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131.
Sedimentary evolution and environmental history of Lake Van (Turkey) over the past 600 000 years 总被引:1,自引:0,他引:1
Mona Stockhecke Michael Sturm Irene Brunner Hans‐Ulrich Schmincke Mari Sumita Rolf Kipfer Deniz Cukur Ola Kwiecien Flavio S. Anselmetti 《Sedimentology》2014,61(6):1830-1861
The lithostratigraphic framework of Lake Van, eastern Turkey, has been systematically analysed to document the sedimentary evolution and the environmental history of the lake during the past ca 600 000 years. The lithostratigraphy and chemostratigraphy of a 219 m long drill core from Lake Van serve to separate global climate oscillations from local factors caused by tectonic and volcanic activity. An age model was established based on the climatostratigraphic alignment of chemical and lithological signatures, validated by 40Ar/39Ar ages. The drilled sequence consists of ca 76% lacustrine carbonaceous clayey silt, ca 2% fluvial deposits, ca 17% volcaniclastic deposits and 5% gaps. Six lacustrine lithotypes were separated from the fluvial and event deposits, such as volcaniclastics (ca 300 layers) and graded beds (ca 375 layers), and their depositional environments are documented. These lithotypes are: (i) graded beds frequently intercalated with varved clayey silts reflecting rising lake levels during the terminations; (ii) varved clayey silts reflecting strong seasonality and an intralake oxic–anoxic boundary, for example, lake‐level highstands during interglacials/interstadials; (iii) CaCO3‐rich banded sediments which are representative of a lowering of the oxic–anoxic boundary, for example, lake level decreases during glacial inceptions; (iv) CaCO3‐poor banded and mottled clayey silts reflecting an oxic–anoxic boundary close to the sediment–water interface, for example, lake‐level lowstands during glacials/stadials; (v) diatomaceous muds were deposited during the early beginning of the lake as a fresh water system; and (vi) fluvial sands and gravels indicating the initial flooding of the lake basin. The recurrence of lithologies (i) to (iv) follows the past five glacial/interglacial cycles. A 20 m thick disturbed unit reflects an interval of major tectonic activity in Lake Van at ca 414 ka bp . Although local environmental processes such as tectonic and volcanic activity influenced sedimentation, the lithostratigraphic pattern and organic matter content clearly reflect past global climate changes, making Lake Van an outstanding terrestrial archive of unprecedented sensitivity for the reconstruction of the regional climate over the last 600 000 years. 相似文献
132.
Rheomorphic ignimbrite D (13.4 Ma, Upper Mogán Formation on Gran Canaria), a multiple flow–single cooling unit, is divided
into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics
indicate contrasting deformation mechanisms during rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pure
uniaxial flattening perpendicular to the foliation; (b) an overlying shear zone characterized by asymmetric fabrics and a
significantly higher finite strain, with an ellipsoid geometry similar to stretched oblate bodies; (c) a central zone with
a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component;
and (d) a slightly deformed to non-deformed top zone where the almost random orientation of subspherical pyroclasts suggests
preservation of original, syn-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional
remobilization of a hot pyroclastic flow as shown by kinematic modeling based on: (a) the overall vertical structural zonation
suggested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of
the calculated vertical cooling progression at the base of the flow (formation of vitrophyre) and the related vertical changes
in strain geometry; (d) the complex lithification history; and (e) the consequent mechanisms of deformational flow. Rheomorphic
flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally
6–8°. We suggest that every level of newly deposited pyroclastic flow material of D first passed through a welding process
that was dominated by compaction (pure flattening) before rheomorphic deformation started.
Received: 25 June 1997 / Accepted: 28 October 1998 相似文献