Large-scale Cycle Research Articles

Sequence stratigraphy and carbonate platform organization of the Devonian Santa Lucia Formation, Cantabrian Mountains, NW-Spain

In this paper, the sedimentology and the stratigraphic architecture of the Devonian Santa Lucia Formation in the Cantabrian Mountains of NW-Spain are described. The Santa Lucia Formation consists of 11 different facies that can be attributed to peritidal/lagoonal, intertidal and subtidal facies associations. These facies associations are arranged in small-scale sedimentary cycles. Three different settings of small-scale sedimentary cycles are recognized: intertidal/supratidal, shallow subtidal/intertidal and subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale stratigraphic cycles are stacked into seven medium-scale cycles that in turn are integral parts of three larger-scale cycles. Most of the Santa Lucia Formation (sequences 2–6) forms one major large-scale cycle, whereas sequences 1 and 7 are part of an underlying and an overlying cycle, respectively. Eustatic sea-level changes exerted major control on the formation of these large-scale sequences, whereas the medium-scale cycles seem to be co-controlled by regional tectonism and eustasy. Small-scale cycles seem to be the product of high frequency, eustatic sea-level changes. During the deposition of the Santa Lucia Formation, the morphology of the carbonate platform changed from a gently south-dipping ramp to a rimmed shelf and back to a gently dipping ramp.

The Diurnal Cycle of the Summertime Atmospheric Hydrologic Cycle over the Southwestern United States

Abstract Diurnal variations in the climatological large-scale summertime hydrologic cycle over the southwestern United States are examined using surface and upper-air observations along with regional model output. Rainfall rates are greatest during the daytime, but the hydrologic balance that supports this rainfall changes as the day progresses. During the late morning and early afternoon, the area-averaged rainfall is balanced predominantly by evapotranspiration augmented by low-level moisture convergence; moisture from these two sources is redistributed via eddy diffusion, resulting in an overall moistening of the atmosphere and a divergence of moisture aloft. During the late afternoon, vertical redistribution via eddy diffusion weakens considerably, although precipitation continues at approximately the same rate because of drying aloft, which also supports continued large-scale divergence of moisture at these levels. This large-scale divergence aloft persists at all times of day, suggesting that for the domain as a whole, precipitation is dependent upon low-level moisture sources. At finer scales, these balances are modified principally by the presence of moisture convergence/divergence centered on the elevated regions of the domain, suggesting that local balances may be more complex than the area-average balances described here.

The Summertime Atmospheric Hydrologic Cycle over the Southwestern United States

Abstract In this paper the authors examine the large-scale summertime hydrologic cycle associated with the northwestern branch of the North American monsoon, centered on the southwestern United States, using a suite of surface-and upper-air-based observations, reanalysis products, and regional model simulations. In general, it is found that on an area-averaged basis, seasonal precipitation is balanced predominantly by evaporation; in addition, this evaporation also supports a net, vertically integrated moisture flux divergence from the region of the same magnitude as the precipitation itself. This vertically integrated large-scale moisture flux divergence is the result of an offsetting balance between convergence of low-level moisture and divergence of moisture aloft (<750 mb). Over the western portion of the domain, most of this low-level moisture convergence is related to advection from the Gulf of California and eastern Pacific; over the eastern portion of the domain, low-level moisture convergence is ...

Astronomical forcing in Upper Miocene continental sequences: implications for the Geomagnetic Polarity Time Scale

We present an integrated stratigraphic study of cyclically bedded distal alluvial fan to lacustrine deposits in the late Miocene continental sections of Cascante and Cañizar (Teruel basin, NE Spain). The cyclostratigraphic analysis reveals that different scales of sedimentary cyclicity are present with a thickness ratio of about 1:2:5. Spectral analysis of colour records in the depth domain indicates the presence of a significant peak at ∼2.2 m, which corresponds to the average thickness of the basic, mudstone–carbonate, cycle. Other peaks correspond to the large-scale cycle, which consists of clusters of five basic cycles, and to a cycle twice the average thickness of the basic cycle. Magnetostratigraphic results, in combination with small-mammal biostratigraphy, indicate that the three normal polarity intervals recorded in our sections correspond to C5n.2n, C5n.1n and C4Ar.2n. Assigning the ages of CK95 to the polarity reversals implies significant changes in sedimentation rate, which is not in agreement with the regularity of the sedimentary cyclicity. Thus, spectral analysis of high-resolution colour records in the time domain only produce a spectrum that is consistent with Milankovitch climate forcing, if several of the assigned age tie points are excluded. This indicates that the sedimentary cyclicity in these sections is related to astronomical variations in climate and that the ages of reversal boundaries are in error. Hence, we have calculated the astronomical durations for C4Ar.2n (87 ky), C4Ar.3r (54 ky), C5n.1n (141 ky), and C5n.1r (33 ky), which indeed show significant discrepancies with CK95. The duration pattern of our polarity intervals is confirmed by seafloor anomaly profiles and magnetostratigraphic records of deep-sea cores. Consequently, this study demonstrates that astronomically forcing in continental sequences can be a powerful tool to improve the fundamental dating of the geological record.

Apparently homogenous “reef”-limestones built by high-frequency cycles: Upper Jurassic, SW-Germany

Central European Upper Jurassic sponge-microbial “reefs” are commonly regarded as compact, massive and homogenous facies bodies. A continuous study of borehole cores from SW Germany clearly shows a very systematic architecture of these “reefs” built by sedimentary cycles of various hierarchical levels. Altogether five orders of cycles can be distinguished: decimeter-scale microcycles building meter-scale mesocycles, which in turn are stacked to form several-meter thick macrocycles. These high-frequency cycles in turn are parts of basin-wide medium-scale and large-scale cycles reported in a previous paper. This paper documents the high-frequency micro-, meso- and macrocycles. Microcycles (0.1–0.8 m thick) usually have a sharp erosional base covered by reworked sponges, shells and intraclasts, interpreted as storm events. These bioclastic layers are colonised by a succession of thrombolitic and stromatolitic microbialites. Successive microcycles form distinct stacks that record gradually increasing and decreasing depositional energy, thus defining mesocycles (0.5–4 m thick). Several mesocycles construct macrocycles (3–25 m). The cycle stacking patterns were analysed by means of facies-proportion-diagrams. The dominant microscale cyclicity records changes in water agitation, possibly caused by fluctuating storm wave base. The sponge-microbial mounds could probably build up just to average storm wave base. Within the cycles of larger hierarchical levels the successive smaller-scale cycles show systematic trends in their symmetry and in thickness. This may be caused by variations in the ratio of accomodation space (and associated energy level) to carbonate production. External reef geometries (progradation, aggradation, retrogradation, condensation) can be related to internal cycle stacking patterns. To approximate the duration of the high-frequency cycles two different approaches were used. Firstly, the duration of cycles was determined with the help of published chronostratigraphic time scales. Secondly, cycle duration was estimated using published microbial reef growth rates. Both approximations result in ca. 0.4 Ma for the medium-scale cycles and 0.1 Ma for the macrocycles. Both are regionally correlatable, thus appear to be allocyclic in nature and probably are Milankovitch-driven. In contrast, the microcycles are only locally correlatable and thus indicate an autocyclic (storm-induced) nature. This study poses the question if not more of the apparently homogenous “reef bodies” of other settings and geological ages may possibly also show internal high-frequency cycles.

Continental Triassic in Argentina: response to tectonic activity

In Argentina, continental Triassic sediments occur in many sedimentary basins. They overlie a complex basement consisting of several terranes. The sediments form cycles at different scales. Small-scale cycles, such as point bar fining upward sequences, are common structures. Large-scale cycles, covering several lithostratigraphic formations, and middle-scale cycles, which create the formations or parts thereof, indicate uplift pulses of the basin hinterlands or the respective terranes, respectively. The middle-scale cycles also may be connected with volcanic events. The uplift pulses in the different terranes occurred, with few exceptions, independently of one another and were a main controlling factor in the evolution of the basins. Field work and pebble, modal, and geochemical analyses provide a differentiated view of the evolution of the Triassic sedimentary basins in space and time. In general, the sediments were derived from traceable stable blocks. In the Marayes-Desagüadero basin, the Rincón Blanco section (Cuyo basin), Malargüe basin, and perhaps the Deseado massif, the sources include magmatic arcs and a variety of terranes.

DESCRIPTION AND PALEOBIOGEOGRAPHIC SIGNIFICANCE OF A RARE CENOMANIAN MOLLUSCAN FAUNULE FROM BATHURST ISLAND, NORTHERN AUSTRALIA

A middle Cenomanian faunule from the Moonkinu Formation of Bathurst Island in Northern Australia contains the best-preserved suite of benthic Mollusca known from the Cretaceous of the Australian region. Twenty-four species of bivalves, gastropods, and scaphopods, many exquisitely preserved with original aragonitic nacre, are recognized. Thirteen are new: Nucula s.l. meadinga n. sp. (Nuculidae), Nuculana bathurstensis n. sp. (Nuculanidae), Jupiteria? n. sp. A (Nuculanidae), Varicorbula cretaustrina n. sp. (Corbulidae), Vanikoropsis demipleurus n. sp. (Vanikoridae), Euspira n. sp. A (Naticidae), Amuletum praeturriformis n. sp. (Turridae), Granosolarium cretasteum n. sp. (Architectonicidae), Echinimathilda moonkinua n. sp. (Mathildidae), Acteon bathurstensis n. sp. (Acteonidae), Biplica antichthona n. sp. (Ringiculidae), Goniocylichna australocylindricata n. sp. (Cylichnidae), and Dentalium (Dentalium) n. sp. A (Dentaliidae). Nominal species of Nuculana, Grammatodon, Cylichna, and Laevidentalium also are present. The occurrence of ammonites, including taxa that occur in the type Cenomanian, securely establishes the fauna as middle Cenomanian (Acanthoceras rhotomagense Zone). The Moonkinu Formation and its faunule were deposited in a high-energy, shallow-shelfal setting, as part of a large-scale regressive cycle recognized as the Money Shoals Platform of northern Australia. The assemblage represents a parauthochthonous suite which experienced little or no post mortem transport. Epifaunal and infaunal suspension feeders (some 60 percent) dominate the bivalve fauna with a subordinate representation of deposit-feeding infaunal burrowers (some 40 percent). Nearly all of the gastopods were carnivores with the aporrhaid Latiala mountnorrisi (Skwarko), probably a deposit feeder, the only exception. The scaphopods were probably micro-carnivores. Concentrations of the ammonite Sciponoceras glaessneri are likely the result of mass kills in surface waters. The cosmopolitan nature of the Bathurst Island fauna at the genus-level reflects unrestricted oceanic circulation patterns and an equitable climate on a global scale during the Cenomanian. The retreat and disappearance of the Australian epicontinental sea at the close of the Albian coincided with reduced endemism in the molluscan faunas, after which time the continental shelves hosted a rich suite of cosmopolitan affinity. The high number of endemic species in the Moonkinu Formation probably represents an early stage of broad-scale genetic separation among Southern Hemisphere molluscan stocks, a trend that became increasingly pronounced through the Late Cretaceous. The new records of Varicorbula, Amuletum, Granosolarium, Echinimathilda, and Goniocylichna represent the oldest occurrences recorded for these genera and are suggestive of Southern Hemisphere origins.

Eutrophication and Self-purification: Counteractions Forced by Large-scale Cycles and Hydrodynamic Processes

Eutrophication and Self-purification: Counteractions Forced by Large-scale Cycles and Hydrodynamic Processes

Large-Scale Cycle Architecture in Continental Strata, Hornelen Basin (Devonian), Norway

ABSTRACT Nine large-scale stratigraphic cycles, each about 100 m thick, along the northern margin of the Hornelen basin, western Norway, record systematic expansions and contractions of alluvial-fan, braidplain, and lake facies tracts. The braidplain facies tract occupies the basin center, from time to time expanding toward the margins, and constitutes the deposits of axial or longitudinal drainage from an eastern source. The alluvial-fan facies tract, which comprises the deposits of high-gradient transverse drainages, occupies a narrow belt close to the fault-bounded basin margin. The lake facies tract is between the alluvial-fan and braidplain facies tracts. Physical correlation of strata shows that the large-scale cycles are symmetric in all facies tracts. The symmetry of the large-scale cycles, in which the base-level rise and fall hemicycles of each cycle are of approximately the same thickness, indicates that sediment accumulated in all environments during the entire base-level cycle. Absence of regional unconformities (sequence boundaries) also indicates approximately continuous sediment accumulation in the basin during base-level cycles. The base-level fall-to-rise turnaround is represented by an interval of strata, rather than by a sequence boundary, at the maximum expansion of the alluvial-fan facies tract. Condensed sections are also absent. They are represented mainly by intervals of lake strata at base-level rise-to-fall turnarounds. Strata in the base-level fall hemicycle (decreasing accommodation) accumulated as the braidplain and alluvial-fan facies tracts expanded from the basin center and the northern basin margin, respectively. Expansions of these two facies tracts filled the basin, while the lake facies tract contracted. Strata in the base-level rise hemicycle accumulated as the alluvial-fan and braidplain facies tracts contracted sourceward, respectively toward the northern and eastern basin margins, during times of increasing accommodation. This sourceward retreat of the alluvial-fan and braidplain facies tracts was coincident with expansion of the lake facies tract. At the rise-to-fall turnarounds, alluvial-fan and braidplain sediments are stored near their respective sources toward the basin margins, and the centers of mass of these facies tracts are at basin-margin positions. During base-level fall time, the centers of mass of the alluvial-fan and braidplain facies tracts migrate basinward. The alluvial-fan and braidplain facies tracts usually expand toward each other simultaneously, and then move away from each other simultaneously. This in-phase relationship is replaced by a reciprocal stacking pattern when the two facies tracts abut each other near base-level fall-to-rise turnarounds. When the two facies tracts are in contact, the higher-gradient alluvial-fan facies tract initially blocks expansion of the braidplain. As the alluvial-fan facies tract retreats toward the basin margin and fan-margin gradients are reduced, however, the braidplain expands across former fan margins. In the reciprocal configuration, the two facies tracts move in tandem in the same direction. This reciprocal pattern plus changes in aggradation-to-progradation ratio of the alluvial-fan facies tract suggest that alluvial-fan morphology changes during base-level cycles. Large-scale cycle stacking patterns show significant changes in basin-fill architecture through time, including syndepositional structural changes accompanied by changes in stratal geometries, a change from ephemeral to permanent lakes, permanent increase in alluvial-fan gradients, and permanent reduction of alluvial-fan radii and volume at the northern basin margin. Consideration of these changes and the largely in-phase relationships between facies tracts sourced by two separate sediment supplies suggest that an interplay of climate, self-regulatory, and tectonic factors controlled sedimentary accumulation within the Hornelen basin.

Eutrophication and Self-purification: Counteractions Forced by Large-scale Cycles and Hydrodynamic Processes

During the 1990s nitrogen reserves have shown rising trends in the northern Baltic Sea. Changes in denitrification explain some of the fluctuations observed in nitrogen reserves. Although denitrification is an anaerobic process, the most efficient removal of nitrogen by denitrification occurs where the sediment is moderately well oxidized. A dramatic decrease in the ratio of SiO4 to DIN (DIN = NO3 + NH4) in the northern Baltic proper during the period 1973-1999 was recorded. If dissolved silica limits phytoplankton spring bloom, diatom blooms fade and become replaced by flagellates leading to changes in summer phosphate reserves and sedimentation. Seven years have elapsed from the previous strong saltwater inflow and anoxia has spread over large areas. Deep bottom phosphorus levels have started to increase and denitrification capacity is weakening. Thus, all efforts to minimize N- and P-loading will, in the long run, help the Baltic Sea to recover from the unacceptable status of eutrophication.

Sea-level change and rock-record bias in the Cretaceous: a problem for extinction and biodiversity studies

The association between mass extinction in the marine realm and eustatic sea-level change in the Mesozoic is well documented, but perplexing, because it seems implausible that sea-level change could actually cause a major extinction. However, large-scale cycles of sea-level change can and do alter the ratio of shallow to deep marine continental-shelf deposits preserved in the rock record both regionally and globally. This taphonomic megabias alone could be driving patterns of first and last occurrence and standing diversity because diversity and preservation potential both change predictably with water depth. We show that the Cenomanian/Turonian faunal event in western Europe has all the predicted signatures expected if taphonomic megabias was the cause. Grade taxa terminating in pseudoextinction and Lazarus taxa are predominantly found in the onshore facies that disappear for extended periods from the rock record. Before other mass extinctions are taken at face value, a much more careful analysis of biases in the rock record needs to be carried out, and faunal disappearances need to be analyzed within a phylogenetic framework.

Cretaceous-Palaeogene ocean and climate change in the subtropical North Atlantic

Abstract Ocean Drilling Program (ODP) Leg 171B recovered continuous sequences that yield evidence for a suite of ‘critical’ events in the Earth’s history. The main events include the late Eocene radiolarian extinction, the late Palaeocene benthic foraminiferal extinction associated with the Late Palaeocene Thermal Maximum (LPTM), the Cretaceous-Palaeogene (K-P) extinction, the mid-Maastrichtian event, and several episodes of sapropel deposition documenting the late Cenomanian, late Albian and early Albian warm periods. A compilation of stable isotope results for foraminifera from Leg 171B sites and previously published records shows a series of large-scale cycles in temperature and δ 13 C trends from Albian to late Eocene time. Evolution of δ 18 O gradients between planktic and benthic foraminifera suggests that the North Atlantic evolved from a circulation system similar to the modern Mediterranean during early Albian time to a more open ocean circulation by late Albian-early Cenomanian time. Sea surface temperatures peaked during the mid-Cretaceous climatic optimum from the Albian-Cenomanian boundary to Coniacian time and then show a tendency to fall off toward the cool climates of the mid-Maastrichtian. The Albian-Coniacian period is characterized by light benthic oxygen isotope values showing generally warm deep waters. Lightest benthic oxygen isotopes occurred around the Cenomanian-Turonian boundary, and suggest middle bathyal waters with temperatures up to 20 °C in the North Atlantic. The disappearance of widespread sapropel deposition in Turonian time suggests that sills separating the North Atlantic from the rest of the global ocean were finally breached to sufficient depth to permit ventilation by deep waters flowing in from elsewhere. The Maastrichtian and Palaeogene records show two intervals of large-scale carbon burial and exhumation in the late Maastrichtian-Danian and late Palaeocene-early Eocene. Carbon burial peaked in early Danian time, perhaps in response to the withdrawal of large epicontinental seas from Europe and North America. Much of the succeeding Danian period was spent unroofing previously deposited carbon and repairing the damage to carbon export systems in the deep ocean caused by the K-P mass extinction. The youngest episode of carbon exhumation coincided with the onset of the early Eocene Warm Period and the LPTM, and has been attributed to the tectonic closure of the eastern Tethys and initiation of the Himalayan Orogeny.

Astronomical forcing of sedimentary cycles in the middle to late Miocene continental Calatayud Basin (NE Spain)

A high-resolution cyclostratigraphic and magnetostratigraphic study was carried out on cyclically bedded successions of middle Miocene lacustrine to distal alluvial fan-floodplain deposits from the Calatayud basin, in northeast Spain. Eight (partially overlapping) subsections near the village of Orera are correlated in detail using distinct sedimentary cycle patterns or by following marker beds in the field. Together they form the Orera Composite Section (OCS). Sedimentary cycles are recognised on at least two different scales. The basic small-scale cycles in the OCS consist of an alternation of grey and, occasionally, red clays with white, dolomite-rich, carbonate beds. They are arranged in larger-scaled, so-called large-scale cycles based on repetitive changes in the overall carbonate–clay lithology. Two other, but less distinct, types of intermediate scale cyclicity are also recognised. In terms of depositional environment, the cycle hierarchy is interpreted to represent periodic lake expansion over the palaeo-alluvial fan-floodplain area. The palaeomagnetic results yield a reliable magnetostratigraphic record, which confirms the cyclostratigraphic correlations between the subsections in detail. Rock magnetic experiments reveal that haematite is the main magnetic carrier of the primary component. The magnetostratigraphy of the OCS is correlated straightforwardly with the geomagnetic polarity time scale. This resulted in an age of 10.7–12.8 Ma for the entire succession, which is supported by fossil micromammal findings. In addition, it also reveals the presence of two, possibly three, short new polarity intervals. Based on the number of cycles in the OCS the average periodicity of the basic small-scale cycles is approximately 23 000 yr, while the large-scale cycle indicates a periodicity of 400 000 yr. This suggests that these sedimentary cycles are controlled by astronomically induced climate changes causing lake-level fluctuations and thus resulting in the deposition of carbonate-clay cycles. The continental sequences of the OCS provide a unique opportunity to extend the astronomical polarity time scale into the middle Miocene. The overlap of such continental sequences in the Mediterranean area with time-equivalent astronomically induced marine sequences is fundamental for establishing marine–continental, bed-to-bed, correlations and for understanding regional climate change.

Turbidite basin and mass dynamics related to orogenic wedge growth; the Rheno-Hercynian case

Abstract Investigation of the Rheno-Hercynian Turbidite Basin suggests an interrelation between orogenic wedge migration and changes of the basin architecture, facies types and progradation rates of turbidite systems. The orogenic wedge was built dominantly by the Mid-German Crystalline Rise and accreted older parts of the Rheno-Hercynian Turbidite Basin. Turbidite sequences seem to have developed in stepwise prograding sub-basins. They show systematic trends of widely correlative (tens of kilometres) large-scale cycles that show an upward transition from coarse-grained clastics to highly diluted mud turbidites with highstand shedding of carbonates. These observations suggest the existence of accommodation cycles on the assumed shelf along the Mid-German Crystalline Rise. Exposure of the shelf above sea level was associated with bypass and transport of coarse-grained clastics into the basin, whereas flooding is thought to have caused storage of clastic deposits on the shelf and the simultaneous deposition of mud turbidites in the basin. Accommodation cycles, estimated to reflect an average duration of 10 5 –10 6 a, were probably related to cyclic changes of underthrusting in the internal parts of the orogenic wedge with concomitant uplift. The lifetime and onset of new sub-basins, of the order of a few million years, may show stages of imbricate fan formation. A pattern of major change in sedimentation rates, basin geometry and subsidence style of the Rheno-Hercynian Turbidite Basin and the Sub-Variscan Molasse Basin was interpreted to reflect the different change in crustal strength. This occurred because the hinge line between previously rifted and unrifted crust was overrun during wedge progradation. Numerical mass balance that compares exhumed and intra-basinal masses of the Rheno-Hercynian Turbidite Basin, including the proportional fill of the Sub-Variscan Molasse Basin and the Saar-Nahe Basin, indicates a significant lack of sediment mass (i.e. more than one-third of the exhumed masses). In this estimate, syn- to post-depositional truncation was included. As extension in the internal parts of the orogenic wedge has only a minor role, the above deficit is probably related to subduction of sediment during the early stages of convergence.

The Large-Scale Magnetic Field and Sunspot Cycles

Extended abstractWe report on the correlation between the large scale magnetic field and sunspot cycles during the last 80 years that was found by Makarovet al. (1999) and Makarov &amp;amp; Tlatov (2000) in H-αspherical harmonics of the large scale magnetic field for 1915–1999. The sum of intensities of the low modes 1 = 1 and 3, A(t), was used for comparison with the Wolf number, W(t). It was shown that the large scale magnetic field cycles, A(t), precede the sunspot cycles, W(t), by 5.5 years.Let us consider the behaviour in time of the harmonics with low numbers 1 = 1 and 1 = 3. The radial component B(r) of the magnetic field may be expanded in terms of the spherical harmonicswhereθandϕare the latitude and longitude,are Legendre polynomials andandare coefficients of expansion on the spherical functions.The magnetic moments of a dipole (1 = 1) and an octopole (1 = 3) are determined by the following equations:Let us enter the parameter describing their intensity,

Shallowing-upward cycles in pelagic troughs (Upper Jurassic, Subbetic, Southern Spain)

Abundant calcareous tempestites with chert and hummocky cross-stratification (HCS) occur in Upper Jurassic successions (Milanos Formation) of the Subbetic (Betic Cordillera, Southern Spain). These calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts, and have thicknesses between 25 and 75 cm. Beds show symmetrical wave ripple lamination near the top, and are usually capped with wave ripples. According to palaeogeographic reconstructions, these strata were deposited in a subsiding trough in the pelagic domain of the Southern Iberian Continental Margin, which was the northwestern-most border of the Tethys. Calcisiltite and calcarenite beds deposited in this pelagic trough form small-scale to large-scale shallowing-upward cycles. Shallowing-upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part, have also been recognized in some sectors of the Subbetic pelagic basin. The large-scale cycle occurs in the upper part of the Jarropa Radiolarite Formation, which directly underlies upper Oxfordian, Kimmeridgian and Tithonian strata of the Milanos Formation. Two types of large-scale shallowing-upward cycles have been recognized, according to the presence or absence of thick units of oolitic limestone with large-scale cross-stratification. The water depth during deposition ranged from below, but near, storm wave base at the base of the large-scale cycles to very shallow carbonate platform at the top.

Depositional Cycles as a Shelf-to-Basin correlation Tool, Capitan Reef Margin - Ideas from Stratigraphic Computer Modeling: ABSTRACT

Correlations from the Yates shelf through the Capitan reef margin and into the Bell Canyon of the deep Delaware Basin (Guadalupian, Permian Basin) are not known with great detail due to limited biostratigraphic control and the inability to trace beds or time lines from the cyclic shelf deposits, through the massive reef and foreslope, and into basinal siliciclastics. The presence of a strong hierarchy of depositional cycles on the shelf and in the basin suggests that cycles may be useful as a correlation tool. A Yates shelf-to-basin correlation is proposed based on matching 3rd- and 4th-order cycles recognized with a stratigraphic computer model and on cores and outcrop. We believe thick, sand-dominated cycles were deposited in the basin during long-term lowstands; whereas, thin, organic-rich lime mudstone-dominated cycles were deposited during long-term highstands. The correlation scheme hinges on equating 3rd-order condensed intervals of organic mudstone in the basin with thick carbonate {open_quote}banks{close_quote} deposited ad the lower and upper Yates boundaries. Within this tentative large-scale framework, there is a reasonably good match between 4th-order shelf and basin cycles. Some ambiguity remains, however, due to the possibility of missed cycle beats in both the shelf (thin or no cycles, erosion) and basin (nomore » or minor condensed mudstones resulting in sand on sand, erosion) during long-term lowstands. Computer modeling suggests siliciclastics bypassed the Yates shelf and were delivered to the basin as turbidites during high-frequency (5th-order) lowstands, but the amount of bypass during any lowstand was controlled by longer-term cycles. The large-scale basinal cycles are slightly more symmetrical than their shelf counterparts; typically showing a gradual increase in clean sandstone and decrease in siltstone and organic mudstone away from a condensed interval, and the converse towards the next condensed interval.« less

Contrasting styles of fluvial-aeolian interaction at a downwind erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, USA

Abstract Fluvial-aeolian interactions result from both short-term interplay between an ergmargin and adjacent fluvial system, and much longer-term expansion and contraction of the erg in response to climatic, tectonic or eustatic effects. Near Moenkopi, in northeastern Arizona, a zone of intertonguing characterizes the transition between the fluvial Kayenta Formation and the overlying aeolian Navajo Sandstone. Three fluvial to aeolian drying-up cycles form the largest scale interaction and represent the repeated advance of the Navajo erg across the Kayenta alluvial plain, probably in response to an increasingly arid climate. During the aeolian phase of each cycle, small-scale interactions resulted from fore-erg deposition on the alluvial plain, and fluviallyfed interdunal flooding, which both occurred during periods of erg activity. The top of each cycle marked a return to wetter climatic conditions which terminated erg activity, forming a stabilization surface. As a consequence, prograding fluvial systems were able to transgress the denuded surface, covering the area formerly occupied by the erg to form the base of the next drying-up cycle. From a hydrocarbon reservoir viewpoint, the two types of interaction have contrasting geometries. Fluvial deposits of the large-scale cycles have regionally extensive sheet-like geometries and are likely to form widespread barriers to vertical flow. Small-scale interdunal flooding is less significant and forms ribbon-like baffles to flow which extend roughly parallel to the strike of the aeolian cross-strata. The key to differentiating each type of interaction lies in determining whether the fluvial syste coexisted with the active erg, or prograded as a result of erg stabilization .

High and low resolution sequence stratigraphy in lithological prediction—examples from the Mesozoic around the northern North Atlantic

The Mesozoic around the northern North Atlantic was characterized by a succession of extensional tectonic events separated by periods of regional subsidence governed by thermal contraction. Sedimentation was controlled by tectonic subsidence, sediment and water loading, isostatic compensation, sediment influx, climate, and eustatic and relative sea-level variations at several scales. A series of examples from the Upper Triassic–Lower Cretaceous from East Greenland and the Northern North Sea have been selected to illustrate the relative importance of the controlling factors on depositional systems of different temporal and spatial scale. A long-term eustatic rise, superimposed by short-term cycles thus started in the Hettangian and culminated in the Kimmeridgian, whereas the main tectonic events include uplift of a major dome in the Early Jurassic, subsidence of the dome in the Middle Jurassic, and domal collapse and rifting in the Late Jurassic. The syn- to early post-rift Wollaston Forland Group (Volgian–Valanginian) is an example of a strongly tectonically controlled coarse-grained deep-water system deposited on tilted fault blocks. Deposition of the partly contemporaneous Fynselv Member (Raukelv. Formation) took place during uniform regional subsidence mainly governed by thermal contraction; relative sea-level variations appear to be the dominant factor controlling the sedimentary architecture. The two successions differ dramatically in geometry, facies and sedimentary processes. The tilted fault block example seems, however, to include the same number of roughly contemporaneous large-scale cycles as the unit deposited in the uniformly subsiding basin. This suggests that a relative sea-level signal can be recognized also in the tectonically controlled deep-water succession. The effects of sea-level control on contemporaneous shallow marine successions are illustrated by the Middle Jurassic Vardekløft Formation and the Brent Group of the Northern North Sea. The two units show great similarity and contemporaneity in terms of facies, parasequence stacking patterns, and systems tract successions. This allows formulation of a combined age and lithological model, which can be used in prediction of spatially separated systems tracts, some of which may contain unrecognized reservoir bodies. The similar development is highly suggestive of eustatic control. However, both regions were affected by large-scale doming, and the uniform development of the two systems may thus be controlled by relative sea-level changes reflecting large-scale tectonic processes. Finally, a high resolution sequence stratigraphic analysis of the lacustrine Rhaetian–Sinemurian Kap Stewart Formation is presented to demonstrate a hitherto unrecognized play type. Correct identification of sequence stratigraphic elements in this formation allows prediction of significant deltaic reservoir sandstones interbedded with black organic-rich lacustrine mudstones with excellent source rock potential. Combined low and high resolution sequence stratigraphy can be used in large-scale lithological prediction and comparison between systems situated far apart, and in detailed within-system predictions on a reservoir scale.

Tectonic and Climate Changes Expressed as Sedimentary Cycles and Stratigraphic Sequences of the Paleogene Lake Uinta System, Central Rocky Mountains, Utah and Colorado

Lake Uinta strata record both long- and short-term changes in climate and tectonic regime. Late Paleocene to late Eocene deposits are characterized by evaporite units (including halite and bicarbonate salts) and organically derived carbonate with extreme positive {delta}{sup 13}C and slightly negative {delta}{sup 18}O values that serve as evidence the lake was the center of a closed hydrologic system. Large reconfigurations of the lake system were tectonically induced and gave rise to relatively thick, lithologically distinct stratigraphic sequences. Simultaneous climate changes initiated very rapid lake level expansions and contractions as well as shifts in lake-water alkalinity and salinity (or chemistry) which resulted in the development of small- to large-scale sedimentary and stratigraphic cycles. Maastrichtian to earliest Eocene phases formed in local depressions (piggy back basins) on the thrust sheets, and in the incipient Uinta basin. Lake system reached its greatest aerial and volumetric extent in the middle and late Eocene and was centered in the foreland formed in front of high-angle reverse faults that bounded the rising Laramide structural blocks. At this time chemical precipitates, including basin centered carbonate and evaporite facies, formed during episodes of tectonically induced subsidence at the center of the clastic sediment-starved basin. Some fault-bounded marginsmore » of the Uinta basin are marked by synorogenic coarse debris that extends from the mountain front to the clastic sediment-starved lake. Tectonically induced stratigraphic sequences of the Lake Uinta system express environments for several million years whereas climatic cycles reflect much shorter episodes and very rapidly changing conditions.« less