Uplifting mountains and shaking deserts: volcano-tectonic earthquakes revealed by soft-sediment-deformation structures in Upper Cretaceous aeolian deposits
Luciano AlessandrettiLucas V. WarrenFilipe Giovanini VarejãoRaul RassiMaurício G. M. SantosMariana Nunes Marçal SilvaFernando Resende HonoratoMichele J.T. EstradaJoão V.O. Cunha
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ABSTRACT During the last stages of Gondwana fragmentation, large regions of the newly formed South American continent were covered by extensive deserts. Some parts of this continental landmass were synchronously affected by pronounced tectonism and magmatism, which were responsible for reshaping the regional topography. In this context, the southwestern part of the Sanfranciscana Basin in central Brazil is a key area for understanding this particular period in the geodynamic evolution of the South American continent. Aeolian deposits of the Posse Formation in the basin occur in direct association with volcanic rocks of the Upper Cretaceous Mata da Corda Group. Here, we report evidence of synsedimentary magmatism in direct association with soft-sediment-deformation structures, including flame structures, load casts and pseudonodules, water-escape structures, convolute lamination, faults, breccias, and clastic dikes, developed exclusively in aeolian sandstone and siltstone facies. The deformation features are interpreted as indicative of liquefaction, fluidization, and brittle behavior of the loose to partially lithified, wet sandy–silty sediments. The Late Cretaceous aeolian sedimentation is contemporaneous with the uplift of the Paranaíba High and associated magmatism in the Minas–Goiás Alkaline Province. In this context, these significant volcano-tectonic activities are considered to have triggered ductile to brittle deformation in the reported aeolian deposits.Keywords:
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ABSTRACT The Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine‐grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low‐angle to horizontally laminated, fine‐grained sandstone with laminae and lenses of granules and pebbles represents upper flow‐regime, overbank deposition within a braided stream system that was close to a coastline. Medium‐scale, planar‐tabular cross‐stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large‐scale, planar‐tabular cross‐stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage. The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium‐ to very coarse‐grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large‐scale planar‐tabular and trough cross‐stratification reflects deposition within the deepest areas of subtidal channels, whereas medium‐scale cross‐stratification represents deposition in shallower water on shoals separating channels. Fining‐ and thinning‐upward sequences most likely resulted from the longshore migration of channels and shoals. The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine‐grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave‐base by combined‐flow storm currents. The quartz arenite facies is characterized by an absence of fine‐grained units and lithologically consists of a super‐mature, medium‐ to coarse‐grained quartz arenite. Large‐scale planar‐tabular cross‐stratification and abundant low‐angle cross‐stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge‐and‐runnel system or a system of nearshore bars. Large‐scale, planar‐tabular cross‐stratification (upper quartz arenite facies), which forms the top of two 40 m‐thick coarsening‐upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand‐starved shelf as transgression caused the detachment and reworking of shoreface channel‐shoal couplets. Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non‐marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.
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Correlation of facies from hydrocarbon-bearing continental and transitional marine sandstones to time-equivalent high-energy shelf-margin carbonates provide insight into hydrocarbon habitats of the Baltimore Canyon basin. These facies occur within a thick (> 10,000 ft) prograded wedge of shelf sediments in this passive margin basin. Wells drilled to test structural closures in shallow-water ( 5000 ft) drilled off the continental shelf edge to test large structural closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone which was deposited within a shoal-water complex located at the Aptian shelfmore » edge, and (2) coral-stromatoporoid grainstone and boundstone which formed an aggraded shelf-margin complex located at the Kimmeridgian through Berriasian shelf edge. Structural closures with reservoir and top seals are present in both updip and downdip trends. The absence of hydrocarbon shows in downdip carbonate reservoirs suggests a lack of source rocks available to charge objectives at the shelf margin.« less
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ABSTRACT The early Pleistocene Laguna and Turlock Lake Formations and China Hat and Arroyo Seco Gravels along the east side of the San Joaquin Valley, California, were deposited in alluvial fans and marginal lakes. Upward‐coarsening sequences of silt‐sand‐gravel record westward progradation of glacial outwash fans from the Sierra Nevada into proglacial lakes in the San Joaquin Valley. Distinctive sedimentary features delineate lacustrine, prodelta, and delta‐front facies within fan‐margin deposits and lower, middle, and upper‐fan facies within alluvial‐fan deposits. The lacustrine facies consists of a few metres of thinly and evenly bedded, rhythmically laminated claystone and clayey siltstone in varved couplets. Draped lamination, sinusoidal lamination, and load and pillar structures occur in some beds. Siltstone and claystone grade upward to slightly thicker wavy beds of siltstone and very fine‐grained unconsolidated sand deposited in a prodelta setting. Convolute laminae within deformed steeply dipping foreset beds suggest slumping on the prodelta slope. The prodelta facies grades up to the delta‐front facies, which consists of burrowed and bioturbated cross‐bedded fine sand. Deltaic deposits are 5–6 m thick. The lower‐fan facies forms the base of the fan sequence and consists of several metres of irregularly bedded, laminated, oxidized siltstone and fine sand. The middle‐fan facies consists of cross‐bedded, medium‐grained to gravelly sand‐filled channels cut into the lower‐fan facies. Interbedded lens‐shaped siltstone beds 2 m thick and several metres across were deposited in abandoned channels. The upper‐fan facies consists of moderately to strongly weathered clayey gravel and sand containing pebble imbrication and crude stratification. Argillization during post‐depositional soil formation has blurred the distinction between mud‐supported debris‐flow deposits and clast‐supported channel deposits, but both are present in this facies. The deposits described here demonstrate the need for additional fan models in order to incorporate the variety of deposits developed in alluvial fan sequences deposited in humid climates. In previous models based on arctic fans, debris flows, abandoned channels, or widespread siltstone beds are not present in fan sequences, nor are marginal lacustrine and deltaic deposits well represented.
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Outwash plain
Alluvial fan
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ABSTRACT During the last stages of Gondwana fragmentation, large regions of the newly formed South American continent were covered by extensive deserts. Some parts of this continental landmass were synchronously affected by pronounced tectonism and magmatism, which were responsible for reshaping the regional topography. In this context, the southwestern part of the Sanfranciscana Basin in central Brazil is a key area for understanding this particular period in the geodynamic evolution of the South American continent. Aeolian deposits of the Posse Formation in the basin occur in direct association with volcanic rocks of the Upper Cretaceous Mata da Corda Group. Here, we report evidence of synsedimentary magmatism in direct association with soft-sediment-deformation structures, including flame structures, load casts and pseudonodules, water-escape structures, convolute lamination, faults, breccias, and clastic dikes, developed exclusively in aeolian sandstone and siltstone facies. The deformation features are interpreted as indicative of liquefaction, fluidization, and brittle behavior of the loose to partially lithified, wet sandy–silty sediments. The Late Cretaceous aeolian sedimentation is contemporaneous with the uplift of the Paranaíba High and associated magmatism in the Minas–Goiás Alkaline Province. In this context, these significant volcano-tectonic activities are considered to have triggered ductile to brittle deformation in the reported aeolian deposits.
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ABSTRACT The Hensel Formation is the terrigenous component of the last limestone-clastic couplet of the upper Trinity Group (Cretaceous). Most previous studies involving the Hensel have been in depocenters to the southeast of the Llano Uplift. Analysis of the Hensel to the southwest of the uplift has been cursory. Exposures of the Hensel Formation along the upper Llano River drainage basin around Junction, Texas exhibit three distinct lithofacies: 1) a basal terrestrial facies of conglomerate consisting of clast supported, high energy fluvial channel lag lenses exhibiting poor sorting and crude lateral accretionary crossbedding; 2) a middle terrestrial facies of alluvial sandstones and mudstones with paleosol horizons, calcrete and well-developed rhizoconcretion zones; and 3) an upper facies that is composed of fossiliferous calcareous siltstone intercalated with thin fossiliferous limestone beds at the top and barren calcareous siltstone beds in the lower portion. Clasts in the lower and middle facies, and the mudstone of the upper facies, are derived from pre-Mesozoic limestone and crystalline rocks from the Llano Uplift. The Hensel was deposited as a series of coalescing alluvial fans/fan deltas that prograded from the positive Llano Uplift. Fossil ostracodes including Bairdia sp., various agglutinated foraminifers, and small bivalves in the upper facies indicate that it is Cretaceous in age and marine in origin. This, and lithofacies analysis, supports recognizing it as a thin westward extension of the down dip, laterally equivalent Glen Rose Limestone.
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Conglomerate
Paleosol
Red beds
Alluvial fan
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Abstract Well log, well test, and seismic data provide excellent structural and stratigraphic information on subsurface reservoirs, but may not be able to resolve small-scale vertical and lateral attributes which may control oil or gas production. Outcrops can provide this important information provided they are of sufficient areal extent. In this paper, we demonstrate the application of outcrop and behind-outcrop characterization for improved understanding of thin-bedded turbidite reservoirs in the Gulf of Mexico and elsewhere. A 200m high by 10km long coastal cliff section of the Late Miocene Mt. Messenger Formation, New Zealand consists of 200+ meters of thick-bedded turbidite fan sands and associated thin-bedded fan-fringe sandstone/siltstone, overlain by 300m of thin-bedded slope fan, levee/overbank sandstone/siltstone. This section was characterized at the ‘seismic’ to individual bed scales. Also, 47 and 105m deep holes were drilled behind the cliff face, cored, and logged through the slope fan facies with Fullbore Formation Micro Imager* and Platform Express* logs. A slope-fan model is presented which differentiates proximal levee facies composed mainly of discontinuous, erosionally-truncated Bouma Tb-c beds from distal levee/overbank facies composed of more continuous, thin-bedded strata owing to fewer truncations. An associated channel-fill facies is composed of sandstone/siltstone interbeds which become muddier upward. These three facies exhibit distinctive dip and borehole image patterns. The model demonstrates the application of outcrop-behind outcrop characterization for improved understanding of thin-bedded turbidites in analog reservoirs. For example, a permeability vs. lithofacies relationship established for the Mt. Messenger strata is common in Gulf of Mexico thin-bedded turbidites. Also, the thin-bedded levee/overbank reservoir facies of the Ram/Powell Field "L sand" in the Gulf of Mexico exhibits very similar features to those observed in outcrop and behind-outcrop cores and logs. Gas production rates of ~100 MMCFD and 9600 BCPD have been achieved from a horizontal well placed in the "L sand" proximal levee facies. In the broader sense, interactive workstation analyses of lithology, bed thickness, and dip-set groupings seen in the high resolution, behind-outcrop borehole images were used to identify depositional cycles and facies transitions based upon calibration with core and outcrop patterns. The same image analysis methods have been applied to other exploration and production wells in the Gulf of Mexico; one Neogene turbidite example is illustrated.
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Siltstone
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