Maria Mutti

Global dominance of coralline red-algal facies: A response to Miocene oceanographic events

Rhodoliths (free-living coralline red algae) can thrive under a wide range of temperatures, reduced light, and increased nutrient levels, and often form a distinct so-called rhodalgal lithofacies that is an important component of Cenozoic shallow-water carbonates. Global distributions illustrate that from the late-early to early-late Miocene (Burdigalian-early Tortonian), rhodalgal facies reached peak abundances and commonly replaced coral-reef environments, accompanied by a decline in other carbonate-producing phototrophs. We argue that the dominance of red algae over coral reefs was triggered in the Burdigalian by enhanced trophic resources associated with a global increase in productivity, as evidenced by a long-term shift toward higher carbon isotope values. Rhodalgal lithofacies expanded further in the middle Miocene when strengthened thermal gradients associated with the establishment of the East Antarctic Ice Sheet led to enhanced upwelling while climate change generated increased weathering rates, introducing land-derived nutrients into the oceans. Globally cooler temperatures following a climatic optimum in the early-middle Miocene contributed to sustain the dominance of red algae and prevented the recovery of coral reefs. The global shift in nearshore shallow-water carbonate producers to groups tolerant of higher levels of trophic resources provides further evidence for increased nutrient levels during that time interval and shows the sensitivity of shallow-water carbonate facies as indicators of past oceanographic conditions.

Nutrient and temperature controls on modern carbonate production: An example from the Gulf of California, Mexico

In addition to salinity and temperature, nutrient concentrations in surface waters are known to have a significant impact on distribution of carbonate-producing biota, but have never been quantitatively evaluated against different temperatures along a latitudinal transect. The western coast of the Gulf of California, Mexico, presents a natural laboratory for investigating the influence of oceanographic parameters such as salinity, temperature, and chlorophyll a, a proxy for nutrients, on the composition of a range of modern heterozoan and photozoan carbonate environments along a north-south latitudinal gradient spanning the entire warm-temperate realm (29°N–23°N). Chlorophyll a, measured in situ at half-hour resolution, is highly variable throughout the year due to short-term upwelling, and increases significantly from the southern to northern Gulf of California. Salinity, in contrast, fluctuates little and remains at an average of 35‰. From south to north, carbonate production ranges from oligotrophic-mesotrophic, coral reef– dominated shallow-water areas (minimum temperature 18.6 °C) through mesotrophic-eutrophic, red algal–dominated, inner-shelf carbonate production in the central gulf (minimum temperature 16 °C), and to molluscan-bryozoan, eutrophic inner- to outer-shelf environments (minimum temperature 13.7 °C). The Gulf of California data, supplemented with oceanographic and compositional information from a database compiled from a spectrum of modern carbonate systems worldwide, demonstrates the significance of nutrient control in the formation of heterozoan, photozoan, and transitional heterozoan-photozoan carbonate systems and serves as a basis for more accurately interpreting fossil carbonates.

δ18O and Marion Plateau backstripping: Combining two approaches to constrain late middle Miocene eustatic amplitude

d 18 Obenthic values from Leg 194 Ocean Drilling Program Sites 1192 and 1195 (drilled on the Marion Plateau) were combined with deep-sea values to reconstruct the magnitude range of the late middle Miocene sea-level fall (13.6‐11.4 Ma). In parallel, an estimate for the late middle Miocene sea-level fall was calculated from the stratigraphic relationship identified during Leg 194 and the structural relief of carbonate platforms that form the Marion Plateau. Corrections for thermal subsidence induced by Late Cretaceous rifting, flexural sediment loading, and sediment compaction were taken into account. The response of the lithosphere to sediment loading was considered for a range of effective elastic thicknesses (10 , Te , 40 km). By overlapping the sea-level range of both the deep-sea isotopes and the results from the backstripping analysis, we demonstrate that the amplitude of the late middle Miocene sea-level fall was 45‐68 m (56.5 6 11.5 m). Including an estimate for sea-level variation using the d 18 Obenthic results from the subtropical Marion Plateau, the range of sea-level fall is tightly constrained between 45 and 55 m (50.0 6 5.0 m). This result is the first precise quantitative estimate for the amplitude of the late middle Miocene eustatic fall that sidesteps the errors inherent in using benthic foraminifera assemblages to predict paleo‐water depth. The estimate also includes an error analysis for the flexural response of the lithosphere to both water and sediment loads. Our result implies that the extent of ice buildup in the Miocene was larger than previously estimated, and conversely that the amount of cooling associated with this event was less important.

Early Marine Lithification and Hardground Development on A Miocene Ramp (Maiella, Italy): Key Surfaces to Track Changes in Trophic Resources in Nontropical Carbonate Settings

ABSTRACT Phosphatic and carbonate hardgrounds occur in a Lower Miocene nontropical carbonate succession in the Maiella carbonate platform margin, located in the Central Apennines. Multiple diagenetic events occurred at or near the sea floor before the deposition of the overlying sediment and included precipitation of inclusion-rich calcite, micrite, iron oxides, and phosphates. Later diagenetic features are limited to chemical compaction and precipitation of clear calcite cement. We relate the development of these features to a two-step model, involving progressive intensification of upwelling on this carbonate margin, which was triggered by regional changes in water circulation and modulated by sea-level changes, leading initially to precipitation of inclusion-rich calcite. With an increase in trophic resources related to the paleoceanographic conditions on the ramp, increased flux of organic matter to the sea floor led to temporary formation of microbial biofilms. These conditions were associated with extensive precipitation of micrite and phosphate microspherules in the uppermost sediment. The lack of sedimentation provides the precondition to accumulate and preserve evidence for organic-matter utilization in the uppermost sediment layer. Our study suggests that hardgrounds in nontropical carbonates might be used as indicators of circulation changes and can provide a useful link to major environmental changes in the ocean-margin environment. The occurrence of microbial micrite and phosphate microspherules precipitated in the absence of sedimentation near the sea floor as a response to higher nutrient supply provides a new and important criterion to differentiate nontropical carbonate facies deposited under the influence of higher nutrient supplies rather than temperature alone. Furthermore, our study shows that local depletion in 13C at a key stratigraphic surface does not necessarily reflect meteoric exposure but may be related to microbial activity at the sea floor.

Depositional geometries and facies associations in an Upper Cretaceous prograding carbonate platform margin (Orfento Supersequence, Maiella, Italy)

ABSTRACT Although progradation of platform margins is a well documented process, its timing with respect to sea-level cycles is controversial. This outcrop study was designed to test the relation between prograding stratal patterns and sea-level cycles, using a sequence stratigraphic analysis and direct investigation of the sedimentological facies associations. The upper Campanian-Maastriehtian Orfento supersequence in the Maiella platform margin is an outstanding outcrop example of a large-scale prograding carbonate wedge, composed of smaller-scale, vertically and laterally stacked sequences. The evolution of the Orfento supersequence was characterized by an initial phase of aggradation, followed by progradation. At the scale of the supersequence, basinal aggradation preceded progradation of hallower-water facies, because excess relief had to be buried prior to progradation. Seven depositional facies were distinguished on the basis of depositional geometries, stratal patterns, and sedimentary features. These facies record different depositional environments on a low-angle ramp, ranging from deep-water pelagic wackestones to wave-dominated upper shoreface grainstones. Facies distributions record the overall transition from pelagic sediments, turbidites, and megabreccias into progressively shallower-water shoreface complexes. The prograding units consist of shingled offlapping sigmoidal complexes, composed of rudstones and grainstones, deposited in shallow, wave-dominated environments, and their deeper-water equivalents. Sigmoidal sequences are separated by erosive unconformities, which bring upper-shoreface facies into direct contact with underlying lower-shoreface or outer-shelf facies, indicating a sea-level fall (forced regression). Meteoric diagenesis associated with the unconformities also supports lowering of relative sea level. Depositional facies patterns within sequences record a relative rise and highstand of sea level. Repetitive downward shifts bring the shoreline successively farther basinward, indicating long-term lowering of sea level during the late stages of progradation, producing downstepping progradation. Our data suggest that changes in relative sea level, acting on a low-angle ramp, control the overall architecture of sequence distribution, whereas changes in sediment supply con rol changes in facies association within each sequence.

Outcrop analog for an oolitic carbonate ramp reservoir: A scale-dependent geologic modeling approach based on stratigraphic hierarchy

Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation. This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models.

Chemostratigraphy in Miocene heterozoan carbonate settings: applications, limitations and perspectives

Abstract The temporal variability of geochemical proxies can be used in time intervals characterized by global changes in marine chemistry to achieve improved stratigraphic correlation. The application of this approach in rocks lithified by cementation requires particular attention, as the original isotopic signature may have been modified by diagenetic processes and, when bulk samples are used, could reflect facies-specific compositional changes as opposed to primary changes in the water column. This paper examines sedimentological and chemostratigraphic records from outcrops in the central Mediterranean and cores drilled on the Marion Plateau by the Ocean Drilling Program (ODP) Leg 194, where heterozoan carbonates accumulated during the Miocene. Specifically, the paper addresses how facies and preservation of original marine signatures differentially affect the quality of the dataset. The analysis indicates that, in general, heterozoan systems, relative to their tropical counterparts, show good preservation of marine signatures. Chemostratigraphy offers a viable low-resolution alternative for dating platform sediments considering the general lack of biostratigraphic markers in these settings. It is stressed, however, that care must be taken when interpreting these values, especially when the dataset is at a low resolution or when post-depositional dolomitization took place. Furthermore, chemostratigraphy in shallow-water environments cannot be done without detailed facies analysis, as facies changes may impact bulk-rock stable isotope values.

Characterization of a deeply buried paleokarst terrain in the Loppa High using core data and multiattribute seismic facies classification

The recognition of paleokarst in subsurface carbonate reservoirs is not straightforward because conventional seismic interpretation alone is generally not sufficient to discriminate karstified areas from their surroundings. In the Loppa High (Norwegian Barents Sea), a protracted episode of subaerial exposure occurring between the late Paleozoic and mid-Triassic—Late Permian to Anisian—resulted in a significant overprinting of the previously deposited carbonate units. Here, we map the extension of the karstified areas using an integrated approach consisting of (1) a core study of critical paleokarst intervals, (2) a three-dimensional (3-D) seismic stratigraphic analysis, and (3) a 3-D multiattribute seismic facies (SF) classification. A core retrieved in the flat-topped Loppa High revealed breccia deposits at least 50 m (164 ft) thick, which probably resulted from cave collapses following the burial of the karst terrain. The SF classification was tested on a 3-D cube to (1) discriminate the respective SF related to the breccia deposits compared with other SF and (2) to estimate their spatial extent. Seismic-facies analysis suggests that breccias occupied the topmost area of the structural high, extending up to 12 km (7 mi) in width, 46 km (29 mi) in length, and tens of meters in thickness. The inference of such a large amount of breccia suggests that a significant part of this terrain was derived from the amalgamation of successive cave-development events—including periods of subaerial exposure and subsequent burial and collapse—resulting in a coalesced collapsed paleocave system. Previous observations from the Loppa High revealed the presence of karst plains associated with sinkholes, caves, and other dissolution phenomena associated with the breccia facies, further suggesting that a large volume of carbonate rocks in this area was affected by subaerial exposure and karstification. Our integrated approach and proposed karstification model could be applied to similar sedimentary basins that accommodate deeply buried carbonate successions affected by protracted episodes of subaerial exposure, where only few wells as well as 3-D seismic data are available.

Numerical simulation of fluid-flow processes in a 3D high-resolution carbonate reservoir analogue

A high-resolution three-dimensional (3D) outcrop model of a Jurassic carbonate ramp was used in order to perform a series of detailed and systematic flow simulations. The aim of this study was to test the impact of small- and large-scale geological features on reservoir performance and oil recovery. The digital outcrop model contains a wide range of sedimentological, diagenetic and structural features, including discontinuity surfaces, shoal bodies, mud mounds, oyster bioherms and fractures. Flow simulations are performed for numerical well testing and secondary oil recovery. Numerical well testing enables synthetic but systematic pressure responses to be generated for different geological features observed in the outcrops. This allows us to assess and rank the relative impact of specific geological features on reservoir performance. The outcome documents that, owing to the realistic representation of matrix heterogeneity, most diagenetic and structural features cannot be linked to a unique pressure signature. Instead, reservoir performance is controlled by subseismic faults and oyster bioherms acting as thief zones. Numerical simulations of secondary recovery processes reveal strong channelling of fluid flow into high-permeability layers as the primary control for oil recovery. However, appropriate reservoir-engineering solutions, such as optimizing well placement and injection fluid, can reduce channelling and increase oil recovery.

Unconventional reservoir potential of the upper Permian Zechstein Group: a slope to basin sequence stratigraphic and sedimentological evaluation of carbonates and organic-rich mudrocks, Northern Germany

The Late Permian Zechstein Group in northeastern Germany is characterized by shelf and slope carbonates that rimmed a basin extending from eastern England through the Netherlands and Germany to Poland. Conventional reservoirs are found in grainstones rimming islands created by pre-existing paleohighs and platform-rimming shoals that compose steep margins in the north and ramp deposits in the southern part. The slope and basin deposits are characterized by debris flows and organic-rich mudstones. Lagoonal and basinal evaporites formed the seal for these carbonate and underlying sandstone reservoirs. The objective of this investigation is to evaluate potential unconventional reservoirs in organic-rich, fine-grained and/or tight mudrocks in slope and basin as well as platform carbonates occurring in this stratigraphic interval. Therefore, a comprehensive study was conducted that included sedimentology, sequence stratigraphy, petrography, and geochemistry. Sequence stratigraphic correlations from shelf to basin are crucial in establishing a framework that allows correlation of potential productive facies in fine-grained, organic-rich basinal siliceous and calcareous mudstones or interfingering tight carbonates and siltstones, ranging from the lagoon, to slope to basin, which might be candidates for forming an unconventional reservoir. Most organic-rich shales worldwide are associated with eustatic transgressions. The basal Zechstein cycles, Z1 and Z2, contain organic-rich siliceous and calcareous mudstones and carbonates that form major transgressive deposits in the basin. Maturities range from over–mature (gas) in the basin to oil-generation on the slope with variable TOC contents. This sequence stratigraphic and sedimentologic evaluation of the transgressive facies in the Z1 and Z2 assesses the potential for shale-gas/oil and hybrid unconventional plays. Potential unconventional reservoirs might be explored in laminated organic-rich mudstones within the oil window along the northern and southern slopes of the basin. Although the Zechstein Z1 and Z2 cycles might have limited shale-gas potential because of low thickness and deep burial depth to be economic at this point, unconventional reservoir opportunities that include hybrid and shale-oil potential are possible in the study area.