Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics,Levant Basin

In the Levant Basin, submarine channels are abundant around the Nile deep-sea fan (NDSF), an area which is also affected by salt tectonics related to the Messinian salt giant. Here we focus on the relationship between submarine channels and obstacles formed by salt tectonics. Initially, we use methods developed for terrestrial morphological analysis and quantify channel sinuosity, width and slope in search for consistent relationships between morphometric parameters and channel response to obstacles. However, this traditional analysis did not yield robust conclusions. Then, we apply two new morphometric parameters suggested here to express the distortion of channels by obstacles: incident angle (α), defined as the acute angle between the regionally influenced channel direction and the strike of the tectonic obstacle and diversion angle (Ω), defined as the angle between the direction of the regional bathymetric slope and the average direction of the channel. These parameters illustrate the influence of the regional-scale basin geometry and the superimposed tectonic-influenced seabed patterns, on channel development. We found hyperbolic relationships between incident angle (α) and diversion angle (Ω) in which channels flowing approximately parallel (α ≈ 0°) to tectonic folds are (obviously) not diverted; channels nearly orthogonal (α ≈ 90°) to obstacles, crosscut them right through and, again, not diverted much. In contrast, channels with a general direction diagonal to the obstacles (α ≈ 40°), are diverted by ten degrees (Ω ≈ 10°). This diversion accumulates along large distances and significantly influences the regional development of channels around the NDSF. Noteworthy, this phenomenon of channel diversion, indirectly deteriorate normal slope-sinuosity relationships known from terrestrial studies. In light of these findings, we suggest that these new parameters can be applied to other basins, where submarine channels interact with seabed obstacles.     

Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river

This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ¹³C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.     

杭州湾北部潮流深槽区细颗粒物质输运与再悬浮过程

于2005年5月大潮期间在杭州湾北部潮流深槽区的4个站位进行潮周期观测,获得了流速、悬沙等数据,并对其进行了分析,计算了水沙通量和再悬浮通量。分析结果表明,该深槽区涨潮流速大于落潮流速,涨潮历时小于落潮历时,潮差自湾口向湾内方向增大;悬沙的组分以粉砂为主,分选较差,偏态以负偏为主,这些特征与底质一致;深槽中部和东部的悬沙沿岸线向湾内方向输运;深槽西部和东部外侧的悬沙输运方向与余流方向一致,分别向湾内和南部输运;除转流和流速加速初期外,垂线流速分布符合Kûrmûn-Prandtl模型,摩阻流速与垂线平均流速变化趋势一致。计算得到的表观粗糙长度在涨落潮时段的水流加速或减速阶段都呈增大趋势,且数值较大,这难以把它简单地归结为床面形态的作用,表观粗糙长度的变化趋势可能是高悬沙浓度和浓度成层性共同作用的结果,但对这一假说的验证还有待于进一步的现场观测和机制分析。计算所得的再悬浮发生的周期性与实测悬沙浓度的周期性相符,而且最大悬沙浓度的出现滞后于最大再悬浮通量,说明再悬浮作用对水层中悬沙浓度的变化具有重要影响。