Unconformity

In central Utah, the major pre-Mississippian unconformity is fairly well understood at most of the localities where it is recognized. However, the unconformity is more enigmatic in Rock Canyon of the central Wasatch Range. At this locality, dolomitization of most pre-Mississippian rocks obscures stratigraphic identification of Devonian and older units. The absence of any identifiable angular relationship further complicates resolution. Because of this, both identification of the stratigraphic level of the unconformity and, consequently, its magnitude remain controversial. Large-size dolomite samples taken in Rock Canyon at closely spaced intervals for the 3.6-m directly below definite Upper Devonian rocks yield microfossils, including conodonts, in the uppermost 1.6-m of that interval that indicate no unconformity exists between the Cambrian Maxfield Limestone and the Upper Devonian-Lower Mississippian Fitchville Dolomite at the horizon previously identified as unconformable. Rather, an unknown thickness of dolomitized Upper Devonian Pinyon Peak Formation and probable older rock (possibly Bluebell Dolomite and Victoria Formation) occurs between the top of definite Maxfield and base of the
Fitchville. The identification of the unconformity horizon remains unknown. Our preliminary work outlines a promising procedure for future understanding of the magnitude and stratigraphic level of
the unconformity.

Variably deformed metasedimentary rocks (interbedded metaquartzite, phyllite, slate, etc.) of Paleozoic age exposed at Kg. Kuala Abang, Terengganu, are separable into two units on the basis of discernible differences in structural features. The lower unit exhibits two phases of folding; the earlier southeasterly plunging reclined fold (F1) is refolded by a later southerly plunging upright fold (F2). The divergent axial trends indicate that the two phases of folding are not coaxial. The upper unit appears to have experienced only one phase of folding corresponding to the F2-fold of the lower unit. This is also evidenced by the marked differences in the occurrence of quartz veins in the two units. The lower unit displays two sets of quartz veins corresponding to the axial planes of Fl and F2, whereas quartz veins parallel to the F2 axial plane are only present in the upper unit. The difference in the structural features between the two units suggest that the deposition of the upper unit postdated the F1 folding phase implying the probable existence of an unconformity in the Paleozoic metasedimentary sequence. The age of this probable unconformity remains uncertain and can be resolved only when the ages of the metasediments at Kg. Kuala Abang are known.

The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed, or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics.
We combine existing tectono-stratigraphic data with new structural and geophysical datasets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the c. 2960–2750 Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded and unconformably overlain by a c. 2730 Ma, syntectonic clastic sequence, deposited in shallow marine to subaerial conditions, and derived from the erosion of the underlying greenstones. This c. 2730 Ma regional unconformity predates the oldest-known Yilgarn structures, therefore its tectonic significance is so far unknown.
At around the same time, at deeper crustal levels, the c. 2728 Ma Yarraquin pluton was being emplaced along an active, large-scale shear zone network. Our meso- and microstructural analysis shows that the bulk of the fabric in the granite and its country rocks developed during pluton emplacement, and was largely assisted by magma-present shearing. Overall, these structures reflect an important event of syn-emplacement crustal shortening. The regional unconformity and the syndeformational emplacement of the Yarraquin pluton are both expressions of a c. 2730 Ma regional deformation event associated with significant crustal thickening, marking the onset of the Neoarchean Yilgarn Orogeny.

Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (Athabasca basin, Canada) have been analyzed by Raman spectrometry in order to identify trace gases in their vapor phase at room temperature. About 80% of fluid inclusions have detectable gases. The most common gases are H 2 , O 2 , CO 2 , CH 4 , C 2 H 6 and N 2. So-called " NaCl-rich " and " CaCl 2-rich " brine inclusions have similar gas contents. U-bearing fluid inclusions have similar gas compositions when compared to U-absent fluid inclusions. Radiolysis (i.e. production of H 2 and O 2 from H 2 O when fluids have been in contact with previously deposited UO 2) and fluid-rock interactions are the most probable origins for H 2 , O 2 and CO 2 , CH 4 , C 2 H 6 and N 2 respectively. The relative abundance of radiolytic O 2 is related to distance to ore and could be used as vectors towards mineralization. Other trace gases may be indicative of ore-forming processes, specifically of fluid-rock interaction and UO 2 deposition.

An undated limestone section in Sungei Bilut near Raub, previously mapped as Carbo-Permian "Raub Group", shows two phases of non-coaxial folding. The presence of polyphase deformation, not evident in other dated "Raub Group" sediments indicated an older age which was confirmed as either Devonian or lower Lower Carboniferous by conodont studies. The lower part of the "Raub Group" can thus be recognised using structural criteria and  it seems likely that an unconformity is present within the strata previously mapped as "Raub Group".

The application of high‐resolution seismic geomorphology, integrated with lithological data from the continental margin offshore The Gambia, northwest Africa, documents a complex tectono‐stratigraphic history and seascape evolution through the Cretaceous. This reveals the spatial‐temporal evolution of submarine canyons by quantifying the related basin depositional elements and providing an estimate of intra‐ vs extra‐basinal sediment budget. The margin developed from the Jurassic to Aptian as a carbonate escarpment. Subsequently, an Albian‐aged wave‐dominated delta system prograded to the paleo‐shelf edge. This is the first major delivery of siliciclastic sediment into the basin during the evolution of the continental margin, with increased sediment input linked to exhumation events of the hinterland. Subaqueous channel systems (up to 320 m wide) meandered through the pro‐delta region reaching the paleo‐shelf edge, where it is postulated they initiated early submarine canyonisation of the margin. The canyonisation was long‐lived (ca. 28 Myr) dissecting the inherited seascape topography. Thirteen submarine canyons can be mapped, associated with a Late Cretaceous‐aged regional composite unconformity (RCU), classified as shelf‐incised or slope‐confined. Major knickpoints within the canyons and the sharp inflection point along the margin are controlled by the lithological contrast between carbonate and siliciclastic subcrop lithologies. Analysis of the base‐of‐slope deposits at the terminus of the canyons identifies two end‐member lobe styles, debris‐rich and debris‐poor, reflecting the amount of carbonate detritus eroded and redeposited from the escarpment margin (blocks up to ca. 1 km3). The vast majority of canyon‐derived sediment (97%) in the base‐of‐slope is interpreted as locally derived intra‐basinal material. The average volume of sediment bypassed through shelf‐incised canyons is an order of magnitude higher than the slope‐confined systems. These results document a complex mixed margin evolution, with seascape evolution, sedimentation style and volume controlled by shelf‐margin collapse, far‐field tectonic activity and the effects of hinterland rejuvenation of the siliciclastic source.

The genesis and evolution of cold-water coral banks along the Northeastern Atlantic margin is known to be influenced by several factors, among which the palae-otopography and nature of the coral settling surface, the presence of bottom currents and sediment supply. ...

Fluid circulation at basement/cover unconformities is of first importance for metal transfer and especially the formation of Pb–Zn, F, Ba and U-deposits. This is typically the case for world-class Proterozoic U deposits (Canada, Australia, Gabon) in basins, which show many similarities with younger Pb–Zn–F–Ba systems (Irish Paleozoic Pb–Zn deposits, F–Pb–Zn–Ba deposits related to extensional tectonics from Spain, western France and Silesia and fluid movements related to continental rifting in the Rhine graben). As fluid mixing near the basement/cover unconformity is one of the key factors for ore formation, a series of parameters have been considered for both systems: the time gap between basin formation and metal deposit, the origin and nature of the ore fluids, the temperature of fluid end members and the style of migration. Results show great similarities in all fluid systems: (i) a wide range of fluid salinity indicating the lack of homogeneity of fluid chemistry at the scale of the reservoirs, (ii) the deep penetration of brines through faults and dense networks of microfractures within the basement below the unconformity, (iii) local fluid–rock interaction leading to porosity increase and significant fluid changes in fluid chemistry, (iv) a pulsatory fluid regime during fluid trapping, (v) anisothermal fluid mixing revealed by a systematic temperature gap between brines and recharge fluids, (vi) stages of fluid movements facilitated by discontinuous opening related to later tectonic/telogenetic stages linked to major geodynamic events, typically without related sedimentation and burial (exception in a few cases characterized by the synchronous production and penetration of surface brines and ore genesis). By analogy with younger systems, the conditions of burial and penetration of brines in the Archean basement suggest that thermal convection drove the brine movements, and was possibly linked to extensional tectonics in a part of the giant mid-Proterozoic U-deposits.

ABSTRACT: Stratigraphic cores and sections, geological maps and Digital Elevation Models (DEMs) were used for the threedimensional reconstruction of a portion of the regional unconformity, which separates the Last Glacial Maximum (LGM) sedimentary complex from the post-LGM one. The investigation focused on the Brenta megafan, a ca. 3000 km2 wide alluvial system that extends from the Alpine piedmont to the Venice Lagoon. During LGM the Brenta megafan experienced a major aggradation phase, which led the system to achieve its maximum extent. Fluvial downcutting started after 17.5 cal ka BP and led to the formation of incised valleys (IVs) in the plain, which reached depths up to 20-30 m. The interfluves between IVs experienced soil formation (Calcisols, Luvisols) until their burial by later deposits, or until present in the case of wide exposed stretches of LGM plain.
A DEM of the present surface of the Brenta megafan and a geological map of the LGM deposits were created in a Geographic Information System (GIS). Seventeen cores and sections with information on the nature (erosive vs. hiatal with soil formation) and depth of burial of the post-LGM unconformity, as well as on the stratigraphy of the alluvial succession, were introduced in the GIS as stratigraphic control points (SCPs). Contours of the base of post-LGM sediments with 2-m interval were available in the Venice Lagoon and its mainland.
Different interpolation and statistical methods were applied for modeling the unconformity in the pedogenized interfluves and in IVs.
Contours were interpolated with Triangulated Irregular Network (TIN) technique. SCPs were interpolated with Inverse Distance Weight (IDW) algorithm with the function cos3. Linear trend equations between the exposed top of the valley fill, i.e. the topographic surface in the DEM, and the unconformity elevation were calculated in different reaches of IVs.
Data processing resulted in the production of a DEM of the post-LGM unconformity in the whole Brenta megafan with a cell size of 30x30 m. Morphometric analysis of the DEM enabled to evaluate the gradients of the unconformity in long and transverse profiles, as well as to calculate its overall extension (1491 km2), the areas characterized by buried soils (970 km2), and those which correspond to an erosive surface in IVs (521 km2). The difference between the DEM of the unconformity and the present topographic surface allowed for the first calculation of the volume of alluvial and coastal sediments which lie on top of the unconformity (10.4 km3), and of those which fill the IVs
(4.3 km3).
The unconformity was traced and modeled across a whole range of sedimentary environments, from braided gravel-bed channel belts (in the proximal sector) down to coastal barrier-and-lagoon systems. The method may be applied to neighboring megafans of the Venetian-Friulian and eastern Po Plain. Moreover, it can be extended offshore, in the shelf of Northern Adriatic Sea, to the tip of the deltaic systems, providing a valuable tool for basin-scale stratigraphic correlations and quantitative estimation of erosive processes and sedimentary storage.

The Châteaupanne Unit belongs to the South Armorican domain of the Armorican Massif (France), which is part of the Variscan belt. This unit includes two Lower Devonian plant levels and one of them corresponds to the Basal Member of the Chalonnes Formation. A sedimentological and palaeontological analysis of these fossiliferous deposits from the Châteaupanne quarry (Montjean/Loire, Maine et Loire, France) is presented here for the first time. The age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the Armorican Massif. Their presence suggests an emergence event that has never been described before. Our study highlights the promising potential of the Basal Member of the Chalonnes Formation to aid in understanding these occurrences, and provides new insights into the history of the Variscan belt.