当前位置: X-MOL 学术Geology › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
Active faulting controls bedform development on a deep-water fan
Geology ( IF 5.8 ) Pub Date : 2021-12-01 , DOI: 10.1130/g49206.1
Vittorio Maselli 1 , Aaron Micallef 2, 3 , Alexandre Normandeau 4 , Davide Oppo 5 , David Iacopini 6 , Andrew Green 7 , Zhiyuan Ge 8
Affiliation  

Tectonically controlled topography influences deep-water sedimentary systems. Using 3-D seismic reflection data from the Levant Basin, eastern Mediterranean Sea, we investigate the spatial and temporal evolution of bedforms on a deep-water fan cut by an active normal fault. In the footwall, the fan comprises cyclic steps and antidunes along its axial and external portions, respectively, which we interpret to result from the spatial variation in flow velocity due to the loss of confinement at the canyon mouth. Conversely, in the hanging wall, the seafloor is nearly featureless at seismic scale. Numerical modeling of turbidity currents shows that the fault triggers a hydraulic jump that suppresses the flow velocity downstream, which thus explains the lack of visible bedforms basinward. This study shows that the topography generated by active normal faulting controls the downslope evolution of turbidity currents and the associated bedforms and that seafloor geomorphology can be used to evince syn-tectonic deposition.Deep-water fans form the largest sediment accumulations on Earth (Menard, 1955; Jobe et al., 2018) and are archives of past tectonic and climatic events (Blum et al., 2018). Deep-water fans are primarily fed and shaped by turbidity currents, which are sediment-laden turbulent flows that move down subaqueous slopes under gravity (Meiburg and Kneller, 2010) and globally represent the dominant mechanism for transporting sediment, organic matter, and pollutants from continents to the deep sea (e.g., Zhong and Peng, 2021).Turbidity currents reaching a supercritical status (i.e., densimetric Froude number [Fr] > 1) form bedforms that are considered to be building blocks of deep-water depositional systems (Covault et al., 2017). These bedforms are thought to be responsible for the inception of new slope channels and canyons (Fildani et al., 2013); they shape channel-lobe transition zones and fans (Postma et al., 2016) and focus the accumulation of plastic litter at the seafloor (Zhong and Peng, 2021). Deep-water fans are dissected by a variety of erosional and depositional features, which are often interpreted as upper flow regime bedforms, such as antidunes and cyclic steps (Wynn and Stow, 2002; Fildani et al., 2006; Normandeau et al., 2019; Maier et al., 2020). While antidunes are formed by supercritical turbidity currents (Fr > 1), cyclic steps are related to transcritical flows, as each step is bounded at its upstream and downstream end by a hydraulic jump, which is a short zone over which the flow experiences a rapid transition from shallow and supercritical (Fr > 1) to thick and subcritical (Fr < 1) (Parker and Izumi, 2000; Fildani et al., 2006; Cartigny et al., 2011; Kostic, 2011). It has been proposed that seafloor rugosity, either generated internally (Guiastrennec-Faugas et al., 2020; Heijnen et al., 2020) or caused by external factors (Ercilla et al., 2002; Covault et al., 2014; Maier et al., 2017), controls the velocity of (near) supercritical and transcritical flows and thus promotes the formation of antidunes and cyclic steps (Cartigny et al., 2014). Fault topography is known to influence turbidite deposition at the fan-scale (Ge et al., 2017, 2018), but the control of active tectonics on turbulent gravity flows and associated bedforms is poorly known. For example, it is unclear how the vertical displacement of an active normal fault, which we term a dynamic knickpoint, influences sediment deposition through supercritical and transcritical turbidity currents and how bedforms record this influence.We integrated 3-D seismic reflection data with numerical modeling to show how a dynamic knickpoint influences sediment deposition on a deep-water fan in the Levant Basin (Fig. 1; eastern Mediterranean Sea). By investigating the interaction between active faulting and sediment transport processes, we provide new insights into the influence of autogenic and allogenic forcing factors on turbidite deposition, which has important implications for understanding how tectonic signals are preserved in the depositional record.The Miocene to recent stratigraphy of the Levant Basin is characterized by evaporites overlain by up to 1.5 km of clastic deposits (Oppo et al., 2021). Tectonic forces and differential sediment loading led to the deformation and basinward movement of the evaporites (Allen et al., 2016) and generated margin-parallel, salt-detached growth faults (Fig. 2). The presence of fault escarpments at the seafloor (Figs. 1A and 1B; and side-scan sonar images of Elias et al. [2007]) indicates that fault movement is still ongoing.We interpreted 1650 km2 of 3-D, post-stack Kirchhoff, time-migrated seismic reflection data (Fig. 1). The dominant frequency within 50 ms below seafloor is ∼90 Hz, and the vertical resolution is ∼4.5 m assuming a P-wave velocity of 1600 m/s. The bathymetry was derived by picking the first reflection and is presented at a scale of 25 m × 25 m. Visibility, sensu Brown (2011), extends the ability to see geological features on this surface to ∼10 m. The uncertainty in the calculation of seafloor gradients is 5%–10%. We used a velocity of 1500 m/s for time to depth conversion of the seafloor surface. Since seabed sediment sampling is lacking, we used the root-mean-square (RMS) amplitude attribute of the seafloor reflection as a proxy for sediment grain size, with high amplitude indicating coarser deposits, as suggested in other studies (Chen and Sidney, 1997; Maselli et al., 2019).We also conducted a series of numerical simulations of turbidity currents running on a simplified topography derived from a bathymetric section along the fan axis (Figs. 1A and Fig. 1D). The model uses Reynolds-Averaged Navier–Stokes (RANS) methods with k – ε Renormalization Group (RNG) turbulence to simulate the control of the fault throw and seafloor slope on the hydraulic and depositional/erosional processes of turbidity currents (see Section S1 in the Supplemental Material1). Based on constraints from Covault et al. (2014), the flow is 15 m thick with an initial velocity of 1.5 m/s and sediment concentration of 0.4% (0.1% silt, 0.2% very fine sand, 0.1% fine sand). These values are consistent with field data from the La Jolla Fan (southern California; Maier et al., 2020), where muddy sediments are interbedded with well-sorted sands, and the Congo Canyon (west Africa; Simmons et al., 2020), where dilute flows (<10 g/l) have been captured at ∼2000 m water depth (w.d.).In the study area, three shelf-incising canyons merge at the base of the slope and feed a deep-water fan (Fig. 1). The canyons’ thalwegs have a gradient of ∼3.7°, which sharply decreases to ∼1.2° at ∼1400 m w.d. (Section S2 in the Supplemental Material). This change in gradient marks the landward limit (i.e., apex) of the fan. Landslide blocks are widespread within the lower reaches of the canyons (Fig. 1B). Northwest-dipping and southwest-northeast–striking elongate escarpments represent the seafloor expression of normal faults (Fig. 1) as shown in seismic profiles (Fig. 2). The main growth fault within the study area, which we name the Bedform Bounding fault (BBf), generates a 35-km-long and up to 18-m-high fault escarpment at the seafloor, which dips at 35° seaward (Figs. 1 and 2).The fan is characterized by seafloor gradients of ∼1.2° and ∼0.2° landward and seaward of the BBf, respectively. In the footwall of the BBf, two types of bedforms are visible at the seafloor: type 1 in the axial fan section, and type 2 on its external portions (Fig. 1B). Type 1 bedforms are crescentic in shape, 35–50 m high and 830–1465 m long, and have an aspect ratio of ∼30. In cross section, the bedforms are asymmetric and have lee and stoss side gradients of 12.2°–23.7° and 4.1°–6.4°, respectively (Fig. 1); these values decrease in an upslope direction (Section S3 in the Supplemental Material). The escarpment of the BBf is also partially reworked by coalescent type 1 bedforms (Section S4 in the Supplemental Material). Type 2 bedforms have slightly sinuous crests and are 5–30 m high and 910–1670 m long (Fig. 1), which gives an aspect ratio >50. In cross section, the bedforms are asymmetric with lee and stoss side gradients of 7.3°–2.6° and 3.6°–0.5°, respectively (Section S3 in the Supplemental Material). The BBf marks a sharp transition from the bedform fields to a seismically featureless seafloor in the nearest 4 km seaward of the BBf (Fig. 1). This region has an overall convex-up morphology (Fig. 2; Section S4 in the Supplemental Material), which corresponds to a sediment apron (Fig. 2, line 1; Section S5 in the Supplemental Material). Farther basinward, a new series of sediment waves with kilometer-scale wavelength, visible in the RMS map (Fig. 1C), develops where there is an increase in seafloor slope to ∼0.6° (Section S6 in the Supplemental Material).Seafloor RMS amplitude is low across much of the slope and abyssal plain, whereas high values occur along the canyons and on the fan at the footwall of the BBf (Fig. 1C). Intermediate amplitude values are visible across the landward-facing side of the sediment apron (Fig. 1C). RMS amplitude values extracted along two seismic sections oriented perpendicular and parallel to the slope across the fan (Fig. 2, lines 2 and 7, respectively) show sharp changes on the fan when crossing the erosional side of type 1 bedforms, on the escarpment related to the BBf, and on landslide blocks that are visible at the seafloor (Figs. 1B and Fig. 1C; Section S6 in the Supplemental Material). Overall, the RMS values gradually decrease basinward across the fan (Fig. 2).Where type 1 bedforms are present (Fig. 2, lines 2 and 3), seismic reflections are wavy to hummocky with alternating high and low amplitudes (Fig. 2, facies A). Seismic packages are separated by multiple erosional surfaces, which are also wavy. Truncated reflections, which dip landward, are visible on the lee side of type 1 bedforms at the seafloor (Fig. 2; see red arrows in line 3). Where type 2 bedforms are present (Fig. 2, lines 5 and 6), seismic reflections are undulating to subparallel, dip landward, and form alternating high- and low-amplitude packages, which are separated by linear to undulating erosional surfaces that dip landward (Fig. 2, facies B). Truncated reflections, which also dip landward, are visible on the lee side of type 2 bedforms at the seafloor (Fig. 2; see red arrows in line 6). The transition from seismic facies A to B, which also corresponds to the change from type 1 to type 2 bedforms at the seafloor, is clearly visible on a section oriented perpendicular to the main slope direction (Fig. 2, line 7).On the hanging wall of the BBf, seismic packages thin seaward and are characterized by low-amplitude continuous, inclined to divergent, reflections (Fig. 2, line 1), and alternate with higher amplitude reflections (Fig. 2, facies C). Seismic facies change abruptly from A to C at the BBf fault (Fig. 2, lines 1–6). The overall wedge-shaped geometry of seismic strata indicates syn-depositional growth associated with normal fault activity.We interpret type 1 bedforms as partially depositional cyclic steps because of >10° dipping lee and stoss sides, erosional lee sides, and backstepping stratigraphy, and type 2 as antidunes because of gentler gradients, larger aspect ratio, and reduced lee side erosion (Slootman and Cartigny, 2020). Cyclic steps and antidunes coexist at the seafloor (Fig. 1C) and make up the stratigraphy of the fan; vertically stacked and upslope migrating cyclic steps and antidunes accumulate on the axial and external portions, respectively (Fig. 2). The coexistence of cyclic steps and antidunes landward of the BBf suggests that the flows are supercritical to transcritical on the fan. We argue that the flow relaxation due to loss of confinement at the canyon mouth generates spatial variation in the flow velocity of the turbidity currents, which is responsible for the synchronous deposition of the two types of bedforms: a higher flow velocity in the axial fan generates cyclic steps, whereas a lower velocity at the sides forms antidunes. Such flow relaxation mechanism has been reproduced in flume tank experiments, which suggests that turbidity currents, when exiting from a confined system, may experience higher flow velocity along the axial portion, where bed erosion may occur due to an increase in basal shear stress (Pohl et al., 2019).RMS amplitude response, and thus the inferred sediment grain size, is highest where cyclic steps are located, lowest outside the fan, and intermediate where antidunes are present. Flume tank experiments indicate that cyclic steps in fine sand develop considerably steep lee sides (Cartigny et al., 2014), which agrees with the 24° dipping lee faces we observe in type 1 bedforms (cyclic steps).The disappearance of bedforms and the decrease in seafloor gradient from ∼1.2° to ∼0.2° across the BBf indicate that the fault has had a dominant control on the behavior of turbidity currents and the sedimentological properties and geometry of the fan. Modeling results of a turbidity current show that the flow is supercritical over the fan and that the knickpoint formed by the BBf triggers a larger hydraulic jump at its base, which reduces flow velocity and Froude number and hampers the development of visible bedforms seaward of the fault (Fig. 3).The formation of hydraulic jumps in turbidity currents due to seafloor irregularities, either generated by tectonics or mass-transport deposits, has been investigated in different settings (Ercilla et al., 2002; Covault et al., 2014; Ge et al., 2017; Howlett et al., 2019). Our results indicate that sedimentary bedforms may record the interaction between turbidity currents and topography generated by active faulting at the seafloor (i.e., a dynamic knickpoint) and thus can help to quantify intervals of syn-tectonic deposition in the stratigraphic record. We show that tectonically induced rugged topography may suppress supercritical to transcritical flows and thus influence the distribution of sedimentary facies within the fan, which has important implications for reservoir properties in tectonically active settings and for the sequestration of land-derived material in the deep ocean.The reduced seafloor gradient in the hanging wall (∼0.2°) could also control the transition to a Froude subcritical flow and thus explain the lack of visible bedforms, as discussed by Zhong et al. (2015). Recent studies from offshore California based on bathymetric data with (sub)meter-scale resolution, however, have shown that small-scale bedforms can be widespread over the surface of deep-water fans, which suggests that supercritical and transcritical flows may still develop in low-gradient settings for a given flow depth and sediment concentration (Maier et al., 2020; Fildani et al., 2021).Our study shows that deep-water fans are sculpted into supercritical and transcritical bedforms and that the topography generated by active normal faulting triggers a hydraulic jump that suppresses the flow velocity of turbidity currents downstream and thus hampers bedform development and influences facies distribution. This outcome has two key implications: (1) the nature and distribution of bedform fields can be used to quantify intervals of syn-tectonic deposition in the stratigraphic record, which thus supports the interpretation of turbidite fans in other active settings such as rifted margins and salt-dominated basins; and (2) sediments, and associated organic matter and anthropogenic pollutants, are vigorously transported into the deepest reaches of the oceans. Our results also indicate that the loss of confinement at the canyon mouth can generate spatial variation in the flow velocity of the turbidity currents (i.e., flow relaxation) and lead to the synchronous deposition of antidunes and cyclic steps.We thank Wissam Chbat and the Lebanese Petroleum Administration for data access and for allowing publication of this work, and Schlumberger for granting Petrel academic licenses. We are grateful to editor Gerald Dickens, and to Andrea Fildani, Matthieu Cartigny, and an anonymous reviewer for suggestions that greatly improved the manuscript. V. Maselli acknowledges support from the Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2020–04461). A. Micallef acknowledges support from the Horizon 2020 European Research Council program (677898, MARCAN).

中文翻译:

深水扇主动断层控制床型发育

受构造控制的地形影响深水沉积系统。利用地中海东部黎凡特盆地的 3-D 地震反射数据,我们研究了被活动正断层切割的深水扇形体的时空演化。在下盘中,风扇分别沿其轴向和外部包括循环台阶和反沙丘,我们将其解释为由于峡谷口处的约束丧失导致流速的空间变化。相反,在悬壁中,海底在地震尺度上几乎没有特征。浊流的数值模拟表明,该断层引发了水跃,抑制了下游的流速,从而解释了盆地缺乏可见的床型。这项研究表明,活动正断层产生的地形控制了浊流和相关床型的下坡演化,并且海底地貌可用于证明同构造沉积。深水扇形成地球上最大的沉积物堆积(梅纳德, 1955 年;Jobe 等人,2018 年)并且是过去构造和气候事件的档案(Blum 等人,2018 年)。深水扇主要由浊流供给和形成,浊流是载有沉积物的湍流,在重力作用下沿着水下斜坡向下移动(Meiburg 和 Kneller,2010 年),在全球范围内代表了运输沉积物、有机物和污染物的主要机制。大陆到深海(例如,Zhong 和 Peng,2021 年)。达到超临界状态的浊流(即密度弗劳德数 [Fr] > 1) 形成被认为是深水沉积系统基石的床体(Covault 等,2017)。这些床型被认为是新斜坡通道和峡谷形成的原因(Fildani et al., 2013);它们塑造了通道-叶过渡区和扇形(Postma 等人,2016 年),并集中了海底塑料垃圾的堆积(Zhong 和 Peng,2021 年)。深水扇被各种侵蚀和沉积特征所分割,这些特征通常被解释为上流状态的床型,例如反沙丘和循环台阶(Wynn 和 Stow,2002 年;Fildani 等人,2006 年;Normandeau 等人, 2019 年;迈尔等人,2020 年)。虽然反沙丘是由超临界浊流 (Fr > 1) 形成的,但循环步骤与跨临界流有关,因为每个步骤在其上游和下游端都受到水跃的限制,这是一个短区域,在该区域上,流动经历了从浅层和超临界 (Fr > 1) 到厚层和亚临界 (Fr < 1) 的快速转变(Parker 和Izumi,2000;Fildani 等,2006;Cartigny 等,2011;Kostic,2011)。有人提出,海底凹凸不平,要么由内部产生(Guiastrennec-Faugas 等人,2020 年;Heijnen 等人,2020 年),要么由外部因素引起(Ercilla 等人,2002 年;Covault 等人,2014 年;Maier 等人,2020 年) al., 2017),控制(近)超临界和跨临界流动的速度,从而促进反沙丘和循环台阶的形成(Cartigny 等,2014)。已知断层地形会影响扇尺度的浊流沉积(Ge et al., 2017, 2018),但是对湍流重力流和相关地床的活动构造的控制却鲜为人知。例如,目前尚不清楚活动正断层的垂直位移如何通过超临界和跨临界浊流影响沉积物沉积,以及床形如何记录这种影响。我们将 3-D 地震反射数据与数值建模相结合,我们称之为动态拐点显示动态临界点如何影响黎凡特盆地深水扇上的沉积物沉积(图 1;地中海东部)。通过研究活动断层和沉积物输送过程之间的相互作用,我们提供了自生和异源强迫因素对浊积沉积的影响的新见解,这对于理解沉积记录中的构造信号如何保留具有重要意义。黎凡特盆地中新世至近期地层的特征是蒸发岩覆盖着长达 1.5 公里的碎屑沉积物(Oppo 等,2021)。构造力和不同的沉积物载荷导致蒸发岩变形和向盆地运动(Allen et al., 2016),并产生与边缘平行的脱盐生长断层(图 2)。海底断层悬崖的存在(图 1A 和 1B;以及 Elias 等人 [2007] 的侧扫声纳图像)表明断层运动仍在进行中。我们解释了 1650 平方公里的 3-D,叠后Kirchhoff,时间偏移地震反射数据(图 1)。海底以下 50 ms 内的主导频率约为 90 Hz,垂直分辨率约为 4。5 m 假设 P 波速度为 1600 m/s。水深测量是通过拾取第一次反射得出的,并以 25 m × 25 m 的比例显示。Visibility, sensu Brown (2011),将在这个表面看到地质特征的能力扩展到 10 m。海底梯度计算的不确定性为 5%–10%。我们使用 1500 m/s 的速度进行海底表面的时间到深度转换。由于缺乏海底沉积物采样,我们使用海底反射的均方根 (RMS) 振幅属性作为沉积物粒度的代表,高振幅表明沉积物较粗,如其他研究(Chen 和 Sidney,1997 年) ;Maselli 等人,2019 年。我们还对从沿风扇轴的测深剖面推导出的简化地形上运行的浊流进行了一系列数值模拟(图 12 和 10)。1A 和图 1D)。该模型使用具有 k – ε 重整化组 (RNG) 湍流的雷诺平均纳维-斯托克斯 (RANS) 方法来模拟对浊流的水力和沉积/侵蚀过程中断层和海底坡度的控制(参见第 S1 部分)补充材料1)。基于 Covault 等人的约束。(2014),流厚 15 m,初始速度为 1.5 m/s,含沙浓度为 0.4%(0.1% 粉砂,0.2% 极细砂,0.1% 细砂)。这些值与 La Jolla Fan(加利福尼亚南部;Maier 等人,2020 年)和刚果峡谷(西非;Simmons 等人,2020 年)的现场数据一致,其中泥质沉积物与分选良好的砂层互层,其中在约 2000 米水深 (wd) 处捕获了稀流 (<10 g/l)。在研究区域,三个切开陆架的峡谷在斜坡底部汇合,供给深水扇(图 1)。峡谷的 thalwegs 具有~3.7° 的梯度,在~1400 m wd 处急剧下降至~1.2°(补充材料中的 S2 部分)。这种梯度变化标志着扇向陆地的极限(即顶点)。滑坡块在峡谷下游广泛分布(图 1B)。西北倾斜和西南-东北走向的细长悬崖代表了地震剖面(图2)中正断层(图1)的海底表现。研究区内的主要生长断层,我们将其命名为床状边界断层(BBf),它在海底产生了一个 35 公里长、高达 18 米高的断层悬崖,向海倾斜 35°(图 3 和图 2)。 1 和 2)。扇的特点是海底坡度为~1.2°和~0。BBf 分别向陆和向海 2°。在 BBf 的下盘,海底可见两种类型的床型:类型 1 在轴流扇部分,类型 2 在其外部(图 1B)。1 型床体呈新月形,高 35~50 m,长 830~1465 m,纵横比约为 30。在横截面上,床型是不对称的,背风和斯托斯侧坡度分别为 12.2°–23.7° 和 4.1°–6.4°(图 1);这些值沿上坡方向减小(补充材料中的 S3 部分)。BBf 的悬崖也被聚结类型 1 床型部分改造(补充材料中的 S4 部分)。2 型床体的波峰略微弯曲,高 5-30 m,长 910-1670 m(图 1),纵横比 >50。在横截面上,床型不对称,背风和斯托斯侧坡度分别为 7.3°–2.6° 和 3.6°–0.5°(补充材料中的 S3 部分)。BBf 标志着从 BBf 向海最近 4 公里处的床型场到无地震特征的海底的急剧转变(图 1)。该区域具有整体向上凸起的形态(图 2;补充材料中的 S4 部分),对应于沉积物围裙(图 2,第 1 行;补充材料中的 S5 部分)。在更远的盆地,一系列具有千米级波长的新沉积波,在 RMS 图中可见(图 1C),在海底坡度增加到 0.6° 的地方发展(补充材料中的 S6 部分)。海底 RMS大部分斜坡和深海平原的振幅都很低,而高值出现在峡谷和 BBf 下盘的风扇上(图 1C)。在沉积物围裙面向陆地的一侧可以看到中间振幅值(图 1C)。沿垂直和平行于扇侧斜坡的两个地震剖面提取的 RMS 振幅值(分别为图 2,第 2 和 7 行)显示,当穿过类型 1 床型的侵蚀侧时,扇上的急剧变化,在与悬崖相关的到 BBf,以及在海底可见的滑坡块上(图 1B 和图 1C;补充材料中的 S6 部分)。总体而言,RMS 值逐渐降低穿过扇向盆地(图 2)。在存在类型 1 地貌的地方(图 2,第 2 和 3 行),地震反射呈波浪状到丘状,具有交替的高低振幅(图 2) , 相 A)。地震包被多个侵蚀面隔开,这些侵蚀面也是波浪形的。在海底 1 型床型的背风侧可以看到向陆地倾斜的截断反射(图 2;见第 3 行中的红色箭头)。在存在类型 2 地貌的地方(图 2,第 5 和第 6 行),地震反射呈波状向近平行,向陆地倾斜,并形成交替的高低振幅包,它们被向陆地倾斜的线性到起伏的侵蚀面隔开(图 2,相 B)。在海底 2 型床型的背风侧可以看到截断的反射,它也向陆地倾斜(图 2;见第 6 行中的红色箭头)。地震相 A 到 B 的转变,也对应于海底从类型 1 到类型 2 的变化,在垂直于主坡方向的剖面上清晰可见(图 2,第 7 行)。在 BBf 的挂壁上,地震包向海较薄,具有低振幅连续、倾向于发散、反射的特征(图. 2,第 1 行),并与更高幅度的反射交替出现(图 2,相 C)。BBf 断层处的地震相从 A 到 C 突然变化(图 2,第 1-6 行)。地震地层的整体楔形几何形状表明与正断层活动相关的同沉积生长。我们将类型 1 地床解释为部分沉积循环台阶,因为 >10° 倾斜的背风侧和 stoss 侧、侵蚀背风侧和后退地层,以及由于更缓和的梯度、更大的纵横比和减少的背风侵蚀,类型 2 作为反沙丘(Slootman 和 Cartigny,2020 年)。循环台阶和反沙丘在海底共存(图 1C),构成扇的地层;垂直堆叠和上坡迁移的循环台阶和反沙丘分别在轴向和外部积聚(图 2)。BBf 向陆的循环台阶和反沙丘的共存表明,流动在扇上是超临界到跨临界的。我们认为,由于峡谷口失去约束导致的流动松弛导致浊流流速的空间变化,这是两种类型床型同步沉积的原因:轴流风机中较高的流速产生循环步骤,而两侧较低的速度形成反沙丘。这种流动弛豫机制已在水槽水槽实验中重现,这表明浊流、当离开受限系统时,沿轴向部分可能会经历更高的流速,由于基础剪切应力的增加,可能会发生床侵蚀(Pohl 等人,2019 年)。 RMS 振幅响应,从而推断沉积物粒度, 在循环台阶所在的位置最高,在风扇外最低,在存在反沙丘的地方居中。水槽水槽实验表明,细砂中的循环台阶形成了相当陡峭的背风面(Cartigny 等,2014),这与我们在 1 型床型(循环台阶)中观察到的 24° 倾斜背风面一致。整个 BBf 海底梯度从~1.2° 降低到~0.2° 表明断层对浊流的行为以及扇的沉积学特性和几何形状具有主导控制。浊流的模拟结果表明,流动在风扇上是超临界的,由 BBf 形成的临界点在其底部触发了更大的水跃,这降低了流速和弗劳德数,并阻碍了断层向海的可见床型的发展(图 3)。已经在不同的环境中研究了由于海底不规则性导致的浑浊流水跃迁的形成,无论是由构造还是大规模运输沉积物产生的(Ercilla 等人,2002 年;Covault 等人,2014 年; Ge 等人,2017 年;Howlett 等人,2019 年)。我们的结果表明,沉积床形态可能记录了浊流与海底活动断层产生的地形之间的相互作用(即,一个动态的拐点),因此可以帮助量化地层记录中同构造沉积的间隔。我们表明,构造诱发的崎岖地形可能会抑制超临界至跨临界流动,从而影响扇内沉积相的分布,这对构造活动环境中的储层性质和深海陆源物质的封存具有重要意义。正如钟等人所讨论的那样,上盘中降低的海底梯度(~0.2°)也可以控制向弗劳德亚临界流的过渡,从而解释了缺乏可见床形态的原因。(2015)。然而,最近来自加利福尼亚近海的基于(亚)米级分辨率的测深数据的研究表明,小规模的床型可以广泛分布在深水扇表面,这表明对于给定的流动深度和沉积物浓度,超临界和跨临界流动仍可能在低梯度环境中发展(Maier 等人,2020 年;Fildani 等人,2021 年)。我们的研究表明,深水扇被雕刻成超临界和跨临界床型以及活动正断层产生的地形触发水跃,抑制下游浊流的流速,从而阻碍床型发育并影响相分布。这一结果有两个关键意义:(1) 床型场的性质和分布可用于量化地层记录中同构造沉积的间隔,从而支持对其他活动环境中浊流扇的解释,例如裂谷边缘和以盐为主的盆地;(2) 沉积物,以及相关的有机物质和人为污染物,被大量输送到海洋的最深处。我们的结果还表明,峡谷口的约束丧失会导致浊流流速的空间变化(即流动弛豫),并导致反沙丘和循环台阶的同步沉积。我们感谢 Wissam Chbat 和黎巴嫩人Petroleum Administration 用于数据访问和允许出版这项工作,以及 Schlumberger 用于授予 Petrel 学术许可证。我们感谢编辑 Gerald Dickens、Andrea Fildani、Matthieu Cartigny 和匿名审稿人提出的极大改进手稿的建议。V. Maselli 感谢加拿大自然科学和工程研究委员会的发现资助 (RGPIN-2020-04461) 的支持。一种。
更新日期:2021-11-23
down
wechat
bug