Palaeoecology and evolution of marine hard substrate communities

Skeletal concentrations are ubiquitous in Bathonian–Oxfordian shallow water sediments of the pericratonic basins of Kachchh and Rajasthan, western India. By analysing the biofabric of the concentrations and the taphonomic signatures of individual components, it is possible to distinguish between the final concentration process (recorded by the biofabric) and events taking place prior to final deposition (recorded by taphonomic signatures). The skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations. They mainly reflect biological and hydraulic processes, and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient. Plotted against the stratigraphic sections, the skeletal concentrations show, in accordance with other sedimentological data, three orders of bathymetric trends. The first two orders are shallowing–deepening cycles which possibly correspond to eustatic changes in sea level; the third order represents parasequences with a strongly asymmetric sedimentary record. Skeletal concentrations are a useful additional tool in basin analysis, but are best used in combination with other data.

Shell beds as tools in basin analysis : the Jurassic of Kachchh, western India

Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow ( Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13 C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).

Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

Iron formations are economically significant, iron- and silica-rich sedimentary rocks that are restricted to Precambrian successions. There are no known modern or Phanerozoic analogues for these deposits that are comparable in terms of areal extent and thickness. Although many aspects of iron formation origin remain debatable, it is generally accepted that secular changes in the style of deposition are genetically linked to plate tectonic processes, mantle plume events, and evolution of Earth's surface environments. Two types of Precambrian iron formations have been recognized based on depositional and tectonic settings. Iron formations formed proximal to volcanic centers are interlayered with or laterally linked to submarine volcanic rocks and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger sedimentary rock-hosted iron formations are developed in passive-margin settings and typically lack a direct association with volcanic rocks. A full gradation between these two end-members exists in the rock record. Texturally, iron formations are divided into two groups. Banded iron formation (BIF) is predominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is more common in middle to late Paleoproterozoic successions, having been deposited in shallow-marine settings after the rise of atmospheric oxygen at ~2.4 Ga. Secular changes in the style of iron formation deposition have been linked to a diverse array of environmental changes. Geochronologic studies emphasize the periodicity in deposition of giant iron formations, which are coeval with large igneous provinces (LIPs). Giant sedimentary rock-hosted iron formations first appeared ~ 2.6 Ga, possibly when the construction of large continents changed the heat flux across the core–mantle boundary. From ~ 2.6 to ~ 2.4 Ga, global mafic-to-ultramafic magmatism culminated in the deposition of giant sedimentary rock-hosted iron formations in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited immediately before a shift from reducing to oxidizing conditions in the ocean–atmosphere system. Counterintuitively, enhanced magmatism at 2.50–2.45 Ga, which likely delivered large amounts of reductants to shallow-marine environments, may have triggered atmospheric oxidation. After the rise of atmospheric oxygen ~ 2.4 Ga, GIF became more abundant in the rock record than BIF. Iron formations largely disappeared ~ 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global-scale glaciations, during the so-called snowball Earth events. In the Phanerozoic, deeper-water iron formation deposition became restricted to local areas of closed to semi-closed basins, where volcanic and hydrothermal activity was extensive, such as in back-arc basins. In contrast, episodically deposited, basin-scale Phanerozoic oolitic and pisolitic ironstones are linked to periods of intense magmatic activity and ocean anoxia.

Late Paleoproterozoic iron formations and at least some Paleozoic ironstones were deposited at the redoxcline, where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to the production of the conditions necessary for deposition of iron formation. Mantle plume events that led to the emplacement of LIPs also enhanced spreading rates of mid-ocean ridges and resulted in higher growth rates of oceanic plateaus; both processes thus contributed to a higher hydrothermal iron flux to the oceans. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the ocean oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the ocean redox state, iron was deposited mainly proximally, generally as oxides or sulfides; manganese was more mobile. It is concluded that occurrences of BIF, GIF, Phanerozoic ironstones, and hydrothermal sedimentary rocks of exhalative origin (exhalites) surrounding VMS systems are a record of a complex interplay among mantle plume events, plate tectonics, and ocean redox conditions throughout Earth's history, in which mantle heat unidirectionally decreased and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter-term fluctuations.

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

As paleobiology continues to address an ever broader array of questions, it becomes increasingly important to interpret confidently the meaning of the pattern of fossil occurrences as found in outcrop. To this end, sequence stratigraphy is an important tool for paleobiologists because it predicts the distribution of unconformities, facies changes, and changes in sedimentation rate, all factors known from numerous previous studies to affect the quality of the fossil record. Computer simulations now make it possible not only to model sequence architecture within sedimentary basins, but also to model the occurrence of fossils within those basins. These models generate predictions regarding the stratigraphic distribution of first and last occurrences, changes in species abundance, changes in species morphology, and the distribution of gaps in fossil ranges. Although confirmation of some of these predictions has been found in field studies, the extent to which these predictions describe the fossil record in general is still unknown. If the predicted patterns of fossil occurrences are found to be widespread, it will suggest that a relatively simple model of fossil occurrences in outcrops could become a new tool for solving a wide array of paleobiologic and biostratigraphic problems. With such models, paleobiologists and biostratigraphers will be able to use model data to test the accuracy of newly developed methods of analysis.

The quality of the fossil record: a sequence stratigraphic perspective

Jurassic sediments in the shallow pericratonic basins of Kachchh and Rajasthan, western India, exhibit numerous signs of reduced sedimentation, omission, erosion and in situ reworking, in combination with synsedimentary cementation. Hardgrounds developed on carbonate shoals in the Bathonian of Rajasthan, whilst reworked concretion levels are characteristic of offshore siliciclastic sediments of the Callovian of Kachchh. A prominent marker horizon, the Oxfordian Dhosa Oolite Member, occurs throughout much of the Kachchh sub-basin and is a highly condensed unit characterized by hardgrounds, intraformational cobbles, reworked concretions, stromatolitic iron crusts, iron oncoids, and shell lags. Hardgrounds, reworked concretion levels, and condensation horizons are interpreted as the preserved relicts of transgressive pulses. Such pulses were possibly controlled by eustatic rise of sea level. Of at least equal importance, however, was a tectonic control which is demonstrated by the presence of small neptunian dykes, boulder beds derived from small submarine cliffs and rapid lateral facies and thickness changes in the Dhosa Oolite Member. These indications of extensional tectonics are thought to be connected to rifting and initial sea floor spreading between Africa and India.

Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis

Faunal response to transgressive-regressive cycles: example from the Jurassic of western India

Marker Beds in the Jurassic of the Kachchh Basin, western India: Their depositional environment and sequence-stratigraphic significance

The Late Cretaceous (Cenomanian to Coniacian) marine sediments of central India prevalently known as ‘Bagh Beds,’ have been deposited in the E-W extending Narmada Basin. The stratigraphy of these Cenomanian — Coniacian sediments has been reviewed and summarized. The Bagh Beds have been found to consist of three formations: Nimar Sandstone, Nodular Limestone and Corallian Limestone in ascending order. Main emphasis has been given to Nodular Limestone Formation (Turonian), which is the most fossiliferous horizon of the Bagh Beds. Nodular Limestone Formation has more or less alternating bands of varying thickness of nodular limestone and marl. It yielded numerous ammonoid specimens, which have been found to belong to a morphologically highly variable ammoniod taxon Placenticeras mintoi Vredenburg.

Comments on Placenticeras mintoi (Vredenburg, 1906) from the Bagh Beds (Late Cretaceous), Central India with Special Reference to Turonian Nodular Limestone Horizon

Marine Jurassic sediments (Bajocian-Tithonian) of the Kachchh Basin were deposited in a ramp setting. Except during the Middle and Late Bathonian, when a carbonate regime became established, the fill of the basin consists predominantly of siliciclastics. The sediments represent environments that range from coastal plains (rivers and associated flood plains with caliche nodules), deltas, brackish water lagoons, nearshore sand and iron-oolite bars of the inner ramp, generally situated above fair-weather wave-base, to the middle ramp influenced by storm-waves and by storm-generated currents, and finally to the outer ramp which is characterised by low energy, fine-grained sediments. Changes in relative sea level produced a cyclic sedimentation pattern. The rich benthic fauna of macroinvertebrates is dominated by bivalves, followed by brachiopods, gastropods, corals, serpulids, and sponges. The analysis of 370 statistical samples and more than 27, 000 specimens produced more than 40 benthic associations and assemblages. They show a relationship to several environmental parameters, two of which, salinity and climate, are briefly discussed. The spatial distribution of the facies and biota is outlined for two time slices, the Bathonian and the Callovian-Oxfordian, respectively.

Anand K. Jaitly

Papers

Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis

Faunal response to transgressive-regressive cycles: example from the Jurassic of western India

Marker Beds in the Jurassic of the Kachchh Basin, western India: Their depositional environment and sequence-stratigraphic significance

Comments on Placenticeras mintoi (Vredenburg, 1906) from the Bagh Beds (Late Cretaceous), Central India with Special Reference to Turonian Nodular Limestone Horizon

Environments and faunal patterns in the kachchh rift basin, western india, during the jurassic