High frequency eustatic sea-level changes during the Middle to ...

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Flute marks (8F), load casts and gutter casts, aligned NE–SW, occur on the underside of some sandbodies (Makhlouf, 1995), especially the lowermost one, and sparse diagenetic siderite nodules have been found in parts of the succession (Fig. 5). Previous studies have recognised the Db2 sandbodies as erosivelybased, ‘channel-like’ fining-up marine sandbodies, but have come to a variety of interpretations. Powell (1989) attributed them to scour and fill by storm events on a wave- and storm-dominated inner shelf whereas Makhlouf (1998) interpreted them as storm generated offshore subtidal rip current channels. Amireh et al. (2001) interpreted them as subtidal channels formed in response to sea level fluctuations. Although the sandbodies have a variable internal architecture, a common sequence is massive, weakly stratified to structureless, locally deformed fine sandstone (Fig. 8D, E), intercalated with, or replaced by low angle, mostly small- to medium-scale trough and less common planar cross-strata (Figs. 5, 8G). This passes gradationally upwards into locally developed parallel laminations with parting lineations and HCS, followed by wave and current ripple crosslamination, including climbing and in-phase ripple drift types (Fig. 5), with a predominantly northerly directed angle of climb. The low angle cross-stratified sandstones contain sparse mud and silt rip up intraclasts, and the foresets are locally rippled and deformed (Fig. 8D). The HCS sandstones are sparsely burrowed, and interstratified with a few thin beds and drapes of silty shale and siltstone. Some minor discontinuous intrasequence scours are locally overlain by up to 50 cm thick sequences of 1) low angle and parallel laminations overlain by ripple cross-lamination capped by a silty shale drape, or 2) brachiopods and low angle laminations which pass up sequentially into parallel lamination or trough cross-stratification, HCS, parallel laminations and ripple cross-lamination (Makhlouf, 1998) (Fig. 5). The finer interbeds between major sandbodies are commonly laminated and internally structured by cm-thick beds of ripple cross-lamination, including frequent climbing ripple lamination and in-phase ripple drift (Jopling and Walker, 1968) (Figs. 5, 8H) with a northerly directed angle of climb, and cm-thick convolute laminated beds (Figs. 5, 8E). The finer interbeds separating the lower three sandbodies are typically b3 m whereas the upper one beneath the top sandbody is up to 6 m thick and consists of alternating thin beds of rippled and locally burrowed siltstone and fine sandstone (Fig. 5). Body fossils are scarce and consist of brachiopods, bivalves and a few brachiopod moulds. The degree of burrowing and bioturbation is low, though well burrowed beds occur locally (Fig. 8H). Trace fossils include Skolithos, Cruziana, Rusophycus and Harlania. Skolithos burrows are most common in the lower part of sandbodies, and B.R. Turner et al. / Sedimentary Geology 251-252 (2012) 34–48 Fig. 7. Panoramic view of climbing ripple lamination with interpretive sketch at the top of the Lower Db1 member of the Dubaydib Formation, erosively overlain by the middle Db2 member. The climbing ripples pass down dip into in-phase ripple drift lamination to the right of the panorama (see text for details). Location: N29°31′27.0″; 035°45′52.6″. there is a general increase in the abundance of traces and burrows in the upper part of the member, especially in the uppermost 5 m thick sandbody (Fig. 5). This contains trough cross-stratification with abundant intraclasts, shell fragments, bivalves, burrows and most of the Cruziana and Rusophycus traces in the succession (Makhlouf, 1998). The sandbodies are aligned WNW–ESE and were deposited by palaeocurrents flowing predominantly to the NNE, perpendicular to depositional strike, with a minor NW–SE trend parallel to depositional strike (Fig. 5). The lateral extent of the WNW–ESE aligned sandstones, suggests a similarly oriented shoreline with a shallow marine shelf extending to the NNE for several hundred kilometres (Andrews, 1991). The lateral extent of the sandbodies and their overall tabular geometry (Fig. 6B) suggests that they are sand sheets rather than channels (cf. Powell, 1989; Makhlouf, 1995; Amireh et al., 2001). The lower more massive, structureless to weakly stratified, locally deformed sandstones suggest deformation of partially liquefied sediment soon after deposition, together with the effects of differential overloading due to density differences (Allen, 1982; Turner and Monroe, 1987; Boggs, 1995). The thin convolute laminated layers are most common in sediments that have been rapidly deposited, hence their association with climbing ripples (Leeder, 1982) especially in the finer interbeds. They are attributed to deformation of ripple cross-laminated layers, probably due to differential loading effects and gravitational collapse across ripple crests and troughs during liquefaction (Allen, 1968; Leppard in Leeder, 1982). The sandstones overlying the lower more massive, structureless to weakly stratified sandstones are sparsely burrowed, and typically contain low angle cross-stratification, parallel laminations with parting lineations and HCS which together, indicate a high energy, proximal/distal lower shoreface (according to the shoreface terminology of Van Wagoner et al., 1990; Hampson and Storms, 2003; Morris et al., 2006) subject to storm events (storm wave depths typically from 15 to 40 m deep, Walker and Plint, 1992) and deposition from waning flows, probably from oscillatory storm wave currents (Dott and Bourgeois, 1982; Bryant et al., 1988). Storm generated variations on the above bedform sequence include: 1) parallel laminations and low angle cross-stratification, typical of distal lower shorefaces; and 2) low angle cross-stratification and HCS, typical of proximal lower shorefaces (Morris et al., 2006). Low energy post storm activity and a return to normal fairweather conditions are indicated by the overlying oscillatory wave ripples and opportunistic post storm related horizontal burrows and feeding traces on rippled bedding surfaces, and deposition of silt and mud. Trough and planar cross-stratification and ripple cross-lamination reflect bedform migration at or above fairweather wave base in a 39
40 B.R. Turner et al. / Sedimentary Geology 251-252 (2012) 34–48 Fig. 8. Sedimentary features of the middle Db2 member of the Dubaydib Formation. A. Irregular, erosive base of sheet sandstone incised into the top of finer interval of striped fine sandstones and darker siltstones. A thin intraformational conglomerate cuts the sandstone on the left and continues along the base of the sandstone to the right. B. Locally incised base of lowermost Db2 sheet sandstone. The incision, which is up to 3 m deep, is filled with a chaotic mass of sandstone matrix-supported, poorly-sorted conglomerate, composed mainly of subangular to subrounded pebble to boulder size clasts derived from erosion of the underlying Db1 member. C. Brachiopod moulds and shale intraclasts just above the base of the lowermost Db2 sheet sandstone. D. Deformed trough cross-stratification in lower part of sheet sandbody. E. Erosive base of sheet sandstone overlying fine sandstone and siltstone showing flame structures projecting up into the sandstone, parts of which have broken off and sunk into the underlying fine sandstone and siltstone to form small convolute laminated ball and pillow structures. Note the thin layer of deformed and convoluted ripples by top of hammer shaft. F. Low relief flute casts on underside of sheet sandstone. G. Medium-scale low angle trough and planar cross-stratification along erosive base of sheet sandstone overlying finer interbed of striped siltstone and fine sandstone. H. Striped siltstone and fine sandstone with thin in-phase ripple drift at base showing a sharp irregular contact with underlying well burrowed, bioturbated, fine-grained sandstone. All locations in the vicinity of Db2 measured section locality are shown in Fig. 5. shoreface setting. Similar 2D and 3D bedforms are common in modern shorefaces where they form under fairweather conditions (Walker and Plint, 1992). Likewise the greater concentration of Skolithos (Powell, 1989; Makhlouf, 1998; Amireh et al., 2001) in the lower part of sandbodies is a characteristic feature of modern foreshore to shoreface environments at water depths typically between 5 and 15 m (Walker and Plint, 1992). The presence of discontinuous internal erosion surfaces may be due to punctuated stepwise
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