The Navarro Formation produces oil from thin sandstones encased in thick marine shales. Nearby outcrop faunal analysis places these mudstones as being deposited in a middle-shelf depositional environment at 100 km or greater from time-equivalent shorelines based on regional correlation. This study integrates core, drill cuttings, and well logs to identify ten lithofacies grouped into four facies associations (FA-1 to FA-4), representing a dynamic mud-dominated depocenter, with characteristics of both subaqueous deltas and shallow-water contourites. High fluvial inputs from the northwest fed an energetic shelf with storm waves and strong geostrophic currents, transporting fine sediment far across the shelf before deposition as elongate, muddy clinoforms that prograded perpendicular to the currents. Additional accommodation along the Mexia-Talco fault promoted more aggradational parasequence stacking and thicker clinoforms locally. Toesets comprise muddy laminated deposits (FA-1) that grade upward across topsets into bioturbated, silty contourite facies (FA-2, FA-3), forming coarsening-upward parasequences. The clinoform rollover lay near storm-wave base, where storm-ebb flows transitioned into along-shelf geostrophic currents, depositing sandy tempestites (FA-4) thick enough to outpace bioturbation and preserve primary structures, yielding sandstone fairways aligned with rollover zones. This study presents a predictive model for isolated sandstone bodies on mud-dominated highstand shelves, informing unconventional reservoir characterization and hydrocarbon migration pathways.
The upper Miocene succession of the Romagna Apennines includes a variety of carbonate deposits that have been included in the "Calcare di Base" lithostratigraphic unit and usually interpreted as primary evaporites recording the onset of the Messinian salinity crisis (MSC). We carried out an, integrated multidisciplinary (stratigraphic, sedimentological, and geochemical) study on a Messinian carbonate, deposit cropping out in the Romagna Apennines, whose origin and stratigraphic position are still not well defined and for this reason here labeled as incertae sedis carbonates (IS-C). In this study we compare the IS-C with other well-known carbonate deposits of the Northern Apennines that differ in stratigraphic position, age, and origin: i) the PRE-C, interbedded within the euxinic shale unit underlying the primary evaporites (pre-MSC); ii) the PLG-C, hosted in the Primary Lower Gypsum (PLG) deposits of the Vena del Gesso Basin (MSC stage 1); and iii) the RLG-C, associated with the Resedimented Lower Gypsum (RLG) deposits of the Gabicca Riviera in the outer Adriatic foredeep (MSC stage 2). The IS-C carbonates, which are stratigraphically sandwiched between upper Tortonian-lower Messinian deepwater deposits below and upperpost Messinian Lago-mare deposits above, are qharacterized by a brecciated texture, and by the presence of both rigid and deformed carbonate clasts, similar to those described for the Calcare di Base of Sicily (CdB type 3), pointing to a clastic origin and emplacement from subaqueous gravity-flow deposits. Petrographic analysis enabled the recognition of eight main lithofacies. No carbonate extraclasts and primary gypsum facies have been found, but locally some secondary nodular gypsum beds occur lateral to the carbonate deposits. The stable-isotope compositions are facies dependent, as reflected by the peloidal grainstone clasts showing more positive 80 values and more negative 8C values, suggesting deposition under strong to moderate evaporative conditions with minor diluted water inputs, similar to that found in the PLG deposits. Conversely, the matrix shows negative values for both isotopes that could probably be related to moderate diagenetic processes. With respect to PRE-C, PLG-C, and RLG-C types, the IS-C carbonates are characterized by a higher content of fecal pellets whose morphological apalysis suggests that they could have been produced by assemblages of opportunistic planktonic metazoans adapted to schizohaline conditions, likely including Artemia salina. No clear in situ peloidal carbonate deposits have been found, suggesting the complete penecontemporaneous dismantlement during MSC stage 2 of carbonate factories developed on the culmination of anticlines related toractive thrust faults.
Improved understanding of mixed-process deltaic systems and their stratigraphic architectures is important for predicting reservoir heterogeneity and coastal geomorphic changes. This study reveals that the lower part of the upper Eocene Maadi Formation in the north Eastern Desert (Egypt) includes stacked deposits of a mixed-energy, sand-dominated deltaic system. The succession consists of six main architectural elements: i) offshore muds, ii) prodelta, iii) delta front, iv) delta plain, v) tide-dominated estuary, and vi) transgressive storm beds. These elements accumulated in two high-frequency transgressive-regressive sequences; each consists of regressive prograding-delta-lobe deposits truncated by transgressive storm deposits. The prograding-delta lobes show mixed storm and tide influences. Storm action prevailed in the distal delta front and waned in the proximal delta front, in contrast to the tidal action that prevailed in the proximal-delta-front elements and the delta-plain elements. The tidal impact increased successively through the studied succession, probably due to tectonically derived tidal amplification. The drowning of the deltaic system with development of an estuary is attributed to fluvial avulsion or autogenic delta-lobe switching. The study introduces a model for variability of the interaction between river, tide, and storm processes and their products in a mixed-energy delta, and the evolution of the mixed storm- and tide-influenced deltaic system. It improves our understanding of the sedimentological parameters and stratigraphic architecture of mixed-energy coastal reservoirs and their heterogeneities.
Rivers that transport fine-grained sandy sediment, and their associated deposits, remain understudied compared to coarser-grained alluvial channels, particularly in the ancient sedimentary record. This study details the sedimentology of a large (mean and maximum bankfull depth of c. 10 and 20 m, respectively), low-slope, finegrained sandy river with low variance in grain size from the Carboniferous (Upper Mississippian) Cypress Formation in the Illinois Basin, USA, at the outcrop and regional scale. A combination of newly collected and existing subsurface (geophysical logs and core) and outcrop data was used to detail the subtle variability in the sedimentology of this fine-grained river, which is shown to be significant for interpreting the depositional setting, paleohydraulics, and reservoir heterogeneity of the Cypress Formation. Thick (up to similar to 50 m) sandstones of the Cypress Formation are very fineto fine-grained and dominated by six lithofacies associated with fining-upward fluvial channel fills deposited during a significant sea-level lowstand. Up to three stories of stacked channel-fill deposits are present, with the upper-most stories being most complete (similar to 14 to 20 m thick) and capped by a basin-wide paleosol. Small (< 0.27 m thick) and low-angle (< 15 degrees) cross-beds, along with planar bedding and attenuated longitudinal bars (similar to 4 m thick) with low-angle (< 10 degrees) master surfaces, dominate the channel fills and are interpreted to be characteristic of fluvial deposition under conditions of high suspended-load transport. As a result, the scaling of cross-set thicknesses to mean bankfull depths provides shallower depth estimates (similar to 4 m) compared with those derived from measured channel-story thicknesses (7-11 m). The erosional bases of channel stories, although subtle, correspond to abrupt grain-size coarsening and up to fourfold increases in permeability, revealing significant heterogeneity that is otherwise obscured and must be incorporated in reservoir and aquifer models to accurately represent flow behavior. Additionally, the well-sorted and texturally homogeneous sandstones comprising channel fills are interpreted to be the result of high rates of suspended-load transport and subsequent deposition, which can mask preserved stratification, and in some cases renders facies as completely massive. Therefore, seemingly massive, well-sorted, very fineto fine-grained sandy facies require additional scrutiny, because their stratified sediments can be easily mistaken as structureless.
Late Paleozoic strata are well preserved in numerous intracratonic basins on the Indian subcontinent, which was once part of East Gondwana. The sources and transport pathways of late Paleozoic sediments in these Gondwanan basins are poorly understood, owing primarily to the lack of geochronological data on detrital sediments. To reconstruct the regional detrital and tectonic history of East Gondwana, we have incorporated multiproxy provenance analyses that includes detrital 40Ar/39Ar geochronology, sandstone petrofacies, and heavy-mineral analyses of late Paleozoic strata of the Barapukuria, Dighipara, and Khalaspir basins of northwestern Bangladesh and the Jharia Basin of east-central India. We present the first detrital thermochronology data from the Jharia, Barapukuria, and Dighipara basins. Sandstone petrographic analysis indicates temporal and spatial variations in the compositions among adjacent Gondwanan basins. Heavy-mineral assemblages suggest amphibolite-and granulite-facies source terranes. Detrital 40Ar/39Ar data from the Barapukuria and Dighupara basins yields mainly Cambrian-Ordovician (524-474 Ma) ages. In contrast, the Barakar Formation from the Jharia Basin shows more scattered ages (1885-460 Ma) with multiple significant peaks in the Neoproterozoic and the dominant peak at Cambro-Ordovician, while the Talchir Formation has a contrasting Neoproterozoic age population of 895 Ma to 810 Ma, with the principal mode at 860 Ma indicating changes in the source terranes. Taken together, data suggest that the Eastern Ghats Mobile Belt of the Kuunga orogenic belt is the primary contributor to the younger early Permian strata, along with minor contributions from the Prydz Bay Belt and northern Prince Charles Mountains of the East Antarctic Craton. On the contrary, the Chotanagpur Granite Gneiss Complex of the Indian Craton and the Prydz Bay Belt on the East Antarctic Craton was the main source of the older Permo-Carboniferous Talchir Formation.
Multiple glaciations characterize the Neoproterozoic icehouse Earth period. The Ediacaran Gaskiers Glaciation corresponds to a localized glaciation limited to > 30 degrees paleo-latitude and relatively short duration (i.e., < 1 Myr) compared to the Cryogenian glaciations, but was significantly followed by rapid Ediacaran biota evolution. The Gaskiers Glaciation (ca. 580 Ma) is best characterized in sedimentary successions exposed on the Avalon Peninsula of southeast Newfoundland, where the Gaskiers Formation records deepwater glaciomarine sedimentation overlain and underlain by deep-marine turbidites. Here, we present a sedimentological and stratigraphic analysis of the correlative shallow glaciomarine Mercantile and Trinity diamictites in the Bonavista Peninsula and associated shallow-marine to fluviatile strata of the Rocky Harbour Group. The glacial Mercantile and Trinity diamictites consist of a clast-rich diamictite with abundant faceted, flat-iron, stoss-and-lee, or bullet-shaped clasts, and rare striated clasts (< 1-2%), overlain by a clast-poor diamictite with dropstones, dump, and grounding structures. The proximal glaciomarine Monk Bay Member underlies the Trinity Diamictite and consists of stacked 5-to 15-m-thick upward-coarsening, shallow-marine clastic parasequences with rare dropstones, each capped by a gravel layer up to 2 m thick. The overlying postglacial Kings Cove North Member comprises a distinctive pistachio silicified tuff, a mudstone succession > 50-m-thick, and an uppermost stratal unit of thick-bedded turbidites. Parasequences of the Monk Bay Member are interpreted as deposits of marine-terminating glacial advance and retreat cycles controlled by climate-driven forcing. The gravel-rich transgressive layer capping each parasequence records wave reworking of likely ice-rafted material. These cycles culminated in the deposition of the Trinity diamictite, marked initially by a continuous rainout, followed by finer-grained fallout of ice-rafted debris. Overlying this, the Kings Cove North Member records glacio-eustatic flooding of the shelf, driven by a glacial retreat that marks the end of the Gaskiers Glaciation on the Bonavista Peninsula and coincident glacioeustatic sea-level rise, which outpaced glacial-isostatic rebound.
Sand injectites form when poorly consolidated sand is fluidized and forcibly intruded into a host sediment. Macroscopic features of injectite complexes record their formative processes, but relatively little is known about the differences in microscopic characteristics of injected versus parent sandstones. Microfracturing of grains in sand injectites has been recognized, and is related to grain-to-grain collisions during transport in turbulent flow. This study documents four injectite complexes and uses optical microscopy and point-counting, together with back-scatter and scanning electron microscopy (SEM) to show that grain fracturing occurred in depositional, partially remobilized, and fully injected sands in the four studied injectite networks. Point-count results show an increase in the proportion of microfractured versus non-fractured quartz grains in injected sandstones (between 0 to 60% more) when compared with samples of depositional or partially remobilized sandstones. SEM data show that the microfractures are commonly oriented across multiple adjacent grains and radiating out from grain point contacts. This geometry and the location of microfractures result from postemplacement mechanisms of grain impingement, dissipation of pore-fluid pressure, and the effect of overburden pressure, rather than being products of flow processes during emplacement. We propose that the effects of overburden pressure are greater on grains in injected sandstones due to more rapid dissipation of pore fluid leading to more abrupt pressure changes than in parent sandstones. Furthermore, the dominance of grain-impingement fracturing in injectites demonstrates that grain-to-grain collision in a relatively lowconcentration turbulent flow during emplacement is unlikely. This supports core and outcrop observations, such as layering, preservation of delicate features on injectite margins, and clasts concentrated at the upper and lower margins of the injectites, suggesting that flow during injectite emplacement is dominantly high concentration and laminar.
Correlation of meandering fluvial strata is difficult because of the highly variable lateral extent and vertical thickness caused by the erosion of the surrounding floodplain from channel downcutting and meander migration. This variability often results in a lack of confidently traceable marker beds leading to errors with stratigraphic correlation, determination of sedimentation rates, and quantification of stratigraphic completeness in fluvial strata. Bentonites are ideal marker beds for resolving fluvial issues of stratigraphic correlation because they preserve either a single or multiple short volcanic events deposited over widespread areas and can be radiometrically dated. Additionally, they preserve other signals that can be used to construct geochemical fingerprints for individual beds, allowing correlation of fluvial strata despite fragmentary preservation. To test the utility of geochemically fingerprinted bentonites as fluvial marker beds, we examined the Campanian-aged meandering fluvial strata of the Dinosaur Park Formation (DPF), which crops out mainly as badlands in the UNESCO World Heritage Site Dinosaur Provincial Park (DPP), Alberta. The DPF lacks a well resolved stratigraphic framework; this is problematic because the DPF in DPP preserves some of the world's most abundant and diverse dinosaur fossil assemblages, which have been used as a basis for many hypotheses related to evolution of dinosaurs and other terrestrial vertebrates. A poorly resolved stratigraphic framework for the DPF in DPP leads to substantial uncertainty regarding stratigraphic placement of dinosaur fossil sites, and therefore potential errors with any evolutionary hypotheses supported by that biostratigraphic data. We sampled 14 bentonite deposits from DPP, five of which have been dated previously and nine of which were previously unsampled. Stratigraphic sections were measured at bentonite collection sites, and elevation of all bentonite beds was recorded to 60.1 m. Geochemical data for sampled bentonites was obtained using electron-probe micro-analysis on preserved biotite, alkali feldspar, and plagioclase phenocrysts. Our results show that bentonites in DPP have unique geochemical fingerprints and physical traits that can be used for their identification and correlation, without the need to be dated. We identify three previously unrecognized and currently undated bentonites in the DPF that will increase geochronological resolution in DPP. We demonstrate that the Plateau Tuff bentonite can be correlated over a 12 km distance across DPP, demonstrating the potential utility of bentonites for both local-and regional-scale fluvial stratigraphic correlation. These results offer a promising first step for resolving stratigraphic correlation issues for the DPF in DPP and provide a correlation framework model that can be applied to other fluvial stratigraphic systems that preserve bentonites.
Gypsum and its many forms are common in modern sediment and the ancient rock record. In particular, Permo-Triassic bedded gypsum and associated red beds from Pangea represent continental environments that persisted during a time of extraordinary climate change and mass extinction. Despite this significance, there is a paucity of work investigating textures of ancient gypsum, and in particular, diagenetic gypsum textures. Here, we describe gypsum textures from the Permian Cloud Chief Formation of the Palo Duro Basin in Texas, midcontinental United States. We interpret the Cloud Chief Formation gypsum to represent continental saline lakes and mudflats, based on their depositional textures and association with red-bed protosols. Additionally, we document, for the first time, the diagenetic features of the Cloud Chief Formation. Diagenesis is extensive, complex, and remarkably heterogeneous, with distinct textures of gypsum interlocking-crystal mosaics varying at a cm-scale and a mm-scale. The diagenetic history of the Cloud Chief Formation gypsum includes partial replacement by dolomite and subsequent gypsum replacement of the replacement dolomite rhombs, a process we name "gypsum dedolomitization." This work advances our understanding of the depositional and diagenetic histories of ancient gypsum and expands the known geographic extent of Permo-Triassic continental saline-lake systems.
Chert-bearing phosphate successions are globally widespread and provide a robust framework for investigating the interplay between silica, redox conditions, and phosphogenesis. The Upper Cretaceous-Paleogene BouCraa phosphate deposit in southern Morocco exemplifies this framework, featuring two distinct silica-rich complexes in a carbonate-poor stratigraphic succession. Siliceous lithofacies from three representative sections were analyzed using optical petrography, X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS). The results reveal two types of siliceous facies, detrital and nondetrital, characterized by variable P2O5 contents. XRD patterns are dominated by chalcedony, quartz and opal-CT, and carbonate fluorapatite (CFA). Silica is mainly of biogenic origin, sourced from the early dissolution of diatoms in a restricted marine environment. The geochemical signature of the silicification and phosphogenesis environments appear to be the same, marked by suboxic to ferruginous conditions, as evidenced by the general enrichment of redox-sensitive trace elements (e.g., Mo, V, U). Two groups of REE signatures were distinguished: the first group, associated with clay-rich and low-phosphate samples, shows moderate Ce anomalies and 8Ce values indicative of suboxic conditions, probably reflecting limited circulation and water-mass turnover. In contrast, the dominant, more phosphate-rich group shows pronounced negative Ce and 8Ce anomalies and HREE-enriched profiles, similar to those of seawater, indicating oxic depositional conditions. These oxygenation episodes were associated with higher bottom-water energy and episodic reworking. Such conditions exerted a fundamental control on silicification, which proceeded in two stages: an early phase of matrix replacement under reducing interstitial conditions, locally intensified by evaporative concentration, followed by a late-stage cementation associated with circulation of phreatic fluid. This study introduces a multiphase silicification model linked to phosphogenesis in a redox-stratified epicontinental setting. The proposed framework is applicable to other siliceous phosphate systems and offers a valuable tool for guiding the exploration of economically significant phosphorite deposits.
The Huayacocotla basin is one of the most nearly complete stratigraphic records of Pangea breakup in Mexico. The lower strata are composed of several-hundred-meter-thick fluvial successions emplaced during the early stage of continental rift. These deposits were assigned to two units: the Huayacocotla Formation, the oldest unit marking the initiation of rifting in eastern Mexico, and the overlying Cahuasas Formation. These units were tentatively distinguished by color, lithologic assemblage, and sediment composition. However, these features are similar in both units, making it hard to construct a solid stratigraphic framework and an accurate geological map representative of the depositional architecture and tectono-sedimentary evolution of the basin. In this work, we document that, at least in the northern part of the Huayacocotla basin, the Huayacocotla and Cahuasas formations were emplaced in fluvial systems with distinct styles. Thus, they are composed of distinct fluvial elements, which permit their easy differentiation. The Huayacocotla Formation is composed of in-channel conglomerate to sandy bars, interpreted as the stratigraphic record of alluvial fans draining local uplifts. The Cahuasas Formation is dominated by floodplain, crevasse splay, and channel deposits, an association typical of anastomosing rivers. Reworked tuff deposits of similar to 244 Ma in the lower part of the Huayacocotla Formation bracket to Middle Triassic time the initiation of rifting in eastern Mexico, like in other parts of Pangea. The occurrence of Triassic alluvial fans sourced from local basement uplifts in the Huayacocotla basin improves our understanding of the sedimentary systems developed along the western margin of Pangea and shows that the supercontinent was drained not only by transcontinental rivers with headwaters hundreds of kilometers inland in the supercontinent interior, but also by local fluvial systems developed across the supercontinent border.
Identification of cyclic facies successions is fundamental in many sedimentological and sequence-stratigraphic interpretations to accurately reconstruct paleoenvironment and paleoclimate, and to make useful predictions of stratal heterogeneity. However, quantitative identification of cyclical strata to provide sufficiently strong evidence for such interpretation and prediction is challenging. A new method evaluates the order present in any stratigraphic section consisting of at least four lithofacies. Transition-probability matrices are calculated over a range of window sizes up the vertical succession to calculate a Markov-order metric, which is then compared with multiple randomly shuffled versions of the same facies units to calculate the probabilities of the observed succession occurring by chance. Applying the method to eight synthetic sections demonstrates that this method can successfully distinguish cyclical sections from disordered and alternating strata. Two summary metrics are tested on the progressively shuffled versions of the eight synthetic sections and are demonstrated to be useful to quantify order and to distinguish cyclical and alternating strata. Application to one siliciclastic and one carbonate outcrop succession demonstrates that the method can identify order in strata at various length scales. Application to a global dataset of 47 vertical carbonate sections demonstrates mostly disordered strata, suggesting that common interpretations of stratal order related to climate settings might represent conceptual bias. Analysis of eight coeval fluvial-deltaic sections suggests that application of the method can reveal potential characteristic length scales of cyclicity, allow more rigorous correlation, and inform more robust interpretations of autocyclic and allocyclic processes.
Sedimentary architecture studies of river-dominated deltas are important for understanding the role of autogenic vs allogenic dynamics, but examples in high-accommodation settings are rare. In this study, we analyze the kilometer-thick stratigraphic record of a river-dominated delta that accumulated during the Miocene-Pliocene (Carpathian foreland of Romania) in two sections, 13 km apart. The sedimentary successions show a high average (1.45 m/kyr) accumulation rate and are correlated along the dominant paleocurrent direction, substantiating a sequence stratigraphic model of the delta. Facies analysis reveals eight facies associations in three major depositional environments: delta top, subaqueous delta, and lake-dominated coast. The reconstructed deltaic system is characterized by crevasse splays and mires on the delta top, friction-to inertia-dominated river-mouth bars in the subaqueous delta, minor wave influence, absence of tides, minor transgressive reworking, and occasional debris flows. The delta morphology may have varied from highly bifurcating and radial to more fixed and elongated. In the record, we recognize a multitude of facies cycles that have a different expression depending on whether they formed in the proximal, medial, or distal domain. These cycles allow the recognition of hundreds of high-frequency sequences (HFSs), representing 1-12-m-thick alternations between delta (lobe) progradation and abandonment. These sequences become indistinguishable downstream at variable distances, extending for up to 25-40 km from the delta top. The reconstructed model of HFS is comparable with other river-dominated deltas but shows a more gradual lateral transition between delta-top and shallow-water deposits. The autogenic mechanisms of delta-lobe switching as well as sediment storage and release may have been responsible for the generation of the HFS. These sequences are stacked progradationally into larger sequences. The progradational nature of the observed HFS is mainly characteristic for outflowing lakes, even though the hydrology of the Dacian Basin changed over the studied time period. The perceived lake-level stability at 1000-10,000 year time scales may occur because of a relatively wet climate as well as the high resolution of the record. The whole studied record reveals net aggradation, indicating a peculiar long-term balance of sediment supply and accommodation. The record includes sequences at several scales and invites further research to distinguish potential cyclic orbital controls.
Despite the importance of loess in Earth's climate system, aspects of loess formation between glacial silt formation and terminal eolian deflation, i.e., the transport, sorting, and distribution of glacially generated sediment by glaciofluvial systems, remain poorly understood. To address this, we present the first high-resolution dataset of source-to-sink particle size and shape to constrain the physical characteristics of fluvially transported fines in a modern proximal glaciofluvial depositional system (south-central Alaska). From the Matanuska Glacier terminus to Cook Inlet estuary (similar to 85 km downstream), fine-grained sediment was sampled from bar-top fines of the Matanuska River, as well as from tributaries that drain diverse (glacial and nonglacial) terrain along the transect. To capture the particle size and shape characteristics of eolian sediment in the system, six Holocene loess deposits and four stationary dust traps were sampled from strategic locations along the Matanuska River. Dual particle size and shape analysis was performed using the SYNC (by Microtrac), and geospatial analysis in ArcGIS Pro 3.3 was performed to classify Matanuska River tributaries as either glacial or nonglacial (defined by a 3% cutoff of glacial coverage in their respective watersheds). We underscore the critical utility of multidimensional statistical analysis in working with high-dimensional granulometry (particle size and shape) datasets due to its unique ability to recognize variance and separation between defined classes. Our results show that fine-grained sediment in the main Matanuska River is silt-dominant, clusters closely with dust-trap, loess, and glacial-tributary sediment classes, and lacks progressive longitudinal (along-transect) morphological variability. We interpret this uniformity to reflect that suspended-load silt (generated by glacial erosion) overwhelms the system, that this sediment is not influenced substantially by downstream sorting, fining, or tributary mixing, and that this sediment can be deflated (as observed during field sampling) at any distance along the transect to source continental loess in the Matanuska Valley. These findings have implications for understanding analogous processes in ancient glacial systems, including the role of fluvial systems in the formation of loess deposits. Our data build on the arguments of Smalley et al. (2009) and others that the distribution of silt by large rivers is a critical step in forming major loess deposits, with the observation that such glaciofluvial systems are inundated by suspended load sediment with the same size and shape characteristics as loess, at least in the proximal system. This suggests that the particles that form loess deposits do not necessarily require (and may diagnostically not experience) substantial sorting by rivers. Furthermore, this has implications for fluvial channel bar analogs in ancient, proximal proglacial fluvial systems, which we would expect to include a higher proportion of grain shapes characteristic of loess, including angular to subangular, silt-size particles as opposed to rounded sand grains more typical of non-glaciogenic or eolian (dune)-influenced fluvial systems. Our results generally support the usefulness of particle shape data in detecting variations in sediment-transport history, although distinguishing glacial from nonglacial depositional paleoenvironments from these data is more of a challenge. Enabled by the advent of high-resolution and rapid dynamic image analysis (DIA) technology, this study is one of few in the initial steps of developing a robust database of particle size and shape from a broad climatic and geomorphic variety of modern fluvial analogs that is required to reconstruct deep time paleoenvironmental conditions. It is also the first source-to-sink study to focus on particle shape (in the fine-grained, < 50 mu m, fraction), which is imperative for distinguishing between glacial outwash and nonglacial fluvial-deltaic sedimentation in historical systems, given that fines produced by glacial grinding are a hallmark of glacial systems.
An undescribed mechanism of subaqueous sediment deposition from sediment-laden glacial icebergs is documented, in which progressive melting of sediment-laden ice results in gradual buoyancy loss and slow sinking to the sediment-water interface. Unlike dropstone impact or iceberg grounding, descent occurs almost imperceptibly and produces little or no deformation of the underlying sediment. Continued in situ melting deposits sediment directly onto the substrate, forming localized, cone-shaped or nest-like accumulations of clasts or mixed sediment. This process was observed during laboratory experiments originally designed to simulate dropstone deposition. In some cases, partial sediment release from the submerged ice reduced the bulk density sufficiently for the remaining ice to regain buoyancy and lift off, leaving annular or nest-like sediment accumulations surrounding a central low. These deposits resemble isolated mounds and clustered coarse-grained features previously attributed to iceberg dumping or grounding but lacking diagnostic deformation structures (e.g., Fecht and Tallman 1977; Thomas and Connell 1985). Although demonstrated using centimeter-scale experimental analogs, the process is applicable to natural icebergs capable of transporting coarse supraglacial debris. The mechanism provides a physically simple explanation for the emplacement of coarse sediment on fine-grained substrates without causing impact structures and should be considered an additional end-member process in glaciolacustrine and glaciomarine environments.
We propose a new tectono-sedimentary evolutionary reconstruction of the Neogene Arunachal Siwalik Foreland Basin (ASFB), illustrating the interplay among episodic tectonism, sea-level fluctuation, and drainage reorganization in shaping the eastern Himalayan foreland basin. Based on integrated bulk-sandstone petrography, conglomerate-clast composition, paleocurrent pattern, and published data of detrital-zircon geochronology and depositional system, our findings reveal that the lowermost shallow marine Dafla Formation (similar to 10-7 Ma) was derived largely from erosion of the Tethyan and Greater Himalayan rocks via a south-flowing Himalayan drainage system. A southwest-flowing axial-stream system contributed a subordinate sediment input from the TransHimalayan arc into the Dafla Sea. The overlying Subansiri Formation (similar to 7 to > 3.5 Ma) was deposited by the westsouthwest flowing axial paleo-Brahmaputra braided-river system via the rapidly exhuming Eastern Himalayan Syntaxis. The Subansiri Formation was derived predominantly from the Trans-Himalayan arc, Greater Himalayan rocks, and Abor volcanics. It was hitherto unknown that the overlying tidally influenced estuarine to fan deltaic Siji Formation (similar to 3.5 to < 2 Ma) was chiefly sourced from the Lesser Himalayan Gondwana rocks by the south-flowing Himalayan drainage. Also, there was a minor contribution from the Trans-Himalayan arc by the antecedent southwest-flowing axial drainage system. This reorganization in paleodrainage indicates exhumation of the Lesser Himalayan Sequence before the deposition of the Siji Formation and plausibly reflects activation of the Main Boundary Thrust around 5 to 4 Ma in the Arunachal Himalaya. This increased thrust loading at the basin margin, coupled with continued subduction of the Indian Plate, contributed to a greater rate of subsidence in the ASFB, creating increased accommodation space. This combined with eustatic sea-level rise led to marine transgression from similar to 4 to 3.5 Ma. From 2 Ma onwards, southward progradation of the Lesser Himalayan gravel front and a fall in relative sea level led to the deposition of the alluvial-fan-dominated Kimin Formation.
The North China Craton (NCC), situated in the southeast of the Central Asian Orogenic Belt, offers an excellent opportunity to explore the subduction-accretion process of the Paleo-Asian Ocean Plate. Tectonic and magmatic evidence indicates that during the Late Carboniferous to early Permian, the northern margin of the NCC was an Andean-type active continental margin, with the Inner Mongolia Paleo-Uplift serving as a magmatic arc. However, the Carboniferous deposits in the forearc region are characterized by shelf-fluvial sediments, traditionally interpreted as indicative of a passive-margin or post-collisional settings. To resolve this apparent discrepancy, we conducted facies and provenance analysis of the Carboniferous deposits in the Aohanqi region to reconstruct the paleogeography and explore the nature of the basin along the northern margin of the NCC. The Carboniferous deposits formed in the offshore, delta, and braided-river environments. The sedimentary facies constitutes a progradational succession, transitioning from the delta-front to braided-river associations along the basin edge, and from the offshore to delta-front associations in the basin interior. Provenance results indicate that the sediments originated from the Inner Mongolia Paleo-Uplift, with sandstone consisting of volcanic lithics (17-75%), quartz (13-68%), and feldspar (6-38%). The detrital zircons yield U-Pb ages of 1.85 and 0.33 Ga, aligning with the Precambrian crystalline basement and intrusive rocks present in the Inner Mongolia Paleo-Uplift, respectively. Geochemical data further corroborate the mixed origin of these rocks. The upward increase in content of quartz and Precambrian zircon, coupled with progradation in basin filling, suggests that the Inner Mongolia Paleo-Uplift underwent tectonic uplift and exhumation during the Carboniferous. Considering the arc magmatic activity in the Inner Mongolia Paleo-Uplift, we propose that the Aohanqi basin represents a shelved forearc basin. This study emphasizes the potential to identify atypical forearc basins by coupling progradational succession and exhumation of active magmatic arcs in ancient orogens.
The development of lacustrine, palustrine, and pedogenic carbonates through a continuum of sedimentary and diagenetic environments results in highly heterogeneous pore networks, which remain poorly characterized in palustrine and pedogenic facies in terms of their influence on petrophysical properties. These preclude any straightforward correlations between their physical properties and geological characteristics. This study investigates the diversity of elastic properties in fifty Cenozoic lacustrine, palustrine, and pedogenic micritic carbonates of the Paris Basin using samples from three boreholes and ten outcrops. This provides: i) a detailed petrographic framework, ii) ultrasonic measurements of seventy-six 1-inch plugs covering seven facies, and iii) a modeling approach based on the effective medium theory and Gassmann predictions. This study exhibits a broad range of porosities (1.7-40.5%), alongside unusually high P-wave velocities for a given porosity (4.1-6.5 km.s(-1)). The seven studied facies (wackestones with intraclasts, shell-rich floatstones, wackestones with root traces, peloidal grainstones, in-situ brecciated limestones, laminar limestones, nodular brecciated limestones) cannot be discriminated based on their elastic properties. Nevertheless, palustrine and pedogenic samples have similar properties, and about half of them display higher velocities than lacustrine facies. Nine diagenetic pathways were identified in these shallowly buried facies (maximum depth of similar to 200m), which synthesize multiple calcite cementation, dissolution, and/or silicification phases, as well as cracks, but do not account for the observed acoustic dispersion. Studied rocks show six dominant pore types: microporosity, microbial framework, gastropod shells or roots moldic pores, root-related framework, and vuggy pores. They are the results of close relationships between depositional environment and diagenetic processes and provide a better explanation for the dispersion of acoustic data. The difference of acoustic values in palustrine and pedogenic versus lacustrine facies is explained by the prevalence of framework porosity inherited from root systems and vuggy pores from significant dissolution phases in palustrine and pedogenic carbonates. Samples with these pore types behave like an effective medium composed of slightly deformed spheres (aspect ratio close to 1), resulting in a stiffer response than standard differential effective medium models predict. Similar elastic behavior is found only in travertines in the literature. As often in pedogenic deposits, velocities are reduced when complete silicification occurs due to the quartz-calcite compressibility difference. In contrast, the micropores and moldic pores, frequently found in lacustrine facies, form a network resembling crack-like inclusions (aspect ratio similar to 0.3), consistent with typical carbonate behavior. This study thus underscores significant contrasts in elastic properties between lacustrine and palustrine carbonates, offering new perspectives for distinguishing them across different scales and for investigating subsurface reservoir characteristics.
Standardizing facies descriptions has proven key to integrating interpretations of depositional processes and environments from sedimentologic observations with geochemistry data for mudstone lithologies. Because of their fine-grained nature, high degree of compaction, and heterogeneous composition, standardizing methods for mudstone descriptions has proven difficult, but it is critical to formulating meaningful interpretations of the processes that govern the accumulation of organic-rich lithologies and their role in both petroleum systems and the global carbon cycle. In this study, we have developed a modified facies classification scheme for mudstone lithologies that incorporates sedimentologic and compositional observation at the hand-sample and thin-section scales with geochemical measurements, including bulk organic and inorganic geochemistry, to characterize these rocks and their variability more completely for improved interpretations of depositional environments during a low-order sea-level transgression. The facies described in this study are of the Cenomanian-Turonian Greenhorn Formation in the USGS #1 Portland Core drilled in Fremont County, Colorado. Strata of the Greenhorn Formation span Oceanic Anoxic Event 2 (OAE-2) and the preceding interval. Lithologies range from organic-rich argillaceous mudstones with varied sedimentary structures to organic-lean, highly bioturbated limestones. Six facies were identified, each differentiated by varied sedimentary structures and geochemical composition. These facies occur in a predictable stratigraphic stacking pattern that represents a low-order sea-level transgression with interpreted depositional environments ranging from terrigenous-dominated pro-delta and muddy continental shelf at the base of the interval to pelagic offshore marine at the top of the Greenhorn Formation. Though the facies are consistent with previous interpretations of depositional environments at this locale in the Cretaceous Western Interior Seaway during the Greenhorn cyclothem, the sedimentary processes governing the accumulation of organic-rich strata that have defined this interval are significantly revised. Variability in the proximity and intensity of bottom currents driven by storms and geostrophic flows were key to the accumulation of each facies, with significant sediment transport occurring even through deposition in the most oxygen-depleted bottom waters. The methodology and interpretations provided here are now being employed to basin-scale predictions of organic enrichment utilizing calibrated petrophysical methods. The approach and results from this study improve understanding of how organic and inorganic carbon was sequestered during perturbations to the global carbon cycle associated with events such as OAE-2.
Terrigenous materials are delivered to the oceans via fluvial and aeolian pathways; the interpretation of terrigenous signals in marine sediments needs an accurate understanding of initial source regions for different terrestrial components. This work aims to evaluate the relative contributions of these transport mechanisms by conducting a source-to-sink analysis of geochemical elements and plant-wax lipids from terrestrial and marine surface samples collected across southern and central Morocco. We analyze the abundance and distribution of elements (Si, Al, Ca, Fe, Ti, and K) and element ratios, and plant-wax derived n-alkane parameters to trace sediment provenance and transport processes. The results indicate a high concentration of terrigenous elements (Si, Fe, K, Ti, and Al) and low concentrations of calcium (Ca) in the continental samples, while the marine sediments show the opposite. This difference is mainly due to the marine biogenic origin of Ca. Element ratios involving Ca (e.g., Fe/Ca and Ti/Ca) are sensitive to dilution effects like enhanced marine biological productivity and carbonate dissolution. Sediment composition reflects weathering and transport dynamics, with Fe/K ratios indicating erosion controlled mainly by physical processes. Aeolian input dominates offshore the Draa basin, while fluvial supply is stronger off the Souss-Massa and Tensift basins. Higher Zr/Al ratios in river sediments (Draa, Souss-Massa) suggest zircon enrichment, whereas lower marine values reflect hydrodynamic sorting. Ti/Zr ratios point to fluvial dominance in the Tensift and Souss-Massa basins but greater aeolian influence in the Draa, while Ti/Al ratios further highlight riverbed sorting before ocean delivery. Additionally, Plant-wax n-alkane concentrations ranged widely, from 40-76,000 ng/g in river sediments to 24-170 ng/g in marine sediments. This strong contrast, together with CPI values of 1.6-57 on land versus 0.5-3.6 offshore, and ACL values of 29.4-30.4 versus 29.0-29.7, reflects both degradation and dilution processes during transport from land to ocean. The study area, encompassing the Tensift, Souss-Massa, and Draa river basins, is a semiarid to arid region influenced by both Saharan dust and episodic river discharge, providing a key region to investigate source-to-sink processes. By integrating inorganic and organic proxies, we show that aeolian signals dominate offshore the Draa basin, whereas fluvial inputs prevail offshore the Souss-Massa and Tensift basins.
