Holocene Examples Research Articles

Identifying animal taxa used to manufacture bone tools during the Middle Stone Age at Sibudu, South Africa: Results of a CT-rendered histological analysis

This paper presents the histological characterisation of a selection of worked bone artefacts from Middle Stone Age layers at Sibudu cave, South Africa. Histographic rendering is achieved using high-resolution Computed Tomography, which is non-destructive and facilitates three-dimensional histologic analysis. Excellent congruency in image quality was achieved with previous studies using this method. The results show that most of the artefact fragments contain mostly primary lamellar tissue, which is the bone tissue best adapted to withstand impact stresses. This indicates that bone with greater elastic properties was chosen. Histological characterisation allows the identification of animal taxa. Based on the sample analysed in this paper, Perissodactyla bone was used predominantly in the older layers at the site. Artiodactyla are represented throughout but appear far more frequently in the later (post-Howiesons Poort onwards) layers. Some of the Artiodactyla specimens have high proportions of Haversian tissue, reducing elasticity. The higher percentages of Haversian tissue in the post-Howiesons Poort artefacts relative to Holocene examples from southern Africa suggests that people may have started experimenting with bone from different animal taxa at this time and had not yet learned to eliminate the mechanically weaker secondary tissue. Apart from mechanical considerations, possible cultural constraints governing raw material selection is also explored.

The potential for perimarine wetlands as an ecohydrological and phytotechnological management tool in the Guadiana estuary, Portugal

Climate and human impact on terrestrial sediment flux, nutrient and pollutant delivery to the coastal zone in the Algarve can be clearly demonstrated with both past and recent examples. A Holocene period of high sediment input, supporting a depositional coastal response to relative sea-level rise, has now given way to a period of coastal retreat where increased damming of rivers has led to reduced river flow and terrestrial sediment input. These changes in water and sediment budget are coupled with increased nutrient and pollutant influx associated with agriculture and industrial/urban development. Solutions to these problems may only be addressed through integrated river basin management, and with ecohydrological principles playing a critical role in the development and routine use of sustainable management techniques. Whilst riparian wetlands and saltmarshes are recognised as ecohydrological management tools with which to regulate hydrology, sedimentation, nutrient status and pollutant sequestration and the conservation of biodiversity, the same is not true of perimarine wetlands that occupy the transition zone between catchment and sea. Evidence from Holocene examples of estuarine and back-barrier perimarine wetlands in the UK suggests that they are well adjusted to the conditions of high relative sea-level rise (3–6 mm yr −1) and low terrestrial sediment supply. Whilst Holocene examples in Portugal are sparse, the present conditions of reduced sediment input due to damming and enhanced relative sea-level rise due to Greenhouse warming now makes perimarine wetlands a viable coastal management option. Here, the benefits and practical aspects of perimarine wetland development are considered with a view to establishing a demonstration pilot project in the Guadiana.

Sedimentary facies and depositional architecture of ice-contact glaciomarine systems

The characteristics of ice-contact glaciomarine systems are reviewed, based on a range of Weichselian and Holocene examples from the fjordal basins in Norway and Svalbard. These depositional systems occur as large morphological ridges, “end-moraines” (including “push-moraines”), which are coarse grained and show large-scale foreset bedding. The paper addresses some of the major problems with regard to the recognition, classification and stratigraphic analysis of these systems. The ice-proximal glaciomarine systems are classified here into three categories: (1) ice-contact submarine fans, (2) ice-contact deltas and (3) glaciofluvial deltas. This classification integrates morphological and sedimentological criteria, and renders these glaciomarine systems not only easier to recognize in the stratigraphic record but also possible to compare with common, non-glacial deltaic systems. The emphasis is on the criteria for the recognition of the ice-contact glaciomarine systems (i.e., ice-contact fans and ice-contact deltas) and their discrimination from common Gilbert-type deltas and fan deltas. The review further focuses on ice-contact glaciomarine systems as highly sensitive recorders of glacier dynamics and climatic fluctuations. It is shown that both the periods of ice-front “stability” and changes in the ice-front position, as well as the magnitude and rate of those changes, are all recorded in the ice-contact depositional system. The sedimentary record further reflects glacio-isostatic sea-level changes and local palaeogeography. A new conceptual model is suggested, which places these ice-contact glaciomarine systems in a dynamic stratigraphic context. The model integrates sedimentary facies and their depositional architecture in an allostratigraphic framework, and is meant to be a guide for the analysis of the sedimentary record of ice-contact systems.

Constructional Azooxanthellate Coral Communities: An Overview with Implications for the Fossil Record

As evidenced by the success of zooxanthellate coral species today, symbiosis with algae confers great advantages to organisms inhabiting the reef environment. Although not as well known as their zooxanthellate counterparts, azooxanthellate coral communities, dominated by so-called corals, can and do build constructionalframework, harboring a variety of associated vertebrate and invertebrate fauna. As has long been noted, when it comes to reefs and constructional roles, zooxanthellate and azooxanthellate corals exhibit mutually exclusive relationships. This is traditionally explained in terms of competition. When zooxanthellate species are absent due to restrictive environmental conditions, azooxanthellate species may inhabit relatively shallow water, even building limited constructional framework. In some non-reef settings where zooxanthellate corals are not living under optimal conditions, azooxanthellate species can be found living alongside them. On the shallow-water reef as a rule, azooxanthellate corals are relegated to cryptic habitats not colonizable by the overwhelmingly dominant zooxanthellate species. A review of 16 well-documented examples of modern constructional azooxanthellate coral communities and their physical and biological characteristics reveals a wide variety of latitudinal and depth ranges, temperatures, species composition, thicknesses, and species diversity. In all major oceans, they are best developed in cold and relatively deep water, as deep as 1500 m, and from the equator to latitudes of over 70?. Besides having a predominantly coral composition, some deep-water, azooxanthellate coral communities with extensive constructional framework have unexpected faunal diversity and presumably also complex interactions. The paleobiogeography and evolution of some of these deeper-water, coral-dominated azooxanthellate communities, known to have inhabited atolls and oceanic seamounts, may have been influenced by plate tectonic seafloor movements. While we are becoming increasingly cognizant of Holocene examples, counterparts in the geologic record are much more poorly known. The apparent paucity of ancient analogs may stem from difficulties in their interpretation and recognition in the geologic record. Judging from the size, facies relationships, faunal content, and diversity of Holocene examples, ancient deep-water coral structures, when preserved in the rock record, could be easily misconstrued as features of shallow-water origin. More careful analysis of deep-water Holocene examples can greatly expand our awareness of ancient analogs. In some cases, depending on the amount and thickness of construction, they could hold potential as hydrocarbon reservoirs. Azooxanthellate scleractinian communities are common in today's oceans and examples of them extend back to the Middle Triassic period. Fossil examples, unfortunately, are rare and sporadic in stratigraphic distribution, with the most notable reports from the Tertiary. Our view of the relative abundance of ancient counterparts may be hampered by poor preservation and problems of recognition.

Fan-Delta Deposition in Lower Cotton Valley Group Sandstones of Northeast Texas: ABSTRACT

Fan deltas have been defined as progradation of alluvial fans into a standing body of water from a proximal highland area. Sedimentary environments associated with fan delta complexes have been described in detail in Holocene examples. The subaerial fan is composed of braided channels, gravelly beaches, flood plains, and marshes. The subaqueous fan includes tidal lagoons, channel-fill complexes, marginal islands, breaker bars, and is also characterized by steep slopes and submarine channels where mass-gravity processes may dominate. Few fan delta complexes have been recognized in the subsurface. The Cotton Valley Taylor B Sandstone is interpreted as the distal part of a prograding fan delta based on the vertical sequence in cores from Kildare field, Cass County, Texas. Three End_Page 423------------------------------ major sandstone facies are distinguished on the basis of sedimentary structures, composition and texture. They are, in descending order, (1) a massive to laminated, well-sorted sandstone, (2) a laminated, pebbly sandstone, and (3) a very fine-grained sandstone interbedded with black shale. Facies 1 is fine-grained (0.21 mm), lacks bedsets, and is mostly massive or indistinctly laminated. Monocrystalline quartz content is high, reaching 95%. Other minerals are minor at 4%, and rock and shell fragments range from 5 to 15%. Facies 1 is only 10 ft (3.1 m) thick and is interpreted as bar or beach deposits formed by reworking of sand by waves at the margins of the delta plain. Facies 2 is composed of conglomeratic bedsets which fine upward from a pebbly erosive base to a parallel or cross-laminated top. Bedsets range from 0.5 to 3 ft (15 cm to 1 m) thick. There is an increase in rock and shell fragments and a decline in monocrystalline quartz from Facies 1, averaging 34% and 63% respectively. The facies remains fine-grained (0.19 mm); however, average grain size and bed thick ess increase upward. Facies 2 is interpreted as a complex of shallow, braided, distributary channels. Thickness is variable but ranges from 10 to 20 ft (3.1 to 6.2 m). Facies 3 bedsets are graded and the sequence of sedimentary structures reflects deposition under conditions of decreasing flow regime. The thin, 1 ft (0.3 m), graded sandstone beds are separated by black laminated shale. This facies is very fine-grained (0.09 mm), relatively poor in quartz, 70%, and rich in matrix which may exceed 30%. Facies 3 is interpreted as thin-bedded turbidites deposited in channels and channel margins downdip from the break in slope at the edge of the delta platform. The thin AE, ABE, BE and CDE bedsets alternate with intervals, 1 to 3 ft (0.3 to 1 m) thick, of bioturbated siltstone or rippled sand tone indicative of normal, shallow marine biotic and current activity. There is a sharp, erosive relationship between Facies 2 and 3. Resistivities are variable and SPs depressed due to extensive carbonate cement (up to 50% total rock) which is also responsible for low porosity and permeability. Dissolution of authigenic carbonates results in secondary porosity up to 17%. Cross-plots of resistivity (Rt) versus porosity are useful in characterizing facies and enable identification of facies in non-cored wells. End_of_Article - Last_Page 424------------

Patch Reef Modeling--A Comparison of Devonian and Recent Examples: ABSTRACT

In reef research, models have been developed to define variations in the lithic and biotic development of facies. Walker and Alberstadt, and Hoffman and Narkiewicz developed models for growth of ancient reef communities. Although these models form a solid foundation by which patch reefs can be classed and zoned, they are neither complete nor accurate for all reef types. A comparison was made of Lower Devonian patch reefs from the Appalachian basin of New York, New Jersey, and Pennsylvania, and Holocene examples from the Bahamas and Florida Keys to identify the structure, orientation, community variability, and succession of the reef biofacies. The complexion and genesis of the carbonate lithofacies were also studied. Results show similarities; these include the size, area distribution, 3-D geometry, wave-resistance potential, lateral sequences of facies, sedimentary textures and structures, vertical zonation explained by growth from low-energy to high-energy regimes, biotic diversity, growth habit and form, and postmortem alteration. Thus, when used in conjunction with the traditional models, the recent can serve as the basis for a general model which include most patch reef types. However, these models should not be used as explicit analogs for all Phanerozoic reefs. Knowing and understanding the limitations of these comparative studies are essential to a fuller comprehension of the potential for variations which exist within and between the traditional models. End_of_Article - Last_Page 1459------------

Endolith microborings and their preservation in Holocene-Pleistocene (Bahama-Florida) ooids

Holocene ooids from Joulters Ooid Shoal (Bahamas) are bored in various ways by blue-green algae that groove along the grain surface, reside just beneath the grain surface, and tunnel extensively a few tens of microns within the grain. The microborings, morphologically distinctive, are documented with scanning electron micrographs of open borings and resin casts. Gentle dissolution of ooid aragonite permits identification of several algal genera by light microscopy and enables comparison with the microboring casts. Pleistocene ooids from the Miami Limestone (Florida) contain natural casts of microborings, some of which are similar in form to Holocene examples. Significantly, these aragonite casts are more resistant to solution than surrounding ooid aragonite. They remain after most of the ooid is leached away and survive replacement of the ooid by low-Mg calcite. Dissolution or precipitation may occur along the walls of microborings, causing morphological alteration during their preservation. This points out a difficulty in the specific identification of endoliths on the basis of fossilized microborings in ancient rocks composed of original aragonite grains.