Sequence Stratigraphy and Depositional Response to Eustatic, Tectonic and Climatic Forcing

Three increasingly realistic models of the stratigraphic distribution of fossils are presented. The first assumes a perfect stratigraphic record, the second allows sampling effects, and the third combines sampling effects, facies control, and stratigraphie cyclicity. When this third model is applied to depositional sequences exhibiting realistic stacking patterns, several patterns emerge. Taxa will typically display a scarce-common-scarce pattern of occurrence within a parasequence. This pattern may be abruptly truncated at a flooding surface, and will be separated by a long gap in adjacent parasequences provided both parasequences contain the proper facies for the collection of the taxon. Even longer gaps will occur if one of the parasequences does not contain the correct facies. For these reasons, first and last occurrences can be offset by up to one full sequence from the true times of origination and extinction. Clusters of first occurrences and last occurrences will tend to occur at major flooding surfaces (particularly within the transgressive systems tract) and at major basinward shifts of facies, such as at the base of the lowstand systems tracts. These clusters will have a predictable composition of taxa with respect to preferred water depth and facies tolerance. All of these patterns will be expressed most strongly for highly facies-limited benthic taxa, but will also be evident for less facies-limited benthic taxa as well as nektic and planktic forms that are ecologically tied to the benthos. Limited field data verify the existence of these patterns in the fossil record.

Miocene depositional sequences are identified based on sequence strati-graphic concepts applied to sixteen cores that transect the Hawthorn Group on the northeast Florida Platform. Sequence components which represent various stages of sea-level cycles are identified and interpreted in a mixed carbonate-siliciclastic platform setting that has been subjected to multiple depositional and erosional events. The ⁸⁷Sr/⁸⁶Sr composition of phosphorite and dolomite is used to determine the age of in-place phosphorite crusts and dolostone beds (condensed sections) and reworked phosphorite and dolostone sand and gravel (unconformities and transgressive surfaces) and to constrain the depositional age of associated lithofacies. A regional sequence stratigraphic framework is constructed and the depositional and sea-level history of the region is interpreted. The Sr-derived ages are used to document the age of highstands because the phosphorite formed from the early diagenesis of organic-rich sediments deposited during periods of high productivity that resulted from intensified and persistent upwelling associated with rising and maximum sea level. Seven major depositional sequences are documented that correspond to local, and possibly eustatic, sea-level fluctuations. At least seven high-stands occurred between 25 and 6 Ma with maximum flooding of the Florida Platform from 17–15 Ma.

Recent advances in basin analysis based on surface data advance sedimentary and paleoecologic research by taking into account the stratigraphic framework. Sequence stratigraphy and high-resolution event stratigraphy are well-known approaches that aid these advances. Ecostratigraphic interpretations have proved to be valuable tools in high-resolution event stratigraphy through the recognition of ecostratigraphic events. We propose the application of ecostratigraphy as a complement in sequence stratigraphy using ecostratigraphic trends. The conceptual basis consists of the assumption of close, though complex, relationships between the accommodation and the ecospace. Systems tracts, a key to sequence stratigraphy, are assumed to be related to shifting ecospaces and ecostratigraphic trends related to eustasy. Ecostratigraphic trends can also provide information about local ecospace deviations.

Appropriate ecostratigraphic sampling programs are of prime importance for ecostratigraphic interpretations based on the combined analysis of the stratigraphic features and the recorded fossil assemblages of megainvertebrates. Some applications demonstrated here are based on 7,000 megain vertebrates sampled bed-by-bed in sections belonging to the Subbetic Zone, Prebetic Zone, Algarve Basin, Iberian Cordillera, and the island of Mallorca on the Iberian Subplate. The five cases studied deal with condensed (ammonitico rosso) and expanded (rhythmic marly-limestones) facies from the Middle Oxfordian to the Lower Tithonian, and concern: a) the ecosedimentary evolution at the stage and substage levels; b) ecostratigraphic interpretations at the ammonite biochrono-zone level; c) comparison of faunal assemblages from distant epicontinental areas; d) relationship between ecostratigraphic interpretations and trends in abiotic components; and e) the influence of tectono-eustatic interactions on trends in the composition of fossil assemblages. We conclude that shifting bio- and lithofacies can be more adequately interpreted by combining ecostratigraphic and sequence stratigraphie approaches.

The stratal architecture of the Upper Miocene coral-reef platform of southwestern Mallorca, Spain, is controlled by high-frequency changes in accommodation and sediment supply (carbonate production), in the absence of significant compaction and subsidence during progradation. In this example, carbonate production and accommodation changes are not independent factors and both are, in turn, controlled by the changes of sea-level and morphology of the depositional profile of the basin floor.

The basic unit of accretion is the sigmoid which stacks in ever larger accretional units: sets, cosets, and megasets of sigmoids. All of these accretional units have the same characteristics in terms of stratal geometries, facies architecture and bounding surfaces, and may be viewed as depositional sequences reflecting different hierarchical orders of sea-level fluctuations. The stratal and facies architecture in sigmoids, sets, cosets, and megasets, reflect higher production of carbonate during sea-level rises and lower production during sea-level Stillstands and sea-level falls. Their stacking patterns allow definition of four reef-platform systems tracts: low-stillstand, aggrading, high-stillstand and offlapping.

On larger scale, progradation of carbonate reef complex is extensive (up to 20 km) toward the south, where the basin was shallow, but progradation is much less (less than 2 km) toward the west, along the margin of the relatively deeper Palma Basin. This results from the steepness and overall morphology of the depositional profile within the context of fluctuating sea level that controls carbonate production. Progradation of the reefal systems is more significant during sea-level falls on a gentle depositional profile. The subsequent sea-level rise creates a wide lagoon which enhances carbonate production and downslope shedding of sediment. A steeper topographic gradient allows only minor reef progradation during sea-level falls and, subsequently, a small lagoonal area is created during flooding of the platform, leading to proportionally small carbonate production and downslope shedding. This example illustrates how a reefal carbonate platform responds to high-frequency sea-level changes and how it differs from siliciclastic systems.

The stratigraphicsequences that occur throughout the sedimentary record are the products of independent variations in eustasy, tectonic movement and rates of sedimentation. The computer program Sedpak, which graphically simulates the sequence stratigraphicfill of basins by changing sea-level position, tectonic movement and sedimentation rate as independent variables, was used to test interpretations of the evolution of Neogene sequences in the offshore Canterbury Basin, eastern South Island, New Zealand. The simulation results for the Canterbury Basin suggest that sequence boundaries were created by changes in depositional base level (i.e. sea level), which in turn were driven by variations in the rate of eustatic and/or tectonic change. Varying the rate of sediment supply in the simulations did not produce sequence-bounding unconformities, but greatly influenced sequence stacking patterns.

Global rates of coal deposition correlate positively with long-term lowering of global sea level, probably because long-term first-order regression leaves sedimentary shelves exposed as coastal plains for development of peat swamps during higher-order short-term transgressions. Global coal depositional rates are weakly correlative with orogenic activity, probably because development of foreland basins favors peat deposition. Global coal depositional rates are not correlative with paleogeographic parameters, although previous work has shown that paleogeography is a major control on location of coal deposits. Global coal depositional rates are correlative with neither abundance of plant species nor changes in abundance of plant species.

The paleoclimatologic significance of these results is that, although coal is clearly a climatically sensitive sediment, its paleolatitudinal sensitivity may diminish during global regression. The geochemical significance is that coal deposition, like sulfate deposition, is a parameter important in governing the atmosphere’s O₂ and CO₂ contents. If, as recent work suggests, both coal deposition and sulfate deposition are functions of sea level change, eustasy may have been a major control on the changing composition of the earth’s atmosphere in the Phanerozoic.

During Early and early Late Triassic times, the Northern Calcareous Alps (Austria, southern Germany) and the Dolomites (northern Italy) were situated at the margin of the western Tethys. In the Scythian, widespread clastic-carbonate deposition on the shelf prevailed. Carbonate ramps revived in the earliest Anisian. From the late Anisian to the early Ladinian, carbonate ramps evolved to rimmed carbonate platforms. The Dolomites comprise five Scythian sequences, controlled by low amplitude sea-level changes and progressively increasing tectonic subsidence rates. During the Anisian to Ladinian, the sea-level fluctuations increased in amplitude. Five Anisian, three Ladinian and two early Carnian depositional sequences developed. Tectonic subsidence rates changed significantly over intervals of 2–5 Ma in the northwestern Dolomites, but developed steadily in the northeastern Dolomites. The Northern Calcareous Alps comprise two Scythian, five Anisian, four Ladinian and two early Carnian depositional sequences. The completely marine succession were only weakly affected by early tectonics. A distinct increase in subsidence occurred in the late Ladinian, leading to the change from distally steepened ramps to rimmed platforms. Only during this time interval, a rapid tectonic subsidence signal overprinted the sea-level signal.

Depositional sequences in the Early to early Late Triassic of the Northern Calcareous Alps and the Dolomites can be correlated, supported by biostratigraphic data. Local controls, for instance varying subsidence rates, were either subdued or can be accounted for by comparing different sections within one study area or both study areas as a whole. Deposition in the northwestern Tethys realm was strongly controlled by basinwide sea-level fluctuations. However, this need not imply eustatic control. In order to assess global sea-level changes, data from the northwestern Tethys have been compared to sea-level data from other Pangean margins. Although biostratigraphic resolution in other basins is limited, depositional sequences of other basins in the northwestern and eastern Tethys, epicontinental seas and the Arctic Sea can be correlated during much of the Scythian to early Carnian: the late Scythian to latest Anisian/earliest Ladinian, the late Ladinian to earliest Carnian and the late early Carnian. Correlative sequences in these basins suggest, that eustatic sea-level changes had a major influence on the development of depositional systems during Scythian to early Carnian times.

Eustatic and epeirogenic trends may be independently quantified given knowledge of flooding patterns for multiple, coexisting, tectonically-independent land-masses. Herein, we develop a method of analyzing paleo-continental flooding records in order to reconstruct both global sea-level trends and individual continental epeirogenic histories. Our method is based on the hypotheses that: 1) co-existing landmasses are likely to have experienced the same range of eustatic fluctuations, 2) differences in flooding are thus primarily a function of differences in coastal hypsometry, and 3) differences in estimated sea-level elevations between individual landmasses and the world may reflect continental epeirogeny. We apply the method to the flooding records of 13 Paleozoic landmasses for which detailed paleogeographic reconstructions are available (Ronov and others, 1984; Khain and Seslavinsky, 1991).

Our analysis indicates that Paleozoic eustatic highstands were probably +100 to +225 m above present sea level, which is substantially lower than previous estimates of +300 m (Vail and others, 1977) to +600 m (Hallam, 1984). Reconstructed epeirogenic histories suggest that Paleozoic continents experienced ±100 m of independent vertical motion relative to global sea level at a 10–40 m.y. timescale. Most large epeirogenic excursions coincided with major tectonic events such as rifting, passive-to-active margin transitions, and continental collisions, and may reflect a range of epeirogenic mechanisms for Paleozoic continents comparable to that documented for modern continents. Close links between eustasy and continental epeirogeny are suggested by the antithetic pattern of Gondwanan crustal motions and global sea-level elevations during the mid-Paleozoic.

Six third-order depositional sequences are documented for Late Cambrian time by interbasinal correlation of cyclic carbonates from tectonic settings in the Appalachian and Cordilleran passive margins, the Texas cratonic embayment, and the southern Oklahoma aulacogen. Paleobathymetric interpretation, integrated with graphic correlation, is used to establish the relative synchroneity of Upper Cambrian depositional sequences and is crosschecked with two quantitative techniques that provide an approximation of the accommodation history independent of fluctuations in carbonate sediment production. The first technique, Fischer plots, graphically illustrates systematic changes in the stacking patterns of meter-scale cycles that presumably reflect third-order changes in accommodation potential. The second technique, subsidence analysis, determines the accommodation remaining after the isostatic and thermo-tectonic components of total decompacted subsidence have been removed. Integrating the three methods enhances the interbasinal correlation of individual third-order depositional sequences and permits the construction of a robust relative sea-level curve for the Upper Cambrian of North America.

Comparison of the relative sea-level curve determined in this study with published curves derived from different regions of North America suggests that all six sequences have correlatives in other areas of the continent, supporting an allogenic control on sequence development. Detailed inspection of individual sequences from separate basins illustrates the influence of intrinsic factors such as tectonic setting, platform morphology, subsidence history, paleotopography, and prevailing oceanographic conditions on the Stratigraphic record. Even though each section is composed of different types of meter-scale cycles and component lithofacies that reflect the environmental dynamics of their depositional setting, similarities in the overall internal architecture of individual sequences are clearly evident, and suggests that continent-wide depositional patterns were controlled by a single allogenic mechanism, most likely eustasy.

The Early to Middle Cambrian Carrara Formation includes two partial third-order sequences and three complete third-order sequences. The first partial sequence, composed of the basal two Carrara Formation members, is completed by the addition of the underlying Early Cambrian Zabriskie Quartzite. The middle six Carrara Formation members compose three complete sequences. The second partial sequence incorporates the uppermost Carrara Formation member and is completed by the addition of the basal part of the overlying Middle to Late Cambrian Bonanza King Formation.

These third-order sequences consist of a basal siliciclastic portion and an upper carbonate portion, corresponding to Grand Cycles. Minimal evidence of erosion or exposure along Carrara Formation sequence boundaries indicates that they are Type 2 unconformities. Carrara Formation Grand Cycles allow sequence-stratigraphic modeling of a depositional system with two sediment sources that respond differently to changes in accommodation. Nearshore marine and non-marine, fine-grained siliciclastics prograded basinward from the craton and were deposited in shelf-margin systems tracts or by-passed the shelf if little to no accommodation space was available on the shelf. In either case, subsequent siliciclastic deposition was retrogradational forming transgressive systems tracts. Siliciclastics were distributed widely in shelf-margin and early transgressive systems tracts, but were not able to maintain wide distribution during later transgressive and highstand systems tracts. Subtidal and peritidal carbonates prograded cratonward from an offshore carbonate bank and were deposited in transgressive and highstand systems tracts. The carbonate bank experienced decreased sediment production rates during accumulation of the shelf-margin systems tract and ever-increasing rates during accumulation of the transgressive and highstand systems tracts. This interpretation of shelf-margin systems tract siliciclastics and highstand systems tract carbonates differs from some interpretations of Grand Cycles in the southern Canadian Rockies. Shelf-margin systems tracts in most (common) Grand Cycles are either very thin and difficult to distinguish from transgressive systems tracts or are missing entirely. In anomalous Grand Cycles, the shelf-margin systems tract is well developed with intertidal to supratidal lithofacies. Both the common and the anomalous types of Grand Cycles are present in the Carrara Formation.

Studies of short term sea-level change have emphasized the effects of climate on the volume of water tied up in continental ice. Here we discuss two different time scales of non-ice-related storage of water on the continents and their implication for sea-level change. Human activities generate a flux of water from continental reservoirs, such as aquifers and wetlands, to the sea. Our calculations suggest that this flux is currently in excess of 1/3 of the sea-level rise rate inferred from tide gauge records. This observation has implications for interpretation of 20th century sea-level rise. Secondly, on time scales of orbital variations, climatically driven changes in non-ice-related continental water storage can produce geologically significant cyclic change in sea-level. This mechanism of sea-level change may have been the dominant source of Milankovitch frequency eustatic fluctuation during periods of Earth history that lacked continental scale ice sheets. In a final comment we consider the impact of fluctuation in lake area on climate models, and on the abundance of modern lake related fauna.

We propose the hypothesis that hypervelocity asteroids and/or comets that collide with the Earth may be responsible for most third-order global sequences. Collisions with asteroids and/or comets greater than 10 km in diameter can produce 10³² to 10³³ ergs of energy which is capable of inducing global release of stress at plate boundary faults. In response, continental margins adjust to near isostatic equilibrium, inducing global marine transgressions. The frequency of terrestrial impact craters coincident with eustatic sea level events lends support to this hypothesis.

Previously, third order (1–3 million years) depositional sequences which characterize most continental margins had been attributed to either glacial eustasy, regional release of stress at continental plate boundaries, tectonically driven variations in sediment supply, or geoidal distortion. However, these mechanisms do not adequately explain global third-order cycles, as much of the Stratigraphic record cannot be equated with periods of glaciation and the latter three effects are local.