Gilbert Deltas Research Articles

Observations de karsts pliocènes fossilisés par des limons éoliens quaternaires dans les monts de Matmata (Sud-est tunisien)

Observations of pliocene karsts fossilized by quaternary eolian silts in the Matmata mountains (South-east Tunisia). The submeridional Dahar chain in south¬ eastern Tunisia is over 200 km long. It is separated from the Mediterranean Sea by the Jeffara plain with some tens of kilometers in width. This landscape continues to the South into Libya, but to the North, the chain ends with the Matmata mountains which form a plateau slightly inclined to the west and some 10 km wide. The eastern scarp shows a mainly calcareous geological stratigraphy from Upper Permian to the Senonian. The Dahar-Matmata structure belongs to the Sahara platform and shows a hiatus during the whole Tertiary, since it was emerged since Upper Cretaceous. The Tunisian Atlas nearby shows a completely different paleogeographic evolution, with a complete Tertiary series and a later Plio-Quaternary structuration. These two paleogeographic domains of Southern Tunisia, the Sahara Atlas and the NE border of the Sahara platform, were influenced by the Messinian crisis (5.9 Ma to 5.3 Ma). This was expressed by the collapse of the Mediterranean Sea level, profoundly modifying the fluvial dynamics with an inversion of the erosional system, from normal erosion to regressive erosion. It results a deepening of canyons in the downstream part and a deepening of the watercourses in the upstream part. The geological structures in the Messinian have been deeply affected by these large eustatic changes, with an incision of cluses in the Atlas and the deposition of a thick clayey-sandy series that we could recently link to deltaic systems and Gilbert deltas. The re-establishment of seaways between the Atlantic and the Mediterranean, and the subsequent infill in the Lower Pliocene (Zanclean transgression), with an important inpact in Southern Tunisia, had multiple consequences in that region. The newly adjusted sealevel, together with a more humid climate that was confirmed by faunal and floral extension oof tropical plants in Northern Africa, stimulated an important karstification of the limestone areas. In the Dahar chain, caves, dolines, karstic depressions orkarstic dry valleys emerged, the most spectacular ones being found in the Matmata Mountains. The karstic depressions are the forms that represent best this Pliocene karstification that surely was interrupted in an early stage, because localized endokarstic forms had not enough time to develop. So the karstification seems to have been active in Matmata from 5.4 to 4.0 million years, i.e. two times as long than the duration of the Messinian crisis. The interruption of karstification is due to an increase in temperature and dryness, which even gets more intense during the Pliocene, pulverizing the soils. Already at the beginning of the desertification, a calcareous crust forms by rapid cristallization of dirt. It is immediately transported from the karstic zones to the Jeffara plain. This transfer fo dissolved calcite was the origin of the resistant calcific crust well known in the Jeffara plain. We now identified the same crust in a karstic depression in the Matmata Mountains, opening the way to new geomorphologic and tectonic interpretations, and a review of the eolian silts formerly attributed to the Upper Pleistocene. Later, during Upper Pliocene-Gelasian, we observe a general tectonic uplift of the Dahar chain and the Matmata Mountains as well as the subsidence of the Jeffara plain at the Medenine fault (NW-SE), prolonging the large Gafsa fault towards the East. The karstic paleoforms were thus uplifted more than 500 m, but nevertheless remain open on the Jeffara plain, as seen by large depressions. As a consequence, the karstic depressions of Matmata played the role of traps for eolian silts blown from the Jeffara plain during the extreme desertification in the Upper Pliocene-Gelasian. The morphological reconstruction since the Messinian shows a succession of important events during the Pliocene that profoundly influenced the Quaternary. All indications permit to reject the hypothesis that the Matmata silts came from the West (Eastern Erg).

Reconstruction of the White Sea Basin during the late Younger Dryas

Pasanen, A., Lunkka, J. P. & Putkinen, N. 2009: Reconstruction of the White Sea Basin during the late Younger Dryas. Boreas, 10.1111/j.1502‐3885.2009.00128.x. ISSN 0300‐9483The Weichselian Scandinavian Ice Sheet (SIS) in the White Sea Basin retreated from its maximum position to the Kalevala end moraine between 17 000 and 11 500 years ago. Even though the deglaciation history is relatively well known, the palaeoenvironments in front of the ice sheet are still poorly understood and partly controversial. In the present paper, we use geomorphological, sedimentological and ground‐penetrating radar survey methods to study glaciofluvial plains and shorelines at the Kalevala end moraine. These data are used to define the shoreline gradient for the area and to numerically reconstruct the palaeotopography and the area and volume of the water body in the White Sea Basin during the late Younger Dryas 11 500 years ago. The results indicate that at three sites glaciofluvial plains represent Gilbert deltas deposited to the same water level next to the ice margin. Using the shoreline gradient of 0.42 m/km, it is shown that the water body in the White Sea Basin was extensive and relatively deep, inundating large, currently onshore, areas on the western side of the White Sea and the Arkhangelsk area to the east. The ice margin terminated in the White Sea, which was connected to the Barents Sea via the Gorlo Strait and separated from the Baltic drainage basin to the south.

Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-to-orthogonal-rifted Kwanza basins

We analyse tectonic and sedimentary field and subsurface data for the Angola onshore margin together with free-air gravity anomaly data for the offshore margin. This enables us to characterize the mode of syn-rift tectonism inherited from the Precambrian and its impact on the segmentation of the Angola margin. We illustrate that segmentation by the progressive transition from the Benguela transform-rifted margin segment to the oblique-rifted South Kwanza and orthogonal-rifted North Kwanza margin segments. The spatial variation in the intensity of post-rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post-rift succession of the coastal Benguela and Kwanza Basins: Upper Cretaceous to Cenozoic uplifted palaeodeltas, erosional unconformities, palaeovalleys, Quaternary marine terraces and perched Gilbert deltas. The onshore Benguela transform margin has a distinctive, mainly progradational stratigraphic architecture with long-term sedimentary gaps and high-elevation marine terraces resulting from moderate Upper Cretaceous–Cenozoic to major Quaternary uplifting (i.e. 775–1775 mm/ky or m/Ma). By contrast, repeated synchronous episodes of minor Cenozoic to Quaternary uplift occurred along the orthogonal-rifted North Kwanza segment with its Cenozoic aggradational architecture, short-term sedimentary gaps and low-elevation Pleistocene terraces. Margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep-sea fans. Along the low-lying North Kwanza margin, sedimentation of the broad Cenozoic to Pleistocene Kwanza submarine fan was probably governed by the width of the Kwanza interior palaeodrainage basin combined with the wet tropical Neogene climate. Along the high-rising Benguela margin, the small size of the Benguela deep-sea fan is related to the interplay between moderate continental sediment dispersal from long-lived small catchments and a warm, very arid Neogene climate. However, the driving forces behind the epeirogenic post-rift uplift of the Angola coastal bulge remain a matter of speculation.

Punctuated drainage of an ice‐dammed quaternary lake in southern south america

.A chain of east‐west elongated lakes fringes the eastern flank of the Andes Mountains between 39 and 52°S in southern Chile and Argentina. These deep lakes fill valleys left by the retreat of outlet glaciers of the Patagonian Icefield about 16000 years ago. Two of the lakes, Lago General Carrera/Buenos Aires and Lago Cochrane, represent the shrunken remnants of a much larger lake (called here the predecessor lake) that developed in a moat alongside the icefield.A series of seven stepped lacustrine braid deltas on the southeastern shore of Lago General Carrera/Buenos Aires mark the progressive, punctuated drainage of the predecessor lake. The deltas were formed by the build‐out of fans of sand and gravel into the lake. They have the form of Gilbert deltas, with virtually flat subaerial topset beds and steeply inclined subaqueous foreset beds. The exposed delta fronts are marked by a series of small terraces produced by wave erosion during falling lake levels. On either side of the fans are embayments with beaches formed by deposition in the lee of the pro‐grading fans. These embayments are filled with ridges of well‐sorted gravel deposited by wave‐driven long‐shore drift.About 13 000 years ago the regional drainage underwent a remarkable change. Rivers flowing eastwards into the Atlantic reversed their direction to flow westwards through the Andes Mountains and into the Pacific Ocean. The watershed moved some 200 km to the east. The cause of this change, and the falling lake levels, was melting of the Patagonian Icefield. As the ice retreated the icefield split into north and south components. This broke the ice dam that impounded the western end of the predecessor lake. The stepped, progressive fall of the lake level suggests that as the ice melted a series of lower overflow sills of hard rock were exposed. Each of the major episodes of sudden drop in lake level was followed by a long period of up to thousands of years when lake level remained constant. Terraces on the delta fronts indicate that each major episode of fall in the lake level was punctuated by up to ten minor episodes possibly representing major storm events or annual cycles of freezing and melting, blocking the overflow sills.

Evidence of different contractional styles along foredeep margins provided by Gilbert deltas: examples from Bowser Basin, British Columbia, Canada

Abstract The northern margin of the early Middle Jurassic Bowser Basin in British Columbia contains large, solitary shelf-type Gilbert deltas and smaller, multistorey Gilbert deltas stacked against inferred active reverse faults. Analysis of the deltas provides evidence of tectonic structures that can no longer be directly observed. Bowser Basin developed as a foredeep in response to overthrusting of Stikinia by Cache Creek Terrane (oceanic crust) during accretion of Stikinia to the western margin of North America. The terrane boundary is currently represented by the King Salmon and associated thrust faults. The shelf-type Gilbert deltas consist of accretional conglomerate packages up to 100 m thick, that are overlain by and laterally associated with cyclothemic shelf facies. Gilbert deltas responded to low accommodation to sediment supply ratios by prograding to the shelf break of the basin, where they fed large volumes of gravel to slope gullies and the deep basin beyond. At one location, fault-block rotation resulted in oversteepening of foresets and internal discordances. Deposition was dominated by non-cohesive debris flows (50% to 60% of foresets) and intervening muddy debris flows. The coarse-grained sediments are texturally and compositionally mature, consisting predominantly of spherical radiolarian chert clasts derived from the overthrust Cache Creek accretionary prism. A spectacular succession at least 700 m thick near Cartmel Lake records the aggradational stacking of 30 small Gilbert deltas. Like the shelf-type Gilbert deltas, sediment gravity flows predominated. Creation of accommodation space for accumulation of the Gilbert delta stack was repeated many times; the locus of accommodation remained in much the same place over time. This implies a high degree of tectonic control over the geometry of the stacked Gilbert deltas. Accommodation to sediment supply ratios for this kind of Gilbert delta were high. Based on their thickness, geometry, and general association with shelf lithofacies, Bowser Basin shelf-type Gilbert deltas probably required relatively large drainage catchments, such as those associated with thrust ramps, fault transfer zones or blind thrusts. In one example, delta progradation direction was at a low angle (15 –27°) to the main structural trends and in this particular case we favour delta accumulation at a thrust fault transfer zone. In contrast, the stacked Gilbert deltas are inferred to be footwall deltas that accumulated close to an active, reverse fault where the stacking geometry indicates little or no basinward shift in the locus of deposition. Drainage basins were relatively small and were frequently rejuvenated. The Bowser Basin Gilbert deltas accumulated at the margin of a foreland thrust wedge, outboard of a major Early to Middle Jurassic, crustal-scale thrust system that carried oceanic Cache Creek terrane rocks in its hanging wall. Coarse-grained sediment, derived either from northwards erosional back-stepping of King Salmon Fault or from older Bowser Basin sediment, was cannibalized and redeposited in the Gilbert deltas and associated lithofacies. Either case provides a mechanism to account for the reworking necessary to produce the high clast sphericity and the monomict chert composition.

Transtensional fault‐termination basins: an important basin type illustrated by the Pliocene San Jose Island basin and related basins in the southern Gulf of California, Mexico

ABSTRACTTranstensional basins are sparsely described in the literature compared with other basin types. The oblique‐divergent plate boundary in the southern Gulf of California has many transtensional basins: we have studied those on San Jose island and two other transtensional basins in the region. One major type of transtensional basin common in the southern Gulf of California region is a fault‐termination basin formed where normal faults splay off of strike‐slip faults. These basins suggest a model for transtensional fault‐termination basins that includes traits that show a hybrid nature between classic rift and strike‐slip (pull‐apart) basins. The traits include combinations of oblique, strike‐slip and normal faults with common steps and bends, buttress unconformities between the fault steps and beyond the ends of faults, a common facies pattern of terrestrial strata changing upward and away from the faults into marine strata, small fault blocks within the basin that result in complex lateral facies relations, common Gilbert deltas, dramatic termination of the margin of the basin by means of fault reorganization and boundary faults dying and an overall short basin history (few million years). Similar transtensional fault‐termination basins are present in Death Valley and other parts of the Eastern California shear zone of the western United States, northern Aegean Sea and along ancient strike‐slip faults.

The Ardèche endokarstic responses to the eustatic variations resulting from the Messinian salinity crisis

The Messinian salinity crisis is typically recorded by evaporites in the abyssal plains of the Mediterranean Sea and by canyons incised into the Mediterranean margins and their hinterlands. However, the impacts of crisis on geomorphology and surface dynamics lasted, until canyons were filled by sediments in the Pliocene (fig. 2). In the mid-Rhone valley, the Ardeche Cretaceous carbonate platform is incised over 600 m by the Rhone Messinian canyon. The canyon thalweg is located -- 236 m bsl (below sea level) in the borehole of Pierrelatte [Demarcq, 1960; fig. 1]. During the Pliocene, this canyon was flooded as a ria and infilled by a Gilbert type fan delta [Clauzon and Rubino, 1992; Clauzon et al., 1995]. The whole Messinian-Pliocene third order cycle [Haq et al., 1987] generated four benchmark levels. The first two are [Clauzon, 1996]: (i) The pre-evaporitic abandonment surface which is mapped around the belvedere of Saint-Restitut (fig. 1). This surface is synchronous [Clauzon, 1996] of the crisis onset (5.95 Ma) [Gautier et al., 1994; Krigjsman et al., 1999] and, consequently, is an isochronous benchmark. (ii) The Messinian erosional surface is also an isochronous benchmark due to the fast flooding [Blanc, 2002] of the Rhone canyon, becoming a ria at 5.32 Ma [Hilgen and Langereis, 1988]. These surfaces are the result of endoreic Mediterranean sea level fall more than a thousand meters below the Atlantic Ocean. A huge accommodation space (up to more than 1000 m) was created as sea-level rose up to 80 m above its present-day level (asl) during the Pliocene highstand of cycle TB 3.4 (from 5.32 to 3.8 Ma). During the Lower Pliocene this accommodation space was filled by a Gilbert fan delta. This history yields two other benchmark levels: (i) the marine/non marine Pliocene transition which is an heterochronous surface produced by the Gilbert delta progradation. This surface recorded the Pliocene highstand sea level; (ii) the Pliocene abandonment surface at the top of the Gilbert delta continental wedge. Close to the Rhone-Ardeche confluence, the present day elevations of the four reference levels are (evolution of base-level synthesized in fig. 4): (1) 312 m asl, (2) 236 m bsl, (3) 130 m asl, (4) 190 m asl. The Ardeche carbonate platform underwent karstification both surficial and at depth. The endokarst is characterized by numerous cavities organised in networks. Saint-Marcel Cave is one of those networks providing the most complete record (fig. 5). It opens out on the northern side of the Ardeche canyon at an altitude of 100 m. It is made up by three superposed levels extending over 45 km in length. The lower level (1) is flooded and functionnal. It extends beneath the Ardeche thalweg down to the depth of 10 m bsl reached by divers. The observations collected in the galleries lead us to the conclusion that the karst originated in the vadose area [Brunet, 2000]. The coeval base-level was necessarily below those galleries. The two other levels (middle (2) and upper (3)) are today abandoned and perched. The middle level is about 115 m asl and the upper one is about 185 m asl. They are horizontal and have morphologies specific to the phreatic and temporary phreatic zone of the karst (fig. 6). In literature, the terracing of the Saint-Marcel Cave had been systematically interpreted as the result of the lowering by steps of the Ardeche base-level [Guerin, 1973; Blanc, 1995; Gombert, 1988; Debard, 1997]. In this interpretation, each deepening phase of the base level induces the genesis of the gravitary shaft and the abandonment of the previous horizontal level. The next stillstand of base level leads to the elaboration of a new horizontal level (fig. 7). This explanation is valid for most of Quaternary karsts, that are related to glacioeustatic falls of sea-level. However our study on the Saint-Marcel Cave contests this interpretation because all the shafts show an upward digging dynamism and no hint of vadose sections. The same "per ascensum" hydrodynamism was prevailing during the development of the whole network (figs. 8 and 9). We interpret the development of the Ardeche endokarst as related to the eustatic Messinian-Pliocene cycle TB 3.4/3.5 recorded by the Rhone river. The diving investigations in the flooded part of the Saint-Marcel Cave and also in the vauclusian springs of Bourg-Saint-Andeol reached - 154 m bsl. Those depths are compatible only with the incision of the Messinian Rhone canyon at the same altitude (-236 m bsl). The Saint-Marcel lower level would have develop at that time. The ascending shaping of levels 2 and 3 is thus likely to have formed during the ensuing sea-level rise and highstand during the Pliocene, in mainly two steps: (i) the ria stage controlled by the Mediterranean sea level rise and stillstand; (ii) the rhodanian Gilbert delta progradation, that controlled the genesis of the upper level (fig. 10).

Clinoform nucleation and growth in coarse‐grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacement

AbstractWe investigate the controls on the architecture of coarse‐grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half‐graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault‐controlled subsidence along the basin‐bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well‐exposed, vertically arranged fluvio‐deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo‐seaward into distal pro‐delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert‐delta facies associations as they are traced palaeo‐seaward. This transition is characterised by down‐transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset–foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault‐controlled subsidence during each PU. During episodes of low slip rate, shoal‐water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long‐term (>100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault.

Sedimentary processes on a Gilbert-type delta in Lake Llanquihue, southern Chile

A small Gilbert-type delta on the southern shore of Lake Llanquihue, southern Chile, was studied over a period of two months. Bottom samples were taken by scuba diving along a 10 x 10 m grid to determine the distribution of sedimentary facies. The wind direction, wave conditions, underwater current directions and the orientation of sedimentary structures were recorded on a daily basis. Observations made during fair-weather conditions indicate that neither waves nor currents have any significant effect on the bottom sediments, and that the only transport is produced by bivalves causing small avalanches on the steep delta slope. This probably prevents the oversteepening and large-scale slope collapse typical of many Gilbert deltas. Observations recorded from shore during one major storm, show an excellent agreement with wave theory and empirical predictions, indicating that storm waves can affect the whole delta front and slope. These waves break about 35 m from shore where there is a clear transition from clast-supported gravel on the inner delta front to matrix-supported gravel and gravelly sand on the outer front. The storm waves are able to transport cobbles up to 40 cm in diameter. During these events, strong lakeward-directed bottom currents enhanced by the effluent plume of the river, transport some of these clasts to the edge of the delta front, where they avalanche down to the foot of the delta. During the waning stages of the storm and shortly thereafter, dense, sediment-laden bottom currents discharged from the river mouth carry plant material down the delta slope and over the pro-delta, burying the cobbles just deposited. A single cycle of delta progradation should produce two coarsening-upward cycles, which might be confused in the rock record with two distinct phases of delta progradation.

Preliminary biostratigraphy and 3D geometry of the Vouraikos Gilbert-type fan delta, Gulf of Corinth, Greece

Abstract Along the southern coast of the Gulf of Corinth, interactions between Pleistocene to Recent normal faults and a number of syn-rift Gilbert-type delta systems can be studied. These relationships are exemplified in the Vouraikos Gilbert delta, which has a complex internal sedimentary and structural geometry that was influenced by basin-forming and intra-deltaic planar and listric syn-sedimentary normal faults, as well as by later planar (second-order) extension faults. A thick alluvial succession, well exposed to the east of the Vouraikos delta, predates the delta system. Preliminary biostratigraphical (palynological) data indicate that the Vouraikos delta was deposited during the Early Pleistocene, and built out into a marine body of water, based on the presence of a newly observed shallow marine limestone, which is associated with distinctive coastal facies conglomerates and sandstones. Accommodation space for the >800 m thick Vouraikos delta was mainly created by up to 1000 m of vertical displacement on the north-dipping Pirgaki–Mamoussia Fault. The delta has a grossly concave-upward basal (diachronous) surface in transverse section, thought to be generated by the fixed location of the principal fluvial gravel input to the system. The largest foresets observed in the delta indicate building into a water-filled basin of 300–400 m depth, and a crudely radial form to the delta, shown by NW- and NE-inclined surfaces. The latest sediments of the delta are flat-lying topsets, which are overlain by thick, highly reddened soil sequences exposed in a number of plateaux. These predate the Late-Pleistocene marine terraces that incise the range front. To cite this article: F. Malartre et al., C. R. Geoscience 336 (2004).

Supply, segregation, successions, and significance of shallow marine conglomeratic deposits

Research Article| December 01, 2003 Supply, segregation, successions, and significance of shallow marine conglomeratic deposits H. Edward Clifton H. Edward Clifton Monterey, California, USA Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2003) 51 (4): 370–388. https://doi.org/10.2113/51.4.370 Article history received: 01 May 2002 accepted: 24 Jul 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. Edward Clifton; Supply, segregation, successions, and significance of shallow marine conglomeratic deposits. Bulletin of Canadian Petroleum Geology 2003;; 51 (4): 370–388. doi: https://doi.org/10.2113/51.4.370 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract The accumulation of gravel in the shallow marine environment generally involves special conditions of sourcing, transport, and concentrating mechanism. Accordingly, the correct interpretation of shallow marine conglomerate can not only provide unique insights to tectonics, delivery systems, sedimentary processes, and sequence stratigraphy, but also facilitate prediction of the distribution and quality of hydrocarbon reservoirs within these deposits. Despite their economic and scientific importance, shallow marine conglomerate remains imperfectly understood relative to shallow marine sandstone. Studies of modern shallow marine gravel are relatively few and sporadic; most of our knowledge derives from the stratigraphic record.The study of shallow marine conglomerate may be facilitated by analysis of the processes whereby gravel is delivered to the marine environment (supply), and concentrated once in place (segregation), as well as the succession in which the conglomerate occurs. Although the source of some shallow marine is obvious for some shallow marine conglomerate, it is less certain for many others. Little is known about the potential extent of longshore gravel transport. Understanding issues of supply may be critical to predicting the distribution of gravel in many ancient shallow marine deposits.The degree of segregation of gravel and sand in the shallow marine environment can critically influence a deposit’s reservoir character. Waves are particularly effective at winnowing sand from gravel, particularly during storms. Waves also tend to drive gravel shoreward and concentrate it on or near beaches. Shallow marine gravel typically accumulates in any troughs or other depressions on the sea floor.The occurrence of gravel in the shallow marine environment is particularly sensitive to changes in the rates of accommodation and sediment supply, which in turn determines the nature of the succession in which conglomerate occurs. Coastal gravel can occur in progradational, transgressive and aggradational successions, each of which bears a distinctive signature. Conglomerate in progradational deposits is mostly in the form of beach foreshore/upper shoreface or Gilbert delta accumulations. In transgressive successions, conglomerate may form transgressive lags or accumulate in tidal inlets. Aggradational deposits of shallow marine conglomerate, which seem to require a persistent balance of relative sea level and sediment input that is generally uncommon, can be difficult to explain. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Role of Turbidity Currents in Setting the Foreset Slope of Clinoforms Prograding into Standing Fresh Water

Clinoforms produced where sand-bed rivers flow into lakes and reservoirs often do not form Gilbert deltas prograding at or near the angle of repose. The maximum slope of the sandy foreset in Lake Mead, for example, is slightly below 1°. Most sand-bed rivers also carry copious amounts of mud as wash load. The muddy water often plunges over the sandy foreset and then overrides it as a muddy turbidity current. It is hypothesized here that a muddy turbidity current overriding a sandy foreset can substantially reduce the foreset angle. An experiment reveals a reduction of foreset angle of 20 percent due to an overriding turbidity current. Scale-up to field dimensions using densimetric Froude similarity indicates that the angle can be reduced to as low as 1° by this mechanism. The process of angle reduction is self-limiting in that a successively lower foreset angle pushes the plunge point successively farther out, so mitigating further reduction in foreset angle.

Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico

Research Article| February 01, 2000 Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico Rebecca J. Dorsey; Rebecca J. Dorsey 1Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272 Search for other works by this author on: GSW Google Scholar Paul J. Umhoefer Paul J. Umhoefer 2Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Author and Article Information Rebecca J. Dorsey 1Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272 Paul J. Umhoefer 2Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Publisher: Geological Society of America Received: 15 Jun 1998 Revision Received: 30 Mar 1999 Accepted: 23 Apr 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (2): 177–199. https://doi.org/10.1130/0016-7606(2000)112<177:TAECOS>2.0.CO;2 Article history Received: 15 Jun 1998 Revision Received: 30 Mar 1999 Accepted: 23 Apr 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rebecca J. Dorsey, Paul J. Umhoefer; Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico. GSA Bulletin 2000;; 112 (2): 177–199. doi: https://doi.org/10.1130/0016-7606(2000)112<177:TAECOS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional deformation along the western margin of the active Gulf of California plate boundary. Sedimentary rocks range in age from ∼5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmarine, deltaic, and marine settings. The basin is divided into the central and southeast subbasins, which have distinctly different subsidence histories and stratigraphic evolution. Sedimentary rocks of the Loreto basin are divided into four stratigraphic sequences that record discrete phases of fault-controlled subsidence and basin filling. Sequence boundaries record major changes in tilting geometries and sediment dispersal that were caused by reorganization of basin-bounding faults. Sequence 1 consists of nonmarine conglomerate and sandstone that accumulated in alluvial fans and braided streams. The sequence 1–2 boundary is a marine flooding surface in both subbasins, and parasequences within sequence 2 consist of progradational Gilbert deltas that are capped by transgressive marine shell concentrations and flooding surfaces. The sequence 2–3 boundary is a low-angle erosional unconformity in the southeast subbasin and a thin interval of downlap in the central subbasin. Sequence 3 is characterized by bioclastic limestones that were derived from the uplifted portion of the hanging-wall tilt block. The sequence 3–4 boundary is an angular unconformity in the southeast subbasin and an abrupt marine flooding surface in the central subbasin. Sequence 4 consists dominantly of in situ shallow-marine carbonate deposits.By comparing parasequences of sequence 2 with marine oxygen-isotope curves, we can discriminate between eustatic and tectonic controls on stratigraphic evolution. In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles that do not match the O-isotope curve, and there are no unconformities. In the southeast subbasin, sequence 2 accumulated at a rate of ∼1.5 mm/yr; it contains 4 paracycles that appear to match the O-isotope curve, and sequence boundaries are unconformities. Thus, we conclude that during sequence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were produced by episodic fault-controlled subsidence; and (2) subsidence in the southeast subbasin was slower than the rate of eustatic sea-level changes, and the internal stratigraphic cyclicity preserves a record of eustatic rather than tectonic events. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico

The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional deformation along the western margin of the active Gulf of California plate boundary. Sedimentary rocks range in age from ∼5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmarine, deltaic, and marine settings. The basin is divided into the central and southeast subbasins, which have distinctly different subsidence histories and stratigraphic evolution. Sedimentary rocks of the Loreto basin are divided into four stratigraphic sequences that record discrete phases of fault-controlled subsidence and basin filling. Sequence boundaries record major changes in tilting geometries and sediment dispersal that were caused by reorganization of basin-bounding faults. Sequence 1 consists of nonmarine conglomerate and sandstone that accumulated in alluvial fans and braided streams. The sequence 1–2 boundary is a marine flooding surface in both subbasins, and parasequences within sequence 2 consist of progradational Gilbert deltas that are capped by transgressive marine shell concentrations and flooding surfaces. The sequence 2–3 boundary is a low-angle erosional unconformity in the southeast subbasin and a thin interval of downlap in the central subbasin. Sequence 3 is characterized by bioclastic limestones that were derived from the uplifted portion of the hanging-wall tilt block. The sequence 3–4 boundary is an angular unconformity in the southeast subbasin and an abrupt marine flooding surface in the central subbasin. Sequence 4 consists dominantly of in situ shallow-marine carbonate deposits. By comparing parasequences of sequence 2 with marine oxygen-isotope curves, we can discriminate between eustatic and tectonic controls on stratigraphic evolution. In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles that do not match the O-isotope curve, and there are no unconformities. In the southeast subbasin, sequence 2 accumulated at a rate of ∼1.5 mm/yr; it contains 4 paracycles that appear to match the O-isotope curve, and sequence boundaries are unconformities. Thus, we conclude that during sequence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were produced by episodic fault-controlled subsidence; and (2) subsidence in the southeast subbasin was slower than the rate of eustatic sea-level changes, and the internal stratigraphic cyclicity preserves a record of eustatic rather than tectonic events.

Role of fault interactions in controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift, Sinai, Egypt

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse‐grained delta system to evolution of a fault array in a Miocene half‐graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall‐sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin‐bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault‐related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point‐sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along‐strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.

Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene Lake Bonneville, Northern Utah and Southern Idaho

This study documents the effects of changing lake level (limnostasy), tectonics, sediment yield, and basin physiography on the facies architecture, sequence stratigraphy, and reservoir quality of three fine-grained deltas deposited along the eastern margin of late Pleistocene Lake Bonneville. Analysis of facies architecture indicates that the Weber and Spanish Fork deltas were strongly wave-modified because they were situated along openly exposed portions of the shoreline. The Bear River delta, nestled in a relatively isolated northeast arm of the lake, records both fluvial and wave processes. These fine-grained deltas were fed by low-gradient rivers that drained large regions that were sparsely glaciated, whereas other contemporaneous, coarse-grained Gilbert deltas were fed by steep-gradient rivers that drained local source areas that were strongly influenced by glaciers. Limnostasy, tectonics, and sediment yield were similar for all three fine-grained deltas, implying that the most influential forcing parameter on sequence stratigraphy is basin physiography. Basin physiography (specifically ramp length and accommodation) most strongly controlled the external and internal geometry of the lowstand systems tract of each delta. Potential hydrocarbon reservoir quality (e.g., grain size and sorting) is largely a product of drainage basin size and stream gradient. Relative differences in ramp lengths seemed to determine thickness and lateral continuity of delta front deposits. Relative differences in accommodation appeared to determine the internal geometry of the delta front deposits, especially in the lowstand systems tract. These deltas can serve as analogs for lacustrine exploration and production where many forcing parameters typically are unknown.

Earthquake clustering inferred from Pliocene Gilbert-type fan deltas in the Loreto basin, Baja California Sur, Mexico

A stacked sequence of Pliocene Gilbert-type fan deltas in the Loreto basin was shed from the footwall of the dextral-normal Loreto fault and deposited at the margin of a marine basin during rapid fault-controlled subsidence. Fan-delta parasequences coarsen upward from marine siltstone and sandstone at the base, through sandy bottomsets and gravelly foresets, to gravelly nonmarine topsets. Each topset unit is capped by a thin shell bed that records marine flooding of the delta plain. Several mechanisms may have produced repetitive vertical stacking of Gilbert deltas: (1) autocyclic delta-lobe switching; (2) eustatic sea-level fluctuations; (3) climatically controlled fluctuations in sediment input; and (4) episodic subsidence produced by temporal clustering of earthquakes. We favor hypothesis 4 for several reasons, but hypotheses 2 and 3 cannot be rejected at this time. Earthquake clustering can readily produce episodic subsidence at spatial and temporal scales consistent with stratigraphic trends observed in the Loreto basin. This model is supported by comparison with paleoseismological studies that document clustering on active faults over a wide range of time scales. Earthquake clustering is a new concept in basin analysis that may be helpful for understanding repetitive stratigraphy in tectonically active basins.

The Provenance of Middle Tertiary Sandstones of the San Emigdio Range and Tehachapi Range, Southern California: ABSTRACT

Eocene to lower Miocene sandstones and shales in the San Emigdio Range of southern California were deposited in fan deltas and braid deltas along the tectonically active southeastern margin of the San Joaquin basin. The sandstones are arkoses and lithic arkoses. Framework grain s include quartz, K-feldspar, plagioclase, and volcanic/ metavolcanic, metamorphic, and sedimentary/ metasedimentary lithic fragments. Modal point-count data plot within the uplifted basement and dissected magmatic arc fields on provenance discrimination diagrams, suggesting dissected Sierran and Mojave block sources. ^Nd(t) values of -7.85 and -6.68 from two sandstone samples in the Tejon Formation (Eocene) also suggest a Mojave block source. Modal averages show distinct differences between sandstones deposited in various subenvironments along the basin margin. In general, eolian (QFL=80,16,4) and beach (QFL=75,13,12) sandstones are more mature than fluvial (QFL=40,47,13), nearshore (QFL=53,40,7), Gilbert delta (QFL=49,39,12), and shelf sandstones (QFL=41,46,13). The relative immaturity of the shelf sandstones probably is a result of sand bypassing from nearshore and deltaic settings onto the shelf during storms. Climate does not appear to have exerted a major control on sandstone composition. Quartz-rich sandstone in the Uvas Conglomerate (lower Eocene) and the upper San Emigdio Formation (upper Eocene) may have resulted in part from hot, humid climate. Feldspar-rich sandstone in the Pleito Formation (lower Oligocene) may reflect climatic deterioration at the Eocene-Oligocene boundary.

Gilbert-Type Deltas in a Miocene Marine Sequence, Cuyama Basin Southern California: ABSTRACT

The Soda Lake Shale Member of the Vaqueros Formation in the Cuyama Valley-Caliente Range area of the southern Coast Ranges of California consists almost entirely of basinal facies, although in the southeastern Caliente Range, the unit contains prominent sandstone bodies that are interpreted as small delta deposits. The Soda Lake in the southeastern Caliente Range consists of four lithofacies: (1) siltstone and mudstone, interpreted as the deposits of a muddy marine shelf or shallow basin, (2) platy shale, interpreted as deep marine basin deposits, (3) thickbedded sandstone, interpreted as gravity flow deposits, and (4) large-scale foreset-bedded sandstone, interpreted as the deposits of small deltas. The abundance of graded beds in the foreset-bedded sandstone bodies suggests that gravity-flow processes were the predominant mode of sediment transport. These deltaic deposits are interpreted as Gilbert-type deltas, not greatly different from the classical Gilbert delta model. The progradational sequence in the southeastern Caliente Range suggests an initial ramp geometry for the basin margin there, in contrast to what appears to have been a steep basin margin in the northwestern part of the basin. The basin margin evolved from a ramp geometry b y a combination of differential compaction of fine-grained basinal deposits versus coarser-grained near-shore deposits and the progradation of small deltas, leading to the formation of a marine shelf and slope.

Coarse-Grained Gilbert Deltas: Relationships to Pleistocene Climate and Tertiary Uplift at the Easte of Lake Bonneville, Northern Utah: ABSTRACT

Coarse-Grained Gilbert Deltas: Relationships to Pleistocene Climate and Tertiary Uplift at the Easte of Lake Bonneville, Northern Utah: ABSTRACT