Subaerial Erosion Research Articles

Deposition and Interpretation of Long, Narrow Sandbodies Underlain by a Basinwide Erosion Surface: Cardium Formation, Cretaceous Western Interior Seaway, Alberta, Canada

ABSTRACT Sand bodies of the Cardium alloformation (Burnstick allomember) occur in three parallel linear trends in the Garrington, Caroline-Crossfield and Lochend oilfields. They are extremely long (up to 100 km), straight and narrow (up to 4 km) and are encased in marine mudstones, suggesting their former interpretation as offshore bars. Our study of 189 cores and over 800 resistivity well logs shows that, below the Burnstick, the Hornbeck allomember consists entirely of dark mudstones deposited in quiet water. The Hornbeck--Burnstick contact, where seen in cores, is extremely sharp. In cross sections, resistivity well log markers are truncated by the lower surface of the Burnstick, and the sand bodies rest in asymmetrical one-sided elongate scours, with erosional relief of up to 7 m. Above he scoured surface, there is a basal lag of mudstone intraclasts and chert pebbles, overlain by a sandier-upward succession of mudstones, sandstones and conglomerates. The sandstones are extensively burrowed but show some cross stratification. The Burnstick sandier-upward succession is overlain by black mudstones of the Raven River allomember. The morphology of the scoured surface, as defined in isopach maps, does not indicate its formation in an open marine setting. The long, straight, asymmetrical scours are parallel to regional strike and do not suggest subaerial erosion. Instead, we suggest that a major relative lowering of sea level first exposed the Hornbeck mudstones to subaerial erosion. Subsequently, marine transgression of this surface removed evidence of its subaerial exposure. Pauses in transgression (stillstands) resulted in incised shoreface profiles (the asymmetrical scours), and at this time rivers supplied coarse sediment to the shoreface. This sediment was reworked alongshore to form what are now the very long, straight narrow sand bodies. Continued transgression resulted in the burial of the shoreface sand bodies by black mudstone (basal part of the Raven River allomember). Incised shoreface environments provide an alternative to the offshore bar settings suggested for other long, linear sand bodies.

Diagenetic Pore Waters Stratified for at Least 35 Million Years: Magnus Oil Field, North Sea: Geologic Note (1)

Isotopically distinct pore waters remained stratified in the Upper Jurassic Magnus Sandstone, reservoir for the Magnus oil field in the northern North Sea, throughout diagenesis. A petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths, kaolinite, magnesian siderite, ankerite, and illite developed across the field. However, the oxygen isotope ratios (delta{18}O) recorded in these diagenetic minerals from the crest of the field are consistently more negative (by about 3 o/oo) than those from the downdip flank. We propose that this consistent difference in delta{18}O values records the persistence of isotopically different and stratified pore waters in the Magnus sandstones. The sandstones w re deposited as marine turbidites, but were subaerially exposed during periods of structural uplift and tilting during the Early and middle Cretaceous. Meteoric water entered the structural crest, resulting in pore-water stratification. Deposition of thick Upper Cretaceous mudstones and marls sealed the Magnus structure during subsequent rapid burial. Diagenetic minerals in the crest of the oil field grew in a pore water with significantly more meteorically derived water than those downdip, assuming that each diagenetic phase developed at a similar temperature in each well. The pore waters remained stratified for at least 35 m.y., from middle Cretaceous (approximately 90 Ma) subaerial erosion of the sandstone until reservoir charging at 55 Ma. The sandstones and mudstones of the Kimmeridge Clay Formation behaved essentially as a closed system during diagenesis, in contrast to the open-system concept envisaged in most other diagenetic reconstructions.

Geology of the Subsurface Igneous Rocks of Sunflower County, Mississippi

ABSTRACT Mesozoic, Paleozoic, and Precambrian igneous rocks have been penetrated in the subsurface of northwestern Mississippi by the Pan American (Amoco) No. 1 Word, Sunflower County. This well encountered Late Cretaceous waterlain volcaniclastics, Late Jurassic trachytes, Middle to Late Ordovician basic intrusives, and Late Precambrian syenite. The Cretaceous volcaniclastic sequence occurs 85 m below the top of the Eutaw Formation and is comparable to rock types penetrated in northwest Mississippi dated at 78.3 to 91.3 Ma. The Jurassic igneous rocks comprise an 805 m trachyte flow complex 270 m below the top of the Cotton Valley Formation and were dated at 174 ± 8 Ma. Macroscopically, they are gray-green to gray-black, very fine to finely crystalline, slightly porphyritic, amygdaloidal flow rocks which have been severely altered and locally intruded by Cretaceous-Age(?) dikes. The Ordovician igneous section is apparently restricted to basic igneous intrusives in the Precambrian syenite and has been dated at 468 ± 19 Ma. The deepest igneous sequence encountered by the No. 1 Word is a 306 m interval of white to greenish-gray, fine- to medium-grained Precambrian syenite dated at 785 ± 34 Ma. The Precambrian section apparently formed during the rifting of North America and possibly represents the peripheral cratonic margin of Paleozoic North America. Tectonic uplift and erosion at the end of the Carboniferous Ouachita Orogeny removed the Paleozoic sedimentary section. The sole relict of this sequence is believed to be an 18 m conglomeratic quartzite of unknown age which lies between the Jurassic and the Precambrian. The Jurassic flows emanated from volcanic features associated with the Mesozoic igneous complex of the Monroe-Sharkey uplift. Several eruptive phases are apparent from petrographic and seismic evidence. This section was exposed to subaerial erosion only briefly before burial by younger Jurassic sediments. Between the Jurassic trachyte and the Cretaceous volcaniclastics are 878 m of sedimentary rocks of the Cretaceous Tuscaloosa Formation, undifferentiated Lower Cretaceous strata, and upper Jurassic Cotton Valley Formation. The Cretaceous volcaniclastics apparently reached the surface as tuffaceous material, were weathered, and redeposited to form the unit preserved today. In conclusion, all of this igneous activity has been concentrated in northwest Mississippi at the confluence of the Precambrian cratonic rift margin, the Paleozoic - Mesozoic intracratonic Reelfoot Rift, and the Mesozoic Gulf of Mexico peripheral fault zone. Further, this tectonism has created important structural and stratigraphic traps that should be excellent exploration objectives.

Detrital record of the early Central American magmatic arc: Petrography of intraoceanic forearc sandstones, Nicoya Peninsula, Costa Rica

The framework compositions of sand-stones from the Costa Rican forearc provide a compositional norm for detrital modes from basins associated with intraoceanic arcs, a poorly documented tectonic setting. Nicoya Peninsula sandstones are made up of mainly volcanic lithic fragments, plagioclase, sedimentary lithic fragments, and mafic accessory minerals. Modal point counts of 70 samples average Q 3 F 32 L 65 , Qm 2 F 32 Lt 66 , Qp 1 Lvm 72 Lsm 27 , and Qm 6 P 94 K 0 , defining these sand-stones as less quartzose than sandstones from most other tectonic settings and less lithic than most quartz-poor sandstones from intraplate oceanic islands. Four newly defined modal parameters quantify temporal changes in composition. These parameters are (1) Vi/V′, the ratio of volcanic fragments with microlitic or felsitic textures to those with lathwork, microlitic, or felsitic textures; (2) Mh/M, the ratio of hydrous to total mafic silicate mineral grains; (3) Po/P, the ratio of oscillatory-zoned to total plagioclase grains; and (4) Sc/S, the ratio of reworked carbonate detritus to total sedimentary lithic fragments. All four parameters increase from negligible values in the basement unit to moderate or high values in cover units. These increases are followed by generally low values in the youngest (Miocene) sandstones. Significant temporal changes are not revealed by previously defined modal parameters that are commonly used to analyze sandstone composition. Petrography of these sandstones provides strong evidence for the initial construction of an andesitic arc edifice by Campanian time (Late Cretaceous). Increases in Vi/V′, Mh/M, and Po/P ratios through most of the section record increased detrital input from evolved volcanic sources. A parallel increase in Sc/S ratios reflects the progressive construction and erosion of a carbonate-rich sedimentary are apron. The Miocene reversal of these trends reflects subaerial erosion of up-lifted basement rocks at the forearc structural high. Miocene sandstones also contain minor plutonic detritus that includes potassium feldspar, signaling incipient unroofing of the arc. Sandstones of the Nicoya Peninsula thus preserve a relatively complete record of the evolution and erosion of the Upper Cretaceous through Miocene intraoceanic arc of southern Central America.

Large‐scale stratigraphic architecture, eustatic variation, and unsteady tectonism: A theoretical evaluation

We incorporate a process‐based depositional model with basin subsidence models to predict stratigraphic records. This allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins. The models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin, the record of episodic tectonism is distinct from that of eustatic sea level change. In the model, sediment transport is approximated by slope‐controlled diffusion; nonmarine transport is treated as more efficient diffusion than marine transport. Three different subsidence and sediment supply models are examined: a simple passive margin basin, a simple foreland basin, and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief. Predicted passive margin stratigraphies, for cases of varying eustatic sea level, are similar to those of natural basins and include progradational packages and subaerial unconformities, which are used to define sequence boundaries that form during sea level fall. We explore the timing relationships between stratigraphic features and a sinusoidal sea level history, showing that the phase relationship depends on subsidence, sediment flux, efficiency of sediment transport, and period and amplitude of sea level. When the basic geometry of the basin is inverted, placing the sediment supply on the side with maximum subsidence as is the case in foreland basins, the sequence character changes markedly: subaerial erosion does not generate unconformities. In the models of a dynamic foreland basin, sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant. For steady thrusting and variable sea level, unconformities that define sequence boundaries form only on the distal or forebulge side of the basin, and the ages of the sequence boundaries correlate to limes of rising sea level. In cases of constant sea level but variable thrusting, subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin, yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle: erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side.

Topography and Significance of a Basinwide Sequence-bounding Erosion Surface in the Cretaceous Cardium Formation, Alberta, Canada

ABSTRACT The Cardium Formation (Turonian-Coniacian, Upper Cretaceous) of the Western Interior Seaway, Alberta, contains six marine sandier-upward successions bounded by seven basin-wide erosion surfaces. In the subsurface, erosion surface E5 is associated with major oil and gas fields and is penetrated by thousands of wells. A topographic map of E5, based on 6126 well logs and over 1100 measured cores, shows a series of subparallel elongate highs (steps) and lows which trend northwest-southeast, parallel to tectonic trends and to the actively rising cordillera to the southwest. The steps are about 10 km apart, and the relative relief, step to adjacent low, is up to 25 m. The steps are asymmetrical, with a steeper face to the northeast. The E5 erosion surface cuts into offshore marine hummock cross-stratified sandstones and interbedded mudstones, and it records a lowering of relative sea level. Our interpretation suggests that erosion was in neither a fully subaqueous nor a fully subaerial environment. Instead, it involved relative sea-level lowering and subaerial erosion, followed by major modification of the landscape during transgressive shoreface erosion. At times of relative stillstand during overall transgression, gravel was supplied to the shoreface and was reworked a short distance alongshore by waves. These conglomerates now form important oil and gas reservoirs. The asymmetry of the steps and lows indicates that the originally flat sea floor was tilted northeastward immediately before erosion, giving a gradient of about 1:1000. During transgression, shorefaces were incised into the tilted surface to form a stepped morphology. Also during transgression, the sea floor was gradually restored to its pre-tilting position. Each of the six marine sandier-upward sequences, along with the erosion surface at the top, occupies an average of 470,000 years, and we estimate that transgression took at least 100,000 to 200,000 years.

Shallow Burial Dolomitization of Mid-Cenozoic, Cool-Water, Calcitic, Deep-Shelf Limestones, Southern Australia

Oligocene to middle Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably replaced by green to orange, Ca-rich, zoned, medium-crystalline, sucrosic dolomite. The degree of replacement varies from scattered rhombs in limestone to complete dolostones a few tens of meters in thickness and a few kilometers in lateral extent. Dolostone texture ranges from dense and well lithified to completely unlithified, resembling a loose sand of dolomite rhombs. Dolomitization is fabric specific; calcite and Mg-calcite bryozoans are either the last components to be replaced or are molds. The timing and locale of dolomitization are tightly constrained; Sr isotopes indicate a middle to late Miocene age while clasts of dolostone in overlying Pliocene limestones above a regional unconformity confirm a shallow-burial, pre-Pliocene origin. {delta}{sup 13}C and {delta}{sup 18}O values support a marine source for the carbonate; the influence of meteoric fluids appears to have been negligible. Quaternary exposure has resulted in local dedolomitization and/or subaerial erosion, especially in the Murray basin. These rocks are excellent analogues for localized, lenticular dolostone bodies in calcite-rich Paleozoic platform carbonates.

Tectonic Sequence Stratigraphy, Early Permian Dry Mountain Trough, East-Central Nevada

The Early Permian Dry Mountain trough (DMT) of east-central Nevada is one of several tectonic basins and associated uplifts that developed along the continenetal margin during the latest Pennsylvanian-Early Permian Dry Mountain tectonic phase. The sequence stratigraphy reflects a combination of eustatic sea level changes and tectonic uplift or subsidence. Fewer than one to only a few million years separate the development of sequence boundaries within the DMT. At this scale, differences among published eustasy curves preclude their use as definitive tools to identify eustatically controlled sequence boundaries. Nevertheless, available data indicate several pulses of tectonism affected sedimentation within the DMT. The authors are attempting to develop criteria to distinguish tectonic from eustatic sequence boundaries. Detailed biostratigraphic data are required to provide an independent check on the correlation of sequence boundaries between measured sections. For example, the same age boundary may reflect tectonic uplift in one part of the basin and subsidence in another. The uplift may or may not result in subaerial exposure and erosion. For those boundaries that do not result from subaerial exposure, lithofacies and biofacies analyses are required to infer relative uplift (water depth decrease) or subsidence (water depth increase). There are inherent resolution limitations inmore » both the paleontologic and sedimentologic methodologies. These limitations, combined with those of eustasy curves, dictate the preliminary nature of their results.« less

A simulation model of island reef morphology: the effects of sea level fluctuations, growth, subsidence and erosion

Glacioeustatic sea level fluctuations continually cover and expose reefs, alternately allowing growth or erosion to operate. In a simulation model we examine the simultaneous effects of sea level change, island subsidence, reef growth, subaerial erosion, marine backwearing, and fluvial erosion (from central highlands) on reef development. Using values obtained from the literature, we vary the rates of these processes and compare the reefs produced. Our results indicate that subaerial erosion, subsidence and growth are of comparable importance in determining reef morphology. Fore reef terraces, as developed by the model, are primarily drowned growth features; marine backwearing is of little importance in their development. Reef terraces form readily at depths that never had a stable sea stand, their depth is influenced by growth, subaerial erosion, and subsidence rates. Thus reef terraces often do not indicate former sea stands. We examine the causes of reef drowning and attribute it primarily to rapid subsidence and subaerial erosion, not to truncation through marine backwearing. We propose that reefs deeply submerged today are not necessarily “drowned out”, but may be vertically stable through many sea level cycles. Fluvial erosion is likely an important agent of lagoon formation on high islands in areas with high erosion rates.

Correlation of Sea Level Falls Interpreted from Atoll Stratigraphy with Turbidites in Adjacent Basins: ABSTRACT

Past sea levels can be derived from any atoll subsurface sediments deposited at or near sea level by determining the ages of deposition and correcting the present depths to the sediments for subsidence of the underlying edifice since the times of deposition. A sea level curve constructed by this method consists of discontinuous segments, each corresponding to a period of rising relative sea level and deposition of a discrete sedimentary package. Discontinuities in the sea level curve derived by this method correspond to relative sea level falls and stratigraphic hiatuses in the atoll subsurface. During intervals of relative sea level fall an atoll emerges to become a high limestone island. Sea level may fluctuate several times during a period of atoll emergence to become a high limestone island. Sea level may fluctuate several times during a period of atoll emergence without depositing sediments on top of the atoll. Furthermore, subaerial erosion may remove a substantial part of the depositional record of previous sea level fluctuations. For these reasons the authors must look to the adjacent basins to complement the incomplete record of sea level change recorded beneath atolls. During lowstands of sea level, faunas originally deposited near sea level onmore » an atoll may be eroded and redeposited as turbidites in deep adjacent basins. Three such turbidites penetrated during deep-sea drilling at Sites 462 and 315 in the central Pacific correlate well with a late Tertiary sea level curve based on biostratigraphic ages and {sup 87}Sr/{sup 86}Sr chronostratigraphy for core from Enewetak Atoll in the northern Marshall Islands. Further drilling of the archipelagic aprons adjacent to atolls will improve the sea level history that may be inferred from atoll stratigraphy.« less

Post-Miocene diagenetic and eustatic history of Enewetak Atoll: Model and data comparison

The post-Miocene diagenetic and eustatic history of Enewetak Atoll was investigated using a one-dimensional forward model. Comparison of model and data suggests that the post-Miocene history of Enewetak Atoll was dominated by multiple episodes of meteoric phreatic diagenesis attendant with high-frequency (10{sup 4} to 10{sup 5} yr) fluctuations in sea level and a subsidence rate of 39.0 m/m.y. Sensitivity testing indicates that subaerial erosion results in the preservation of additional subaerial unconformities because stratigraphic shortening permits a succeeding sea-level rise to flood the exposure surface and deposit sediment, whereas without subaerial erosion this sea-level rise would be recorded as a paleophreatic lens. Model results indicate that less then 10% of lapsed time is recorded by sediment deposition during periods of high-frequency changes in sea level. Incompleteness of the stratigraphic record suggest that magnetostratigraphy may give erroneous ages for shallow-marine carbonate sequences deposited during times of high-frequency changes in sea level and frequent magnetic polarity reversals.

Geochemical and Mineralogical Evidence for Relative and Eustatic Sea Level Changes in Eocene to Oligocene Carbonates in Alabama: ABSTRACT

Eocene to Oligocene marine shelf carbonates cored in Baldwin County, Alabama, were subdivided into a sequence stratigraphic framework and sampled at closely spaced intervals to determine characteristic geochemical and mineralogical responses to fluctuations in eustatic and relative sea level. Except for the Eocene/Oligocene boundary interval, the carbonates were deposited landward of their respective depositional shelf edges. Sequence boundaries, transgressive surfaces, and surfaces of maximum starvation were delineated by lithologic and textural variations and gamma logs. Comparison of the Baldwin County core data with correlative Deep Sea Drilling Project/Ocean Drilling Program (DSDWODP) records reveal more clearly defined stable isotopic data trends due to greater sampling density per unit time. In the sequence stratigraphic framework the {delta}{sup 13}C signature mimics relative sea level changes (paleobathymetry) and changes in the O{sub 2} minimum; whereas, the {delta}{sup 18}O signature records the eustatic sea level history. Because the stable isotopes are recording different aspects of sea level history, it is necessary to evaluate isotopic signatures independently. Although minor diagenetic perturbations in the stable isotopic signals are found, the depositional signal clearly dominates the diagenetic signal. The paleogeographic position of this core gives a more accurate sea level record than either more landward sections presently exposedmore » in outcrop that have undergone intense subaerial alteration and erosion or DSDP/ODP records that may be so condensed that the record appears incomplete in conventional sampling schemes . Authigenic mineralization related to sea level changes can be used to characterize surfaces within the depositional sequence framework. Trace elemental data shows that hydrologic flow, porosity, and cementation trends are subsequently controlled by the sequence framework.« less

堆積シーケンスとその境界の認定と意義

The depositional sequence concept was established in newly developed sequence stratigraphy, as an unconformity-bounded stratigraphic unit formed during one complete sea-level cycle. This paper reviews general meanings of “sequence”, the definition of depositional sequences, their hierarchial patterns and recognition, and sequence boundaries problems, from a viewpoint of sedimentary geology based on outcrops and bore-hole samples.Though the word, “sequence” has many meanings generally applied to successive geologic events and processes in chronologic order, a depositional sequence is defined in a special sense, as “a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their correlative conformities”. The depositional sequence as one of hierarchial transgressive and regressive units (T-R units), has the first- to forth-order operational units, that is, the megasequence, supersequence, sequence and parasequence in descending order. A sequence boundary with a significant hiatus (=unconformity) is formed by subaerial exposure, concurrent subaerial erosion and partly submarine erosion during eustatic falls or low-stand sea level.The latter half of this paper emphasizes the difference between sequence boundaries and ravinement surfaces. The ravinement surface formed as one of diastems or “transgressive surfaces”, is an erosional surface by shoreface retreat during the following transgression after a sea-level fall. In general, it is lithologically more distinct than the underlying sequence boundary. The right recognition of the difference leads correct reconstructions of sedimentary history.

Maastrichtian to early Tertiary stratigraphy and palaeogeography of the Central and Northern Oman Mountains

Abstract Maastrichtian to Tertiary sedimentary rocks outcrop extensively around the periphery of the Oman Mountains. They form part of a geologically distinct suite of strata deposited after obduction of the Semail ophiolite and thrusting of the Hawasina and Sumeini imbricates onto the eastern edge of the Arabian plate. The lithostratigraphy of Maastrichtian and early Tertiary rocks of the central and northern Oman Mountains is revised and a number of new formations are erected. The proto-Oman Mountains were emergent in the early Maastrichtian and were subject to subaerial erosion and weathering. NW and W of the mountains, gradual subsidence followed during the Maastrichtian, allowing for formation of a transgressive, onlapping sequence represented by the Qahlah (fluviatile to shallow marine terrigenous clastics) and Simsima (shallow shelf carbonate) Formations. More rapid subsidence NE of the mountains produced fan delta and submarine debris flow/turbidite apron sequences comprising the Al Khawd and Thaqab Formations respectively. A low-angle unconformity separates Tertiary strata from older units. During the Palaeogene, basinal and slope facies with debris flows and turbidite sequences were deposited directly upon the unconformity surface in rapidly subsiding basins NW (Muthaymimah Formation) and NNE (Ruwaydah Formation) of the Oman Mountains. Elsewhere, shallow carbonate shelf facies were extensively developed in less rapidly subsiding areas during the Palaeogene. The Palaeocene and early Eocene is represented by the Jafnayn Limestone Formation. This is succeeded and overlapped by a variety of later Eocene units. ENE and E of the mountains early Eocene regression is marked by restricted facies of the Rusayl Formation. Overlying the Rusayl Formation are shallow, open shelf limestones of the Seeb Formation which is rich in alveolinid and nummulitic foraminifera. On the Arabian side of the mountains the Rusayl Formation is not developed and the Jafnayn Formation is succeeded either by the Seeb Formation or fine-grained, faunally restricted limestones and marls of the Fahud Beds, which represent intra-shelf facies.

Late Proterozoic Patsy Springs Canyon, Adelaide Geosyncline: submarine or subaerial origin?

ABSTRACTA significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre‐deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin‐slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon‐filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon‐fill, has a stable isotope signature which may imply a non‐marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon‐filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base‐level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally‐induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian‐style evaporitic lowering of base‐level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations.

Pleistocene geology and geomorphology of the Smoking Hills Upland and lower Horton River Arctic coast of mainland Canada

The Smoking Hills Upland and lower Horton River valley, on the Arctic coast of Canada 300 km east of the Mackenzie Delta, retain an extended record of subaerial erosion of nearly flat-lying Cretaceous shales. This erosion led to the development of (i) a very gently sloping low-relief upland surface (Early Pleistocene(?)), (ii) a slightly steeper intermediate surface, and (iii) younger steep valley walls, terraces, and broad valley bottoms. No direct glacial contribution to any of these landforms can be recognized. An early interglacial(?) fluvial episode is recorded in plateau-cap sediments. Suggestions of an early (mid-Pleistocene or earlier) glaciation overwhelming the Smoking Hills Upland are found in (i) anomalies in drainage patterns, (ii) disturbances in bedding, believed to have been caused by ice thrusting, and (iii) local occurrences of diamictons. Later, probably Early Wisconsinan, glaciation left meltwater channels in peripheral areas. The Late Wisconsinan ice sheet did not reach the Smoking Hills Upland but may have had an indirect influence by modyfying discharge and sediment transport of Horton River.

Late Quaternary evolution of the northern Aegean shelf

A dense network of 3.5 kHz and Uniboom seismic profiles revealed the geological evolution of the Ierissos-Alexandroupolis Shelf area of the northern Aegean during the latest sea-level rise. Near the end of the Pleistocene, ca. 14,000 yr B.P., the sea was at about − 120 m, and almost 5300 km 2 of shelf was exposed to subaerial erosion. Two permanet lakes existed in the Ierissos and Kavalla Gulfs, as well as a number of ephemeral lakes scattered throughout the rest of the area. Kavalla Gulf was drained by the Nestos River which joined the Strymon River at the outer Strymonikos Plateau. On the flat Samothraki Plateau were dune fields, marshes, and a number of minor seasonal rivers, while the Evros River flowed east of the plateau. By ca. 13,000 yr B.P. the sea had risen to − 70 m and covered 30% of the previously exposed shelf. The lakes at Ierissos and Strymonikos Gulfs were shallower and the sea approached to within about 5 km of them. Kavalla Gulf and the adjacent eastern Strymonikos Plateau lay 10 m above sea level, while the sea intruded along river mouths at the Samothraki Plateau. During this transgression of the sea river-bed gravels and sands were covered by silts and clays. At the Pleistocene-Holocene transition, ca. 10,500 yr B.P., the sea lay at about − 50 m and in areas of steep slope the coast was close to its present position. At Kavalla Gulf, the sea advanced along the paleochannel of the Nestos River, and the island of Samothraki was separated from the mainland. By ca. 7500 yr B.P. the sea was only 15 m below its present level and the northeastern Aegean shelf assumed nearly its present morphology. The Nestos River changed course to the east and Thassos Island was separated from the mainland. Coarse sediment formed wedges nearshore, whereas fine-grained sediments were distributed widely by current action. In many areas, relict sediments are present.

Critical Depth for the Survival of Coral Islands: Effects on the Hawaiian Archipelago

Coral islands drown when sea level rise exceeds the maximum potential of coral reefs to grow upward (about 10 millimeters per year). During the Holocene transgression (18,000 years ago to present) sea levels rose at rates of up to 10 to 20 millimeters per year, and most coral island reefs situated deeper than a critical depth of30 to 40 meters below present day sea level drowned. Coral islands that did not drown during the Holocene transgression apparently all developed on antecedent foundations shallower than critical depth. During low stands in sea level during the Pleistocene, these islands were elevated and subject to subaerial erosion. Today, in the Hawaiian Archipelago, the depth of drowned banks is inversely related to summit area; smaller banks are progressively deeper, evidently because of erosional truncation during low sea level stands. Bank summit area may therefore be an important factor determining the failure or success of coral islands.

Sedimentary Architecture of Marginal and Shallow Marine Depositional Systems Tracts in Upper Cretaceous Series of San Juan Basin: ABSTRACT

The Upper Cretaceous series of the San Juan basin consists of five (or more ) depositional sequences bounded partially by subaerial unconformities and partially by correlative, basinward conformities. Three depositional systems tracts account for nearly all the preserved strata: highstand, lowstand, and transgressive. At relative sea level highstand, sediments are accommodated at the most landward sites within the basin (e.g., early Gallup Sandstone tongues). This systems tract is characterized by prograding bay-head deltas and successive open-marine deltas. During the ensuing sea level fall, much of the highstand system is removed by subaerial erosion. At relative sea level lowstand, sediments accumulate at the most basinal sites in the form of prograding strand plains, associated shelf sand sheets and plumes, and a minor amount of fluvial channel and overbank deposits. During transgression the coastal plain is aggrading and hence traps most terrigenous clastics before they reach the open coast. The attendant erosional retreat of the shoreface, however, releases coarse sediment (sand) to the shelf, where it accumulates in large-scale shelf sand ridges. These ridges are the principal petroleum reservoirs in the Cretaceous rocks of the basin (Tocito Sandstone Lentil, parts of the Twowells Tongue of the Dakota Sandstone, etc.). During the transgressionmore » a carbonate-dominated condensed section encroaches landward, stratigraphically above the transgressive depositional systems. This surface attains its maximum landward extent some time between peak rate of sea level rise and highstand. The three principal surfaces of the depositional sequence - the sequence boundary, the transgressive ravinement surface, and the condensed section - all have clearly defined sedimentological characteristics.« less

Unconformities, Diastems, and Condensed Section in Upper Cretaceous Series of San Juan Basin: ABSTRACT

The Upper Cretaceous series of the San Juan basin consists dominantly of terrigenous clastics derived from the Sevier orogenic belt to the west. The marine and marginal marine deposits range in age from middle Cenomanian to Campanian. Several distinct chrono- and lithostratigraphic breaks occur within this succession, the origin of which must be precisely determined to correctly interpret the genetic stratigraphy of the basin fill. Major chronostratigraphic breaks are produced both by subaerial erosion on the basin flank during relative sea level fall (depositional sequence boundaries) and in the form of omission surfaces in the marine section. The most distinctive lithostratigraphic breaks form where depositional systems representing great differences in physical energy become juxtaposed. These two categories of surfaces may or may not coincide. Depositional sequence boundaries are best identified where paleosols have formed on marine sediments or where an abrupt shallowing of facies can be recognized (equivalent to the seismic stratigraphic pattern of downward shift in onlap). Sequence boundaries are associated with regional incision of the coastal plain, while their marine portions generally are conformable. Omission surfaces record intervals when marine sedimentation was absent. These may be firmgrounds, hardgrounds, or erosional surfaces, often recording significant biostratigraphic breaks. The mostmore » significant omission surfaces in the Cretaceous rocks of the San Juan basin may have formed at times of rapid rise of relative sea level, when terrigenous sediment supply to the marine environment was effectively shut off. Such surfaces, therefore, represent the top of marine condensed sections and probably formed sometime between maximum rate of sea level rise and highstand.« less