Shallow Lacustrine Environments Research Articles

Stratigrafia e sedimentologia dei depositi fluviolacustri plio-pleistocenici dell'area nord occidentale del Valdarno superiore (Toscana)

In this paper are exposed the results of statigraphical and sedimentary analysis carried out in the north-western side of the Plio-Pleistocene fluviolacustrine upper Valdarno basin. The deposits of Montevarchi and Torrente Ciuffenna Successions were mapped. These deposits originated during the second and third phases, respectively, of a three-phases sedimentary history. They are separated by an erosional surface extended on the whole basin. According to the existing literature the chronostratigraphic gap extends from the upper part of the Early Pleistocene to the lower part of the Middle Pleistocene. The studied Successions can be classified using the unconformity-bounded stratigraphic units nomenclarure. The Sinthem term is proposed replacing the Succession term. The sedimentary evolution of the studied area is reconstructed starting from the Upper Pliocene. During the deposition of the Montevarchi Sinthem, a fluviolacustrine setting developed in this area. Facies analysis made it possible to recognize coarse-grained fan deltas prograding into a shallow lacustrine environment. The fan delta succession records significant tectonically induced variations of the clastic imput. The lower part of the fan delta succession is characterized by calcareous-arenaceous conglomerates of northwestern provenance. A subsequent rise of the Pratomagno chain lead to a predominance of arenaceous clastic material of eastern provenience. In the fan deltas deposits mammal fossils belonging to the Late Villafranchian and probably to the Early Galerian have been recovered. According ro these data the fluviolacustrine succession developed until the upper part of Lower Pleistocene. The unconformity separating the second and third phase may cover a smaller span of time in the studied area. In the third depositional phase an alluvial setting replaced the fluviolacustrine one previously developed. The alluvial fan succession building up the Torrente Ciuffenna Sinthem, filled the basin during the Middle Pleistocene. The erosion of the northern threshold closing the basin for the most part of the Pleistocene, induced in the Late Pleistocene the beginning of the erosive phase. On the whole the depositional evolution of the studied area was tectonically controlled. The climatic influence over the sedimentation was of lesser importance.

Volcaniclastic sedimentation in a shallow-water marginal basin: the Early Miocene Koura Formation, SW Japan

The Early Miocene Koura Formation, SW Japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics, and constitutes part of the volcanic zone of SW Japan of that time. Reworked volcaniclastic sequences are thick and mostly coarse-grained, and based on conventional facies concepts, are interpreted as shoreface to beach deposits, turbidites, and debris-flow deposits. Thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash-flow emplacement, or in some cases both. Andesite pyroclastic facies include surge and fall deposits. Facies relationships, together with the fossil record, indicate deposition in a shallow lacustrine environment. The deposition rate of the Koura Formation was probably high enough to rapidly fill the shallow lake, but sedimentation was balanced by rapid subsidence of the lake, similar to lakes present within the modern volcanic zones of Taupo, New Zealand and Hohi, Japan. The Koura Formation grades upward into marine deposits, and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back-arc or intra-arc rifting.

Litoestratigrafía y evolución paleogeográfica del Mioceno del borde NE de la Cuenca de Madrid (Prov. Guadalajara)

Miocene deposits of the NE area of the Madrid Basin are studied in this paper. Difierent stratigraphic units are defined and a model of paleogeographic evolution of the NE area of the Madrid Basin is proposed. The three Units previously defined in the Neogene deposits of the Madrid Basin are recognized in this area. These Units, which are bounded by sedimentary discontinuities, have been called: Lower, Intermediate and Upper Miocene Units. Age of these units can be assesed to upper Agenian to middle Aragonian for the Lower Unit; middle Aragonian to 10wer Vallesian for the Intermediate one and lower Aragonian to Turolian for the Upper Unit. Vertical succesions into each of the Units are formed by one single fining-thinning sequence in the Lower and Upper Units and by two fining-thinning sequences in the Intermediate one. Each of the sequences show the progressive retreat of the alluvial systems and later developmet of shallow lacustrine environments. Sedimentation of Lower and Intermediate Units took place in a elosed basin in which tectonic activity events in the margins of the basin resulted in the progradation of the alluvial fan systems and in the retreat of the shallow lacustrine environments. Two different progradation events have been detected. The first corresponds to the bottom of the Intermediate Unit and the other corresponds to the middle part of this Unit. Both events can be recognized in the marginal deposits (sudden increase in grain size) as well as in the more distal ones (detrital deposits are overlying carbonate facies). Location of the different alluvial fan systems is controlled by the paleomorphology and structurallineations of the margin of the basin (Central System and Iberian Range). These alluvial fan systems evolved laterally to wide f100d plains in which shallow lacustrine environments were developed. These lacustrine environments had their greater development in the top of the Intermediate Unit.

Hummocky cross‐stratification and post‐vortex ripples: length scales and hydraulic analysis

ABSTRACTDuke (1987) argues that the structure reported by Greenwood & Sherman (1986) as hummocky cross‐stratification from a shallow lacustrine environment is, instead, bedding formed by simple post‐vortex oscillation ripples. His primary concern is with the length scale of the structures identified in box cores. However, using measured flow data, it is shown here that existing models of bedform generation based upon wave orbital velocity, orbital diameter and grain diameter do not support a simple post‐vortex ripple interpretation. In particular, the relative steepness of the structure refutes such an origin. The structural characteristics of the lacustrine cores most closely match those associated with classic hummocky cross‐stratification and were produced under a combined oscillatory/quasi‐steady flow field.

Condiciones de formación de Paligorskita-Sepiolita en litofacies dolomíticas de la cubeta de Piedrabuena. Campo de Calatrava (Ciudad Real)

Mineralogical, textural and chemical features of two drilling cores that cross over dolomite marls and green clays from Piedrabuena basin (Campo de Calatrava, Central Spain), are studied. The experimental results let us to stablish three stages showing a shallow lacustrine environment with climatic and tectonic changes: a) Lower stage (drying). Massive dolomicrites with increasing- desiccation features at topo Mineralogical association: dolomite-phyllosilicates (sepiolite-palygorskite-illite). b) Middle stage (expansive-retractive). Dolomicrites and intradolomicrites, sometimes bearing clasts, with early dedolomitization textures. Mineralogical association: dolomite-phyllosilicates (palygorskite-sepiolite-illite-dioctahedral smectite)-calcite-quartz. c) Upper stage (freshening). Dolomicrites witb calcite related to dedolomitization processes and thin clay layers witb clasts (quartz, quartzite, mica and carbonates). Mineralogical association: dolomite-ealcite-phyllosilicates (palygorskite-illite-dioctahedral smectite-kaolinite)-quartz. Illite, dioctahedral smectite and kaolinite show a clearly detrital origin from surrounding materials (Palaeozoic quartzites and slates), whilst palygorskite and sepiolite origin is related with early diagenetic processes. So, sepiolite in dolomicrites is associated to desiccation conditions and palygorskite seems have been Cormed after alteration of precursor phases, dioctahedral smectite mainly, under floods conditions. Moreover, both sepiolite and palygorskite show a relation among their genesis, amorpbous silica contents (up to 5.5%), and dedolomitization processes, in a Mgrich environmenl.

Palynostratigraphic investigation of the Albert formation (Lower Carboniferous) of New Brunswick, Canada

Abstract The Albert Formation, in the southeastern part of New Brunswick, contains oil shales from which oil and gas are produced (Stoney Creek field). The fauna and palynoflora of the formation suggest a terrestrial environment of deposition; there is no evidence of marine influence. The strata indicate a relatively shallow lacustrine environment with an associated fluvial deltaic sequence. The preservation of palynomorphs in subsurface core samples from the lower member and middle oil shale member of the formation (Dawson Settlement and Frederick Brook Members, respectively) varies from very poor to very good, whereas preservation of palynomorphs in outcrop samples from the upper member (Hiram Brook Member) is very good. The poor preservation is probably the result of deposition under anoxic conditions, with the subsequent growth of pyrite on the exines; corrosion by microorganisms such as anaerobic bacteria or fungi may also have occurred. No significant qualitative taxonomic variations were discernibl...

Paleoenvironments of Root Casts from the Koobi Fora Formation, Kenya

A study of fossil mots and root casts from East Turkana, Kenya, has shown them to be useful paleoenvironmental indicators. Fossil root casts were obtained from the Pleistocene, Upper Member of the Koobi Fora Formation, deposited within the East Turkana Embayment of the Lake Turkana Basin. Lithofacies and biofacies analysis of the study locality allows the discrimination of several important environments which contain root casts, notably fluvial channels and shallow lacustrine conditions. Five common root cast morphologies have been observed: 1) horizontal, planar root mats; 2) vertical or vertically branching structures; 3) horizontal or horizontally branching structures; 4) diagonally branching structures; and 5) root balls (concretionary structures). Root cast morphology is shaped both by the original root morphology and post-mortem, diagenetic development. Line transect surveys of fossil root casts reveal two striking lithofacies-root cast associations. Laminated siltstones of the shallow lacustrine environment primarily contain root mats and horizontal roots, whereas sandstone-conglomerates of the fluvial paleochannels contain predominantly vertical and diagonal root casts, with no root mats. Studies of plant associations in the Lake Turkana area, and work on modern root morphologies suggest a probable explanation for these associations. In arid environments, water availability is a dominant factor in root morphology. Plants growing along intermittent streams or floodplains must frequently rely on perched water bodies or deeper phreatic water, therefore sending out long, vertical tap roots to take advantage of this resource. Plants living in permanent, shallow lacustrine environments or other areas where phreatic water is quite shallow have no such requirement, since their roots are nearly continuously immersed in ground water; during floods the plant stalk itself is subaqueous. The roots of these plants tend to extend laterally in thin mats over large areas. Thus, a study of root cast morphologies may allow for an assessment of paleogroundwater conditions in nonmarine sediments.

Sedimentary Cycles in the Green River Formation (Eocene), Uinta Basin, Utah

ABSTRACT Several cyclic patterns occur in deposits of fluvial, shallow lacustrine, and deep lacustrine environments in the Parachute Creek Member of the Green River Formation (Eocene), northeastern Uinta Basin, Utah. Individual cycles may be traced laterally, and ideal patterns correlated. Changes in the level of Lake Uinta, possibly resulting from climatic fluctuations, are postulated as the direct cause of the cycles. The presence of such correlative cycles in different facies imposes restrictions on possible modes of origin, and aids in the reconstruction and interpretation of Lake Uinta.

A hypothesis for the change of ocean levels depending on the albedo of the polar ice caps : Bloch M.R., 1965. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1 (2): 127–142.

Upper Miocene and Lower Pliocene shallow lacustrine deposits from the Florina–Ptolemais–Servia Basin in northwest Greece show a distinct m-scale sedimentary cyclicity of alternating marls and lignites or clays, which were shown to be primarily related to precession-induced climate variations ([van Vugt, N., Steenbrink, J., Langereis, C.G., Hilgen, F.J., Meulenkamp, J.E., 1998. Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record. Earth Planet. Sci. Lett. 164 (3–4) 535–551; Steenbrink, J., Van Vugt, N., Hilgen, F.J., Wijbrans, J.R., Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine succession of Ptolemais (NW Greece): Discrepancy between 40Ar/39Ar and astronomical ages. Palaeogeogr. Palaeoclimatol. Palaeoecol. 152, 283–303; Steenbrink, J., Van Vugt, N., Kloosterboer-van Hoeve, M.L., Hilgen, F.J., 2000. Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece). Earth Planet. Sci. Lett. 181 (3–4) 161–173]). Three additional sections and drill cores from the same basin are studied using a combination of cyclostratigraphy, magnetostratigraphy and 40Ar/39Ar dating. Together with the earlier studied sections, they provide a continuous succession that documents the depositional history of the intramontane Neogene basin fill. Sedimentation in the basin started at around 8 Ma ago with deposition of coarse-grained terrigenous-clastics in an alluvial environment. From ∼ 7.1 to ∼ 6.1 Ma ago, diatomaceous marls were deposited in a lacustrine environment, frequently interrupted by deposition of clays and local xylite-type lignite seams. Hereafter, deposition of alternating coarse- and fine-grained terrigenous-clastic alluvial sediments took place, followed by a gradual shift towards lacustrine sedimentation at around 5.5 Ma. Deposition of alternating fine-grained clastics (silts and clays) and biochemical sediments (carbonates, marls and thin lignites), with episodic periods of sub-aerial exposure continued until ∼ 5.23 Ma. From then onwards up to ∼ 3.9 Ma, regular alternating lignites and marls were formed in a shallow lacustrine environment. Precession-induced fluctuations of regional climate primarily defined the m-scale variations in lithology as observed in all members of succession, whereas the 100- and 400-kyr eccentricity cycle had a clear impact on larger-scale lithological alternations in parts of the succession as well. A marked similarity in the ages of fundamental changes in depositional environments between the Florina–Ptolemais–Servia Basin and the marine basins in the Mediterranean is observed, suggesting (but not necessarily implying) that they have a common, possibly tectonic origin.