Miocene clastic wedge of the south Pyrenean basin constitutes a reference model to understand the progressive evolution of sediment provenance and source-to-sink dynamics in a foreland basin. We present new detrital zircon (DZ) U-Pb and U-Pb-He (ZHe) double dating data from the Jaca basin and the Ebro basin, providing insights into the evolution of the sedimentary systems that record a major tectonic and drainage reorganization from the late Eocene to Miocene. Three distinct DZ U-Pb signatures have been identified: (i) Variscan dominated; (ii) mixed Cadomian-Variscan; (iii) Cadomian dominated; and two DZ ZHe signatures (i) Pyrenean dominated; (ii) pre-Pyrenean dominated. Coupling DZ U-Pb, ZHe, and petrographic data allows us to discriminate among distinct Pyrenean sources as well as to understand how DZ signatures are propagated in a source-to-sink system. Our results indicate that while the eastern Jaca basin was fed from eastern source areas located in the central and eastern Pyrenees, the western Jaca basin was fed from the Basque massifs and the Urbasa-Andía Sierra (Basque-Cantabrian Pyrenees).

Differentially dolomitized carbonate strata in the Western Canada Sedimentary Basin (WCSB) are increasingly targeted for carbon capture, utilization and storage (CCUS), yet few studies have evaluated the petrophysical characteristics of these conventional hydrocarbon reservoirs for this purpose. To address this, this study uses drill core analysis (sedimentology, diagenesis, pore morphology and distribution), together with core plug and production data, to evaluate the properties of five depleted oil and gas fields in the Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group. The Swan Hills and Leduc Formations are comprised of reef, shoal and lagoon deposits that are predominantly fossil-rich (e.g. stromatoporoid-dominated rudstones and boundstones). In contrast, the carbonate ramp deposits of the Wabamun Group are fossil-poor, consisting instead of variably bioturbated carbonate mudstones, wackestones and packstones. Replacement dolomitization is variable throughout each stratigraphic unit, but generally occurs within fossil-rich and/or heavily bioturbated intervals. Fracture densities are broadly comparable in limestone and dolostone. Porosity in the Swan Hills and Leduc Formations is predominantly moldic and vuggy, occurring where fossils (e.g. stromatoporoids) are partially or fully dissolved. Pore space in the Wabamun Group is mostly restricted to intercrystalline porosity within burrows. In general, burial cements (e.g. calcite and dolomite) are volumetrically insignificant and only partially fill pores. Exceptions to this include porosity-occluding cements associated w ith fractures and breccias in the vicinity of faults. Dolomitization and depositional facies are found to exert a strong control on pore morphology, distribution and interconnectivity. Porosity is primarily controlled by the relative abundance of skeletal grains and by the presence of burrows. These highly porous facies acted as fluid pathways during burial diagenesis, resulting in their preferential dolomitization, solution enhancement of pre-existing pores, and creation of volume reduction-related porosity. The high CO2 storage capacity and low unplanned plume migration risk (due to depositional/diagenetic baffles) of dolomitized reefal reservoirs (e.g. Swan Hills and Leduc Formations) make them more attractive targets for CCUS than those with limited capacity and/or potential migration pathways (e.g. fault-related fractures and breccias in the Wabamun Group). These results demonstrate that drill core analysis, in combination with legacy data, can provide valuable insights into the factors that control reservoir CO2 injectivity, plume migration and storage capacity.

An anhydrite caprock overlies 65% of the 329 onshore salt stocks of the U.S. Gulf Coastal Region, South-Central U.S. The caprock consists of a succession of katatectic (downward-building) layers of anhydrite (each typically 1 to 4 cm thick) and an upper, black pyrite lamina (approx. 0.1 mm thick). The older (upper) katatectic layers are usually highly deformed. During four stages, these fundamental layers formed at the crest of shallow (less than 1,200 m) salt stocks. Repetition of sets of the stages resulted in the cyclicity. The closed stage: A flush, nearly planar contact separated mature anhydrite caprock from an underlying salt-stock crest. As the diapir elevated by a few mm/yr, it arched the caprock; it also opened conduits at the stock’s uppermost anhydrite sheath, allowing NaCl-undersaturated water from outside the diapir to contact the salt stock. The open-dynamic stage: Along the perimeter of the uppermost salt stock, a narrow, annular halite cave advanced inward and upward directly below the slightly convex base of the anhydrite caprock. The most NaCl-aggressive water rose by free convection to the highest elevations and dissolved halite. The dense, solute-enhanced water then reversed direction and, again, by free convection, flowed past the stock’s margin into country rock; NaCl-undersaturated water replaced the departing brine. A water-filled, commonly ring-shaped, planar cave up to several meters high expanded over the stock’s crest. Sparse physical supports, buoyancy, arched caprock, and high artesian water pressure prevented the cave’s collapse. The cave’s halite floor dissolved vertically downward faster than it moved diapirically upward. As the crestal halite dissolved, a small amount of residual pyrite and, typically, a layer up to approx. 10 cm thick of residual anhydrite (generally 3-8 wt % of the stocks) accumulated on the cave floor. The open-static stage: The persistent “attack” of halite highs by the NaCl-undersaturated water eventually caused the slope of the cave’s floor to approach horizontality. The density-driven flow slowed markedly, and a static cover of NaCl-saturated brine thwarted the dissolution of the halite floor. The downward movement of the cave’s floor reversed, and it moved slowly upward via diapirism. Concurrently, an anhydrite sheath evolved around the uppermost stock. The accretionary stage: The cave closed within a few thousand years, and active diapirism underplated the admixed residual minerals onto the base of the caprock. The uppermost, now compact, residual anhydrite dissolved within the high-pressure environment, leaving the discrete, topmost, thin, black pyrite lamina. With the accretion of the residual minerals, a katatectic layer formed at the base of a succession of such layers. The closed stage recurred, and a new katatectic cycle began forming. Variations of the four genetic stages probably occurred during the formation of anhydrite caprocks in other worldwide salt dome provinces, but because of insufficient data, they are usually unrecognizable.

Abstract Serpentine-bearing sediment, a rare sediment type that is formed and deposited in divergent, convergent, transform, and collisional plate-tectonic settings, carries important evidence of sediment provenance. Specific sources of serpentine-rich sediment display grain assemblages of distinct character that can be used to infer the serpentinization condition and sediment formation. This study reports quantitative and qualitative results on serpentine components in sandstones from Ocean Drilling Program Legs 149 (Iberia), 210 (Newfoundland), and 125 (Mariana and Izu–Bonin regions), and from serpentine-rich debris flows and arenitic breccias in deep-water successions in the Northern Apennine fold–thrust belt. We propose a textural scheme that offers a visual guidance for evaluating serpentinite grains that can be broadly adopted, is easily reproducible, and reduces user bias in determining compositional modes that allow comparison of serpentinite grain populations in arenites from different depositional environments, provenance, and associated tectonic settings. These data allow us to define a scheme for serpentine-dominated deposits that demonstrates the presence of two main groups of grain textures (pseudomorphic and non-pesudomorphic) with specific mineralogy and crystal shape as a function of temperature and pressure in the source rocks. The quantitative analysis of the serpentine-rich arenites and fine-grained sediments derived from forearc and rifted continental-margin settings shows that the studied samples are characterized by high percentages (c. ≥ 80%) of serpentine detritus and subordinate dense minerals and other lithic fragments, including basalt. In rifted continental-margin settings, the prevalent textures in serpentinite sandstones consist of polygonal mesh, mesh-core, and hourglass that all belong to the pseudomorphic category, which preserves the pre-serpentine features and mineralogy. These textures are typically formed in low-temperature conditions (< 390°C); lizardite is the most common mineral, along with minor chrysotile and, in rare cases, antigorite. In contrast, in forearc settings, serpentine-rich grain assemblages exhibit dominantly non-pseudomorphic, interlocking, and interpenetrating textures, dominantly composed of lizardite and recrystallization of lizardite by antigorite. Minor preserved ultramafic minerals related to dynamic recrystallization might be associated with the diapiric rise and protrusion of serpentine bodies. The Northern Apennines case study adopted to test this model indicates that the relationship of detrital serpentine texture to setting can be employed in provenance studies. Firstly, serpentine-bearing sediments derived from ophiolites deformed in fold–thrust belts have more variable serpentinite content, ranging from a few percent to < 10% for samples from deep marine environments, to typically c. 20 to ≤ 50% for stream and beach samples. This compositional variation arises from mixing of sediments derived from deeper to shallower oceanic lithosphere (peridotites and serpentinites) with material from overlying volcanic rocks and sedimentary cover. The deep-water serpentine-rich sands of the Northern Apennines display variable compositions with intermediate characteristics. The source of the serpentine-bearing deposits is interpreted to be a residual oceanic lithosphere characterized by subcontinental mantle-lherzolite originated in the Middle–Late Jurassic by mantle delamination. The serpentinite-dominated debris flows and sand beds contain serpentine grains that exhibit compositional and textural transitions from pseudomorphic to non-pseudomorphic categories, along with changes in mineralogy from lizardite to antigorite. Serpentinite with pseudomorphic texture is observed in the mantle section away from the deformed area. On the contrary, the presence of serpentine-rich arenites with dominant non-pseudomorphic textures suggests derivation from tectonized serpentine along fault scarps and or as products of serpentine diapirism. The detailed serpentinite texture scheme used to classify sand grains in this study includes pseudomorphic (often lizardite, minor crysotile) and non-pseudomorphic textures, with the latter attributed to temperature- and pressure-controlled recrystallization (often to antigorite) or shearing during or after serpentinization. For comparison of different detrital-serpentinite populations, a new ternary plot is proposed where counted parameters are grouped into three end members: undeformed, deformed, and recrystallized. This plot appears to discriminate different sources of detrital serpentine by tectonic setting (e.g., Iberia and Newfoundland margins vs. Mariana forearc) and shows the potential complexity of serpentinite sources in the Apennine basin example. Additional texturally based petrographic data sets are needed to determine the usefulness of this plot in provenance studies.

ABSTRACT The present study provides insights into the origin of siderite cementation in closely interbedded bipartite mudstone to sandstone Pennsylvanian strata from the Anadarko Basin. Mineralogical, geochemical, and stable-isotope data were collected from 80 siderite samples and their immediate non-siderite-bearing regions. Geometrically, siderite mineralization occurs in the form of concretions or bands, with the latter being the most common textural type and occurring solely in mudstone, whereas the former is found in both sandstone and mudstone. This microtextural and geochemical investigation posits siderite as a derivate of biological processes at the sediment–water interface. Bacteria cell walls denoted by an omnipresent nanoglobule structure dominate the areas of mineralization. Mineral quantifications indicate higher phyllosilicate content within the mineralization compared to the non-mineralized sediment reflecting the role the clay minerals provide as a source of bio-essential cations, labile FeOx, and organic matter needed for microbial colonies to flourish. Following the formation of biological siderite, the energetically favorable mineralization surfaces served as nuclei for further precipitation of mesogenetic inorganic siderite enriched in 16O. The second mesogenetic cementation features rhombohedral siderite overgrowths with increasing Mg-concentration on the outer rims of nanoglobules. The identified bands and concretions were formed during periods of relative sea-level highs, whereas the siderite-cemented intraclasts were eroded and deposited downstream during times of relative sea-level lows. This is corroborated by relatively low (Ca-Mg)/Fe substitution in eogenetic siderite, typical of mineralization in meteoric-water-dominated realms. Finally, based on enrichment in 12C and textural observations, which suggest suboxic geochemical conditions, we conclude that the ability of siderite to form early on allowed it to maintain net rock porosity by encasing quartz and inhibiting its overgrowth process.

Abstract The type locality for the upper Oligocene Nuwok Member of the Sagavanirktok Formation (Carter Creek, North Slope, Alaska, USA) contains an abundant occurrence of glendonite, a pseudomorph after the calcium-carbonate mineral ikaite, which typically forms in the shallow subsurface of cold marine sediments. The region during the time of Nuwok Member deposition was located at a high latitude, similar to today, and the study site is characterized by sands and silty muds interpreted here to have been deposited in coastal and shelfal marine environments. Isotopic (Sr) and biostratigraphic (foraminifera) evidence presented here refine the depositional age of the outcrop to approximately 24 Ma. Glendonites occur in two basic forms: radial clusters, commonly centered around a single larger primary crystal (∼ 10 cm, Type A) and larger single blades generally without accessory crystals (∼ 15–25 cm, Type B). Microscopic examination reveals a sequence of multiple types of replacive calcite that formed as a direct result of ikaite transformation: Type 1 rhombohedral crystals characterized by microporous and inclusion-rich cores and concentric zones, Type 2A, composed of clear calcite that overgrew and augmented Type 1 crystals, and inclusion-rich, microcrystalline Type 2B, which formed a matrix surrounding the rhombs and commonly dominates the outer rims of glendonite specimens. Type 3 calcite precipitated as fibrous, botryoidal epitaxial cement atop previous phases and is not ikaite-derived. These phases are distributed in similar ways in all examined specimens and are consistent with several previously described glendonite occurrences around the world, despite differing diagenetic and geologic histories. Stable-isotope evidence (δ13C and δ18O) suggests sourcing of glendonite carbon from both organic and methanogenic sources. Glendonites of the Nuwok Member can therefore assist in the determination of a more comprehensive ikaite transformation model, improving our understanding of glendonite formation and the sedimentological and environmental context of their occurrence. Oligocene glendonites are uncommon globally; the well-preserved occurrence described here can allow future studies to better reconstruct Arctic environmental conditions and paleoclimates during this time.