Subaerial Erosion Research Articles

Uplift and subduction erosion in southwestern Mexico since the Oligocene: pluton geobarometry constraints

Details of the late Oligocene to Middle Miocene uplift and tectonic erosion episodes of the southwestern continental margin of Mexico can be inferred using Al tot geobarometry of igneous hornblendes, geochronology, and field relations. On the basis of such analyses carried out between Acapulco and Huatulco we find the following: (1) Calc-alkaline batholiths exposed along the coast from Acapulco to Huatulco, mostly in the 35-25 Ma age range, were emplaced at depths between 13 and 20 km. (2) The contact relationships between these plutons and their host rocks, and the exposure of volcanic counterparts, 70 km from the coastline, indicate a landward decrease in the amount of uplift. (3) A comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were, in general, higher between Acapulco and Huatulco than those along the margin between Puerto Vallarta and Manzanillo, 700 km northwest of Acapulco. (4) The uplift of this coastal belt occurred during the late stages of magmatism and after its cessation, triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the Xolapa Complex. These findings, in conjunction with the geometry of the present continental margin, as well as the offshore tectonic and stratigraphic features, support previous interpretations of very active late Oligocene to Middle Miocene subduction erosion after the onset of strike-slip tectonics related to the detachment and subsequent eastward displacement of the Chortis block. Subduction erosion involved both trench sediments and crystalline (continental framework) rocks. Different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism. Subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust, whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework. Comparing the calculated depths of pluton crystallization with the present depth of the continental crust-subducted slab boundary, interpreted using previously published seismic refraction and gravity models, we conclude that onshore basal erosion played a subordinate role during Miocene episodes of subduction erosion. Major removal of lower crustal sections was probably restricted to offshore regions. Plate reconstructions of the Cocos plate and its predecessors with respect to North America indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the Guadalupe plate beneath southwestern Mexico.

Influence of relative sea-level on facies and reservoir geometry of the Middle Jurassic lower Brent Group, UK North Viking Graben

Abstract Detailed sequence stratigraphy of several neighbouring fields in the East Shetland basin indicates that a higher order of cyclicity is superimposed onto the lower Brent Group. Both relative falls and rises in sea-level affected the strongly progradational nearshore/barrier-island system and induced important variations in the geometry, distribution and stacking patterns of the reservoir facies. The majority of the shallow marine strata belong to the transgressive and highstand systems tract and consist of the lower-shoreface Rannoch, the upper shoreface and strand-plain Etive and the back-barrier and lagoonal Lower Ness. High frequency increments of shoreline progradation are preserved in the Rannoch as gently seaward-dipping bedsets. The small-scale units mark asymmetric cycles in grain size and mica content and together, with stratiform cement, impart a strong permeability anisotropy on the Rannoch. The nearshore Etive Formation represents a sediment-laden, barred nearshore complex and is subdivided into (1) a uniformly thick upper shoreface and (2) an overlying emergent strand plain. The two subfacies are commonly separated by a thin interval of foreshore which is locally enriched in heavy minerals. The shoreface deposits show systematic variations in stacking geometries ranging from strongly progradational to aggradational. Two relative falls in sea-level led to the subaerial erosion of nearshore Etive deposits and the juxtaposition of coastal-plain mudstones, coals and fluvial deposits with shale-clast lags on lower shoreface Rannoch strata. Although lithostratigraphically part of the Etive, these deposits bear no resemblance to the well ordered nearshore-Etive deposits. The channel complexes are vertically and laterally heterogeneous and are confined to incised valleys separated by non-depositional interfluves. In Cormorant and Tern fields, well-performance anomalies such as early water breakthrough and unpredictable water injection and oil production rates are associated with these lowstand incised valley deposits. An enigmatic coarse-grained unit occurs in the lower Rannoch in Eider field and is provisionally interpreted as the distal expression of the lowstand shoreface deposits associated with the older incised valley. Following the lowstand deposition nearshore-marine sedimentation reestablished during the transgressive/early highstand systems tract. Under the influence of rising relative sea-level the barrier complex aggraded in place with minimal landward translation of the shoreline position. The volume of sediment supplied to the Etive shoreface was sufficiently high to keep pace with phases of increased accommodation. The shoreline system subsequently stacked with a strongly progradational geometry as the high sediment supply filled the available accommodation space during the latter part of the highstand systems tract.

Influence of Relative Sea Level on Facies, Stacking Patterns and Reservoir Heterogeneity: Middle Jurassic Brent Group, UK North Sea: ABSTRACT

The lower Brent Group, above the genetically distinct lowstand deposits of the Broom Formation, is a progradational sequence set made up of the lower- shoreface Rannoch, the upper-shoreface and beach-plain Etive, and the back-barrier Lower Ness Formations. Much of the reservoir heterogeneity in this sandstone-rich succession can be attributed to facies and stacking-pattern variations within higher frequency systems tracts. Highstand systems tracts have strongly offlapping progradational geometries. Sweep efficiency is influenced by: (1) seaward-dipping, mica-rich bed-sets; (2) high-quality ebb tidal-delta sandstone bodies; and (3) the sharp increase in reservoir quality at the sub-horizontal base of the upper shoreface. The increased permeability leads water in the Etive to overrun oil in the Rannoch. Horizontal wells are targeting unswept reserves in the lower-permeability facies. Relative falls in sea level punctuated highstand progradation and led to the subaerial erosion of the Etive and Rannoch formations. Lowstand deposits are contained within incised valleys and consist of fluvial sandstones, coastal-plain mudstones and coal. Well-performance anomalies and abrupt variations in sandstone thickness are associated with the heterogeneous valley-fills. Shoreface sedimentation resumed during transgressive systems tracts. High sediment flux enabled the nearshore system to stack aggradationally. Diachronous lithostratigraphy, aggradation of the heterolithic back-barrier Ness Formation, interbedding ofmore » Rannoch facies in the lower Etive and increased carbonate cement in the lower shoreface Rannoch mark this systems tract.« less

High-Frequency Depositional Sequences and Stratal Stacking Patterns in Lower Pliocene Coastal Deltas, Mid-Norwegian Continental Shelf

Extensive deltaic and coastal progradation occurred along the mid-Norwegian continental shelf during the early Pliocene. Thirty-eight well-developed, high-frequency (fourth-order) sequences are identified within the deltaic complex on multifold seismic data. The fourth-order sequences are arranged in four oblique progradational and two sigmoid progradational sequence sets. Deposition of the high-frequency sequences and their stacking patterns probably were in response to high-frequency cycles of relative changes in sea level cycles produced by variable rates of subsidence and uplift, superimposed on high-frequency eustatic cycles within a lower frequency eustatic system. The mixed aggrading/prograding sequence sets are interpreted to represent increased space-added accommodation rates and deposition within third-order highstand systems tracts. Conversely, the progradational sequence sets are interpreted to represent decreasing space-added accommodation rates and deposition within the third-order lowstand systems tracts. The recognition of multiple sequence sets likely reflects the effect of long-term relative fall in sea level (tectonic uplift?) superim osed on high-frequency eustatic cycles. The terminal sequence boundaries of the sequence sets are formed by conjunction of third- and fourth-order falling inflection points on the relative sea level curve. These sequence boundaries represent the timing of maximum subaerial erosion and may represent times of greatest potential for the deposition of reservoir sands.

Chronostratigraphic framework for the Thanetian and lower Ypresian deposits of southern England

Abstract Magnetostratigraphic and palynomorph data are used to assess the timing and nature of the late Paleocene-earliest Eocene depositioanl sequences of SE England. Ormesby Clay Formation mudstones in east Norfolk are the oldest upper Paleocene deposits (C26r) of southern England. The base of the type Thanet Sand Formation (= start of the Thanetian Stage) is c. 0.65 million years younger (C26n). A subaerial erosion surface separates the Thanet-Ormesby package from the Lambeth Group sediments. The duration of the hiatus (>0.5 million years) separating the two units can be calculated from the magnetostratigraphic data; C25n is missing from the stratigraphic record across almost all of southern England. However, the Upnor Formation in central London contains a record of C25n. A hiatus of c. 0.4 million years separates the late Paleocene Lambeth Group from the early Eocene Harwich Formation. The Lambeth Group, Harwich Formation and lower London Clay Formation were all deposited during C24r. The start of Chron C24n.3n is positioned at the base of Division B of the London Clay Formation.

A giant landslide on the north flank of Tenerife, Canary Islands

The extent of mass wasting along the north flank of Tenerife has been mapped using swath bathymetry, GLORIA side‐scan sonar, and 3.5‐kHz echo sounder data. The marine surveys show that, north of Tenerife, a giant landslide is exposed over an area of 5500 km2 of the seafloor, more than twice the surface area of the island. The landslide truncates an older ridge and valley topography that is associated with the shield building basalts on Tenerife. We interpret the ridge and valley topography as the result of subaerial erosion. The landslide is estimated to have a length of 100 km, a width of up to 80 km, and a volume of about 1000 km3. It extends onshore into the Orotava and Icod valleys which have been interpreted as of landslide origin. K‐Ar dating of basaltic flows in the steep headwall of Orotava suggests an age of formation for the valley is younger than 0.78 Ma and may even be younger than 0.27 Ma. The Icod valley is located immediately to the north of the most recent volcano on Tenerife, Las Cañadas, and has been associated with the collapse of its caldera, between 1.2 and 0.2 Ma. A young age for the landslide is supported by the 3.5‐kHz echo sounder data which show that the landslide is draped by a thin (< 10 m) layer of younger sediment. The landslide did not form, however, during a single catastrophic event but represents the amalgamation of a number of separate landslides. The occurrence of the ridge and valley topography in water depths of up to 2.5 km suggests that the shield‐building basalts have subsided by at least this amount since they formed, 3.3–8.0 Ma. We speculate that this subsidence is caused by some form of stress relaxation that occurs in the underlying lithosphere. The giant landslide imaged in our sonar data is associated with the late stages in the development of the most recent volcano on Tenerife, Las Canadas, which only began at about 1.8 Ma. Thus landsliding may be a particular feature of the time soon after emplacement when because of incomplete isostatic adjustment, oceanic volcanoes have their greatest elevations above sea‐level and therefore are most susceptible to slope failure.

Discussion: Inferred subglacial meltwater origin of lakes on the southern border of the Canadian Shield

In their paper, the authors, R. Gilbert and J. Shaw , add small "finger lakes" in narrow valleys incised through an up-icefacing cuesta to the lengthening list of landforms, large and small, that they and their like-minded colleagues ascribe to erosion or deposition by high-discharge subglacial drainage. This list has included, previously, drumlins, erosion marks, subglacial erosion ("s-") forms, Rogen moraine, fluvial dunes, tunnel channels, a moraine, and regional landform assemblages. Often in these works the glaciofluvial flooding argument is so convincingly based on the field evidence, and counters the glacial argument so effectively, that one feels the conceptual ground shift and one's stance upon it changed. Occasionally, however, as in Shaw and Gilbert (1990) and the present paper, the argument seems to be based on little more than that Other features have been explained by this process, so why not this feature too? In the present paper, the authors give an excellent detailed description of the distribution and form of long, narrow lake basins incised into a limestone cuesta facing up-ice over the Precambrian shield of southeastern Ontario. They liken them to the New York Finger Lake valleys and those in the Niagara cuesta of Ontario, which have been viewed previously as of glacial origin. "Thus, preferential erosion by fast-flowing ice and the presumed absence of subglacial tributaries convincingly account for the valley forms of the Finger Lakes and other reentrant or through-valleys" (p. 1634). However, they caution: "Any agent that might have preferentially eroded through-valleys could take the place of glaciers in the above discussion" (p. 1634). Any agent, that is, except subaerial fluvial erosion, which they discount on grounds going back to Playfair's law. " . . . if large volumes of subglacial meltwater were to be channelized across escarpments" (p. 1634, original emphasis), valleys would be cut similar, in many respects, to glacial troughs. Thus is the subglacial meltwater hypothesis introduced: not solely from evidence but with much supposition. " . . . tributaries would cut by the same floods anastomose (Shaw 1988; Brennand and Shaw 1994), and " . . .esker ridges [with dendritic patterns and accordant junctions] recordonly the most powerful flood events" (Brennand 1994, p. 9). The argument continues: both through-valley and reentrant valley regions (Finger Lakes and Niagara cuesta) lie astride major flood routes proposed by Shaw and Gilbert (1990). This would be fine if the latter reconstruction met with wide independent acceptance, but it seems unfair to seek such support from it when that paper so summarily dismisses the widely accepted glacial history of, for example, the western borderlands of Lake Ontario. The present argument has, nevertheless, gained momentum, albeit cheaply, leading to: "We suggest that meltwater floods may well explain the prominent valleys we describe here" (p. 1635, emphasis added). This seems strangely shy, but may recognize the thinness of the logical ice at this point. Furthermore, flow was across "a wide area upstream [of the escarpment] " (p. 1635), yet tunnel channels and an esker in one of them lie upstream as well as downstream of the escarpment (Brennand and Shaw 1994, p. 507), indicating channelled, not sheet, flow. "Nearly equal spacing" (p. 1635) of the valleys in question suggests to the authors that linearly concentrated escape of sheetfloods over the escarpment was hydrodynamically, rather than morphologically or structurally, controlled, as was the origin of fluting on the tops of escarpment promontories ("noses") (p. 1635). We are left in the dark, however, as to how this control works. Rather, we leap ahead to: "As the flow streamed around the escarpment and noses into the valleys . . ." (p. 1635). Given the valleys, we can follow the argument into counterrotating vortices, a residual medial ridge in one valley and the localization of valley-floor basins. But, it is exasperating to miss how a high-discharge, high-velocity sheetflood becomes patterned into longitudinal counterrotating vortices when it meets an escarpment. This is not to say that the phenomenon is incredible, but be unlikely" (p. 1635), yettunnel channels said to have been we are entitled t o more by -way of explanation than the . van Dyke reference. More accessible than this and charac-

Late Quaternary Evolution of Amvrakikos Gulf, Western Greece

Amvrakikos Gulf is a Neogene basin, formed during a late extensional tectonic phase within the Plio-Quaternary period. It is a semienclosed embayment, separated from the Ionian Sea by a shallow ( 41 m (below present sea level) were a paleo-lake while the rest were exposed to subaerial erosion. Subsequent offshore depositional sequences accumulated at rates of 1.2–2.3 m/ka.

Stratal Patterns, Sediment Composition, and Sequence Stratigraphy at the Margin of the Vercors Carbonate Platform (Lower Cretaceous, SE France)

ABSTRACT Conventional sequence-stratigraphic concepts relate the large-scale stratal architecture of sediment bodies to changes in relative sea level. This paper evaluates the relationship between stratal geometry and sea-level stand, based on a study of large, semicontinuous outcrops of Cretaceous carbonate platform strata in the Vercors (southeastern France). Multiple lines of evidence for sea-level change are combined, including stratal geometry, detailed quantitative microfacies analyses, and diagenetic patterns at platform-top hardground surfaces. The studied outcrops of the Cirque d'Archiane show two main prograding platform tongues, both over 100 m thick. The stratal geometries at the boundary between these platform tongues, including an apparent pinchout of a wedge of slope sediments, suggest the presence of a major lowstand unconformity. However, this stratal boundary does not coincide with the horizon containing the most extensive meteoric alteration. Furthermore, detailed platform-to-basin correlation shows that the wedge of slope sediments is not basin-restricted, but makes a thin drape over the platform top. The sedimentologic and diagenetic evidence suggest incipient drowning of the platform at the boundary between the two main platform tongues, preceded by a minor exposure event only. Internally, the main platform tongues consist of smaller (10-30 m thick) units that prograde towards the basin and aggrade on the platform top, and which are interpreted as stacked highstand wedges. These wedges are usually topped by hardground surfaces with minor evidence for subaerial exposure. The platform-top horizon with the most extensive subaerial diagenesis and erosion correlates with a distinct but relatively thin unit of lithoclastic debris on the slope. A pronounced scour in the clinoforms of the Vercors does not correlate with a major exposure surface on the platform top. This study shows that stratal geometries alone are a rather ambiguous guide to sea-level history. Without the accompanying sedimentologic and diagenetic evidence for sea-level change, most of the stratal architecture of the Vercors platform can be explained by either changes in accommodation or changes in carbonate production.

Origin and Distribution of Holocene Temperate Carbonates, Hecate Strait, Western Canada Continental Shelf

ABSTRACT The surficial sediments in an area of 4000 km2 in Hecate Strait, western Canada continental shelf, are composed predominantly of CaCO3. The deposit is Holocene and forms a discontinuous unit consisting of coarse shell debris, typically < 1 m thick, overlying Tertiary bedrock. Carbonate sediments are largely restricted to water depths of < 50 m and are closely associated with rock and gravel substrates. The faunas yielding these sediments are predominantly bivalves, barnacles, and bryozoans. Radiocarbon dates on surficial material range from modern to 1500 yr B.P. Vigorous water circulation through tidal and wave action, combined with rocky or coarse-grained substrates in some areas, favors suspension-feedin biogenic carbonate producers. The presence of carbonates in Hecate Strait results from the glacial and postglacial history of the region. Glaciation and subsequent subaerial erosion produced abundant rock and gravel substrates; subsequent transgression starved the region of terrigenous clastic sediments. Carbonate sedimentation on high-latitude continental shelves is restricted to areas where conditions favorable to production of biogenic carbonate coexist with low supply of terrigenous sediment. High-amplitude fluctuations of sea level, such as those produced during glaciations, may favor development of temperate carbonates during transgressive and highstand intervals.

Cyclic changes in the sedimentary environment during the last interglacial/glacial cycle; coastal Jameson Land, East Greenland

Raised successions of sediments from the last interglacial/glacial period along coastal Jameson Land, East Greenland, comprise a variety of sedimentary facies that reflect cyclic changes of the sedimentary environment. These changes are controlled by the fluctuations of relative sea-level and glacier advances. Each of the sedimentary cycles starts with till deposition succeeded by marine mud sedimentation, subaerial erosion, deltaic and coastal sedimentation. Sea-level changes alters from falling to rising during each sedimentary cycle and indicates that local East Greenland glacier fluctuations generally lead the global eustatic cycles. Most of the deposits cover isotope stage 5, and successive higher sea-level stands during each sedimentary cycle indicate a net growth of the Greenland ice sheet during the Early Weichselian.

Paleokarst, paleosol, and rocky-shore deposits at the Mississippian-Pennsylvanian unconformity, northwestern Arkansas

The Mississippian-Pennsylvanian unconformity in northwestern Arkansas contains pedogenically altered regolith preserved in low-lying areas on paleokarst on the Chesterian Pitkin Limestone. The regolith is succeeded by transgressive rocky-shore deposits of the Cane Hill Member, Hale Formation (type-Morrowan). Pitkin cements reflect extensive, meteoric, phreatic diagenesis prior to karstiflcation, and paleokarst features include large, dissolution-fragmented Pitkin lithoclasts with numerous solution pipes on a low-relief surface. The overlying, pedogenically altered regolith contains calcretized lithoclasts in clay matrix, suggesting a shift from ever-wet conditions to more seasonal rainfall prior to the Pennsylvanian transgression. That shift may represent local climatic change or short-term fluctuations in mid-Carboniferous tropical climate. The latter interpretation may support global climate models that predict warmer, wetter tropics during periods of high-latitude cooling. Basal Cane Hill strata consist of high-energy, shoreface, boulder-cobble conglomerate containing reworked Pitkin clasts. The conglomerate contains an unequivocal rockyshore community consisting of encrusting bryozoans and corals, Trypanites , acrothoracican barnacle borings, and, possibly, the earliest occurrence of Gastrochaenolites . Preservation of poorly indurated paleosol beneath high-energy transgressive deposits suggests very rapid transgression consistent with a glacio-eustatic mechanism. Truncated Pitkin strata were removed by karstification and subaerial erosion during the hiatus and not by erosion during transgression.

Textural and Isotopic Variations in Diagenetic Kaolinite from the Magnus Oilfield Sandstones

AbstractIn Upper Jurassic sandstones of the Magnus Sandstone Member from the Magnus Oilfield in the North Sea, diagenetic kaolinite morphology and isotopic composition vary away from the Late Cimmerian unconformity. Near the unconformity, coarse-grained (15-30 μm) vermiform kaolinite morphology is most common, whereas downdip, 3 km away, fine-grained (∼10 μm) blocky morphology is prevalent. Kaolinite abundances increase downdip, away from the unconformity, and kaolinite near the unconformity has low δ18O (12·5-14·9%o smow) compared to that downdip (15·9−17·5%o). This reflects replacement of marine depositional pore-waters by meteoric water near the erosion surface. However, isotopic temperature estimates suggest that the majority of kaolinite formed at elevated burial temperatures of ~80°C, and not during subaerial exposure of the sandstone. The δD of kaolinite close to the unconformity is also lower than that downdip. Kaolinite morphology and isotopic composition record meteoric water ingress during Late Cimmerian subaerial erosion and retention of meteoric-derived water in the crest of the Magnus structure during burial diagenesis. Kaolinite formation during subaerial exposure is not of significance to the sandstone reservoir quality.

River Response to Baselevel Change: Implications for Sequence Stratigraphy

Baselevel is the imaginary horizontal level or surface to which sub-aerial erosion proceeds. It is sea level. Controversy surrounds the effect of baselevel change on river behavior, the rejuvenation of landscapes, and the delivery of sediment to the shelf-slope depositional system. The effect of baselevel change depends upon many factors, such as rate of change, amount of change, direction of change, river character, and dynamics and erodibility of the sediment source area. In most cases the effects of baselevel change will be moderate, and they can be accommodated by changes of channel pattern, width, depth, and roughness. Therefore, the delivery of large amounts of sediment to a shoreline or continental shelf probably reflects not only baselevel lowering, but significant uplift of the sediment-source area and perhaps climate change.

Differential response of a Devonian‐Carboniferous platform‐deeper basin system to sea‐level change and tectonics, N. Chilean Andes

AbstractDeposition of a 2700‐m‐thick clastic platform succession in a N‐S striking basin in northern Chile began in the Early Devonian during a global sea‐level rise. A transition to terrestrial facies took place at the Early‐Late Carboniferous boundary when the Gondwana glaciation began and global sea‐level dropped. On the platform, interbedded cross‐bedded or bioturbated sandstones, offshore tidal dunes and sand waves, and mudstones and tempestites suggest switching intertidal and shallow or deep subtidal environments. However, evidence for subaerial erosion indicates a significant regression during the Early Devonian.In an adjacent and deeper N‐S striking sub‐basin to the W, up to 3600 m of turbidites were deposited from the Late Devonian to the Late Carboniferous by mainly southerly palaeocurrents. Turbidites accumulated in coarse‐grained proximal sand lobes in the N, and in fine‐grained lobe fringe and basin plain environments in the S, with alternating upward‐thinning and upward‐thickening cycles typical of tectonically controlled aggradational turbidite systems.The sedimentological data indicate that the deeper basin depositional system evolved to a large extent independently from the platform system. Sediment in the deeper basin is less mature and more poorly sorted than that on the platform, suggesting that detritus bypassed the platform and was shed directly from the source areas into the western basin. The only depositional link between the platform and deeper basin systems seems to be longshore platform currents which may have funnelled minor quantities of mature sand into the deeper basin via bypass canyons. Although platform and deeper basin evolved in a common extensional tectonic setting, the platform reflects eustatic changes of sea‐level whereas deposition in the deeper basin records syndepositional tectonics.

Upper Morrowan Valley-Fill, Sandstone Reservoirs at South Guymon Field, Texas County, Oklahoma: An Integration of Core and Wireline-Log Stratigraphic Studies

South Guymon field, located in south-central Texas County, Oklahoma panhandle, has produced more than 238 bcf of gas and 430 MBO and condensate from sandstone reservoirs in the Lower Pennsylvanian, Upper Morrow and Lower Morrow formations. Upper Morrowan hydrocarbon production in the southern part of South Guymon field is from two conglomeratic sandstone units. The integration of core and wireline-log stratigraphic studies indicates that the two sandstones record valley-fill deposition in two distinct depositional sequences separated by a major lowstand surface of erosion unconformity. During periods of subaerial exposure and erosion associated with Upper Morrowan eustatic drops in sea level, fluvial valleys were incised in older Upper Morrowan marine shales and previously deposited Upper Morrowan valley-fill sequences. With the initiation of sea level rise, incised valleys began to fill with fluvial sediments. Fluvial deposits in the southern part of South Guymon field are light-gray, medium-scale, cross-stratified and horizontally stratified, coarse-grained sandstones and conglomeratic sandstones. Core study, dipmeter analysis, and detailed mapping of southern part of South Guymon field suggest these sandstones were deposited by braided and coarse-grained meandering streams. Continued transgression resulted in marine drowning of the incised valleys and development of restricted, quiet-water bays. Quiet-water bay deposits contain commonmore » sand-size uranium-enriched carbonaceous plant debris that results in high gamma-ray log values allowing wireline log recognition of these deposits. Isopach mapping of the cumulative thickness of fluvial and bay valley-fill deposits shows an approximate width of 0.7 to 3 mi (1 to 5 km) and a north-northeast and west-northwest trend for the Upper Morrowan incised valleys in the southern part of South Guymon field.« less

3-D Quantitative Modeling of Reservoir Anatomy from Outcrop Data: The Roda Deltaic Complex, Spain

The current interest of the oil industry toward stochastic reservoir modeling has emphasized the importance of [open quotes]going back to the outcrop,[close quotes] in view of providing reliable geological input to these stochastic models. The roda Sandstone (Spain) offers excellent three-dimensional (3-D) exposures on a kilometric scale. As part of an E. E. C.-sponsored project, a study has been initiated to get a better understanding, through geological and computer modeling, of the architecture of this tide-reworked deltaic complex. This presentation will focus on the method of 3-D modeling and discuss the specific problems that are encountered when using outcrop data. Geological modeling is performed in three stages, the results of which have an impact on each other and lead to a series of iterations until the final result is satisfying: geological field work (outcrop observations, measurement of field sections, and core drills), correlation of genetic units and characterization of their internal heterogeneities (sedimentological and petrophysical facies and porosity/permeability measurements from core plugs and logs), and construction of a 3-D computer model of the geological and petrophysical architecture. Some specific problems are linked to the use of outcrop data. (1) Field sections are not equally distributed all over the studied area;more » moreover, they are rarely vertical and, due to recent subaerial erosion or vegetal cover, are often incomplete. Advanced methods of surface interpolation are needed, based on precise topographic survey and use of surface dips. Special attention must be paid to the interpolation model that is used, particularly in folded areas. (2) Due to weathering, petrophysical measurements on samples taken from outcrops and samples taken from deep core drills may be very different and careful corrections must be applied before modeling petrophysical heterogeneties.« less

Sequence Stratigraphy and Facies Model of an Incised Valley Fill: The Gironde Estuary, France

ABSTRACT The Gironde estuary was formed by the Holocene drowning of a fluvial valley incised during the Wurm global sea-level fall. A depositional sequence accumulated in the valley during the eustatic lowstand, the Holocene rise, and the post-Holocene highstand. The sequence comprises a diverse assemblage of lithofacies that can be grouped into lowstand, transgressive, and highstand systems tracts. The lowstand systems tract comprises a continuous unit of relatively thin fluvial gravel and coarse sand in the thalweg of the incised valley. The transgressive systems tract comprises the bulk of the incised valley fill and forms a landward-thinning wedge of tidal-estuarine sands and muds. In the estuary mouth these are overlain by a thick unit of coarse-grained, estuary-mouth tidal-inlet and tida -delta sands. The highstand systems tract, initiated at about 4000 BP, forms a seaward-prograding, tide-dominated estuarine bayhead delta that has been gradually filling the estuary since the post-Holocene stillstand. Coeval with this filling of the estuary, the adjacent oceanic shoreline has been starved of sediment and is being eroded by waves, indicating that transgressive and highstand systems tracts locally can be synchronous. Several important stratigraphic surfaces punctuate the valley fill: the sequence boundary, the transgressive surface, the tidal ravinement surface, the wave ravinement surface, and the maximum flooding surface. The stratigraphic expression of the sequence boundary depends on its position within the valley. In the thalweg it separates lowstand fluvial deposits from underlying Tertiary carbonates; on the valley walls it is directly overlain by transgressive estuarine sediments. On the interfluves the unconformity continues to be enhanced by modern subaerial erosion. As the interfluves are progressively transgressed by the eroding shoreline, the sequence boundary is expressed as a wave ravinement surface with transgressive marine sediments unconformably overlying Pleistocene or Tertiary ubstrates. The stratigraphic expression of the transgressive surface in the valley thalweg is characterized by onlap of transgressive tidal-estuarine sediments onto lowstand fluvial deposits. On the valley walls the transgressive surface merges with the sequence boundary. Tidal scour at the estuary mouth forms a deeply erosional tidal ravinement surface overlain by thick estuary-mouth wave- and tide-reworked sands. Subsequently these tidal-inlet sands are eroded by waves associated with the passage of the transgressing shoreline to produce a wave ravinement surface. In the distal part of the estuary the maximum flooding surface is expressed as a downlap surface where the regressive highstand estuarine muds prograde over transgressive tidal-estuarine muddy sands or estuary-mouth sands. In the upstream or proximal end of the estuary the maximum flooding surface separates identical facies, i.e., transgressive and regressive tidal-estuarine point bars, and would be very difficult to identify.

Sedimentary Geology and Evolution of an Outcropping Shelf-Margin Delta, Late Proterozoic Wonoka Formation, South Australia

Discrete sand units or accumulations crop out within the late Proterozoic Wonoka Formation, northern Flinders Ranges, South Australia. These sand wedges are unique in their size, contacts, facies, and associations to other sands in the formation and are interpreted as shelf-margin deltas. The wedges can be divided into a main body and a series of discrete feeder channels. The main body consists of basal slump masses, a sandstone-dominated succession, and an overlying shale-dominated facies. Lithofacies grossly define a deepening-upward succession from a lower shoreface to outer shelf setting. The formation and deposition of the feeder channels and sand wedges was controlled largely by relative sea level changes. Initial exposure of the shelf and subaerial erosion produced a regional erosional surface and incised valleys across the paleoshelf. Prior to deposition of the wedges, slope instabilities and minor uplift occurred. Sediment was then transported across the shelf and deposited near or at the shelf break and reworked in a shallow marine environment. Deposition of the shelf-margin deltas continued during relative sea level rise, but ended with shelf drowning.

The Ilchulbong tuff cone, Cheju Island, South Korea

ABSTRACTThe Ilchulbong mount of Cheju Island, South Korea, is an emergent tuff cone of middle Pleistocene age formed by eruption of a vesiculating basaltic magma into shallow seawater. A sedimentological study reveals that the cone sequence can be represented by nine sedimentary facies that are grouped into four facies associations.Facies association I represents steep strata near the crater rim composed mostly of crudely and evenly bedded lapilli tuff and minor inversely graded lapilli tuff. These facies suggest fall‐out from tephra finger jets and occasional grain flows, respectively. Facies association II represents flank or base‐of‐slope deposits composed of lenticular and hummocky beds of massive or backset‐stacked deposits intercalated between crudely to thinly stratified lapilli tuffs. They suggest occasional resedimentation of tephra by debris flows and slides during the eruption. Facies association III comprises thin, gently dipping marginal strata, composed of thinly stratified lapilli tuff and tuff. This association results from pyroclastic surges and cosurge falls associated with occasional large‐scale jets. Facies association IV comprises a reworked sequence of massive, inversely graded and cross‐bedded (gravelly) sandstones. These facies represent post‐eruptive reworking of tephra by debris and stream flows.The facies associations suggest that the Ilchulbong tuff cone grew by an alternation of vertical and lateral accumulation. The vertical buildup was accomplished by plastering of wet tephra finger jets. This resulted in oversteepening and periodic failure of the deposits, in which resedimentation contributed to the lateral growth. After the eruption ceased, the cone underwent subaerial erosion and faulting of intracrater deposits. A volcaniclastic apron accumulated with erosion of the original tuff cone; the faulting was caused by subsidence of the subvolcanic basement within the crater.