排序方式： 共有4条查询结果，搜索用时 0 毫秒
1
1.
2.
Lake Rotorua partially occupies a nearly circular 20 km diameter volcano-tectonic depression formed at c. 240 ka by eruption of the voluminous Mamaku Ignimbrite. Three distinct lacustrine littoral terraces, defined on the basis of contrasting geomorphology and field relations, and separated by tephrostratigraphically dateable unconformities and basin-floor disconformities, fringe much of the lake basin. They are here correlated with former high-stands of the lake which resulted from the blockage and re-establishment of a number of alternative outlets due to tectonic activity and volcanism at both the host and adjacent volcanic centres. The unconformities allow division of the deposits into three allostratigraphic units, each of which is then characterised by elevation and sediment provenance. The < 240 ka, post-Mamaku alloformation comprises the highest terrace (up to 415 mASL), and represents the high-stand of an intracaldera lake accumulated in the newly created basin after the eruption of the Mamaku Ignimbrite. Considerable uncertainty surrounds the initial direction of overflow from this level, but the lake may have drained southwards for a period through the Hemo Gorge, through the Ngakuru Graben/Kapenga Caldera area and into the Waikato River catchment. The second alloformation, consisting of volcaniclastic sediments forming shoreline and littoral terraces at c. 380 m elevation developed after the eruption of the 60 ka Rotoiti/Earthquake Flat pyroclastic flows from the neighbouring Okataina Volcanic Centre blocked northern and southern routes out of the lake basin. A northeasterly outlet subsequently became established at a lower level through tectonic subsidence of the Tikitere Graben, creating a drainage path into the Haroharo caldera from where it flowed into the Bay of Plenty via the Kawerau Canyon. The post-36 ka Hauparu alloformation forms the third shoreline terrace at elevations up to 349 mASL. It is the product of a temporary high-stand from blockage of the Tikitere Graben drainage path by pyroclastic debris from the voluminous 36 ka Hauparu eruption. Subsequently, episodic growth of the Haroharo resurgent dome complex between 25 and 9 ka in the adjacent Okataina Volcanic Centre forced Lake Rotorua to rise above its post-Hauparu lowstand level to an elevation where it could overtop a drainage divide on the northern rim of Lake Rotoiti and gain access to the catchment of the Kaituna River, hence establishing the current outlet channel. 相似文献
3.
Tectonically-complex settings present accommodation and sediment supply changes with patterns and rates for which the current sequence stratigraphy paradigms are not designed. In the Tertiary Piedmont Basin (TPB) and Peri-Adriatic Basin (PAB), outcrop and seismic examples demonstrate that the observed stratal and stacking patterns cannot be entirely explained using conventional sequence-stratigraphic models. Therefore, it is of paramount importance to use a model-independent more comprehensive approach encompassing advanced sequence-stratigraphic concepts combined with process changes, while being able to consider the morphostructural complexity that characterizes these margins and their changes induced by basin reshaping.Abrupt relative sea level falls generated by uplift or basin inversion may exceed several hundreds of meters, resulting in wedge-margin progressive unconformities characterized by subaerial and subaqueous erosional truncation. A progressive increase in sediment supply occurs, expressed by increasing volume and size of mass-transport complexes overlain by forced-regressive deltas, as the maximum sediment supply is delayed until after the main uplift. Different accommodation/sediment supply ratios may also occur at the same time along different margins of the same basin, generating a diachronism in the T-R or R-T cycles, adding further complexity to the variability produced by autogenesis.On clastic shelf margins characterized by an increasing rate of relative sea level rise, such as in case of increasing rollback velocities and related flexural tilting, or following an orogenic collapse, sediment supply may not keep pace with increasing accommodation so that initially retrogradation and basinward condensation occur, marked by omission surfaces. However, when the rate of subsidence increases, the succession is punctuated by multiple subaqueous erosional unconformities marking phases of basinward tilting leading to the oversteepening of basin margins and abrupt deepening. The downwarping usually produces large-scale subaqueous erosional surfaces passing laterally into paraconformities, so hinged-margin drowning unconformities affecting clastic shelves occur, associated with regional stratigraphic gaps.The re-establishment of the slope equilibrium profile implies high volume of sediments eroded from drowned deltas and shelves, feeding turbidites deposited at the toe of above-grade slopes. These turbidites can be therefore considered as high accommodation-high sediment supply systems. This suggests that turbidites are delivered basinward not only due to bypass at sequence boundaries or during the highstand progradation of supply-driven deltas, but also due to abrupt accommodation creation on hinged-shelf margin wedges.The great variability of tectonically-driven unconformities generated under either decreasing or increasing accommodation suggests that the features described in the TPB and the PAB are probably not uncommon, controlled by linked dynamic turnarounds of accommodation, sediment supply and stratigraphy taking place throughout the development of basin reorganizations. 相似文献
4.
Morphology and facies architecture of a falling sea level strandplain, Umiujaq, Hudson Bay, Canada 总被引：1，自引：0，他引：1
Coastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground-probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse-sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing-upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp-based bar-and-beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion. 相似文献
1