A. A. Ekdale

Chondrites: a trace fossil indicator of anoxia in sediments.

The trace fossil Chondrites, a highly branched burrow system of unknown endobenthic deposit feeders, occurs in all types of sediment, including those deposited under anaerobic conditions. In some cases, such as the Jurassic Posidonienschiefer Formation of Germany, Chondrites occurs in black, laminated, carbonaceous sediment that was deposited in chemically reducing conditions. In other cases, such as numerous oxic clastic and carbonate units throughout the geologic column, Chondrites typically represents the last trace fossil in a biotutbation sequence. This indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters.

Selective preservation of burrows in deep-sea carbonates

Abstract Burrow preservation is a function of the rates of burrow production and burrow destruction and the rate of burial. These rates are related to water depth and therefore burrow preservation can be a depth indicator in deep-sea sediments. Box cores from fourteen localities in the eastern equatorial Pacific, taken from water depths of 3500–4850 m, showed a three-layer stratigraphy: a mixed layer with intense bioturbation (upper 5–7 cm), a transition layer (extending down to 20–35 cm), and a historical layer below. To be preserved, a burrow must reach below the mixed layer and into the transition layer. Abundance of vertical burrows is inversely proportional to water depth down to 4500 m, where vertical burrows disappear completely due to sediment shear and/or lack of burrow production. Within box cores the various burrow assemblages display an array of colors. Cross-cutting relationships of differently colored burrows help to establish a bioturbation chronology: burrow color fades with age. Two possible models for the observed burrow stratigraphy are presented: a steady-state model and a historical model. The steady-state model focuses on the activity of record-making burrowers versus the destruction of burrows by homogenizing burrowers, redox diagenesis and sediment flow. The historical model focuses on the effects of the early Holocene dissolution pulse on the observed burrow stratigraphy.

Sedimentology and Ichnology of the Cretaceous-Tertiary Boundary in Denmark: Implications for the Causes of the Terminal Cretaceous Extinction

ABSTRACT Trace fossils and other sedimentologic evidence of bioturbation in latest Cretaceous and earliest Tertiary marine deposits in Denmark provide important clues to understanding the ecology of the sea bottom immediately before, during, and after the terminal Cretaceous boundary event. Trace-fossil associations at all the onshore Danish boundary sites indicate a general upward shoaling of the late Maastrichtian chalk sea, which was accompanied by an abrupt shift from a carbonate ooze sea floor to a marly, clay-rich, unstable substrate at the boundary. Although the substrate condition may have become more stable in the early Danian, water depths never did return to their former deep level. There is evidence of localized anoxia during the deposition of some boundary sediments (in the Fish Clay) in shallow parts of the Danish Maastrichtian sea, but no clear evidence of anaerobic conditions occurring at the era boundary was observed in boundary strata deposited in deeper water near the center of the basin. In fact, earliest Danian marl there is totally bioturbated. A pulse of calcite dissolution in shallow water coincided precisely with the era boundary, and this event played a major role in the formation of the Fish Clay in eastern Denmark, which is a condensed series of smectitic clay-rich layers from which much calcite has dissolved. The effects of this dissolution episode, however, were not so extreme in the marl that was deposited at the same time in slightly deeper water in northwestern Denmark. Evidence from Cretaceous-Tertiary boundary sequences in Denmark and elsewhere suggests that no single catastrophe can account for the major biotic extinctions that occurred at the end of the Cretaceous Period. The primary causal factors of the terminal Cretaceous extinction event appear to be drastic global sea-level regression occurring simultaneously with extensive volcanism on land and a strong pulse of calcite dissolution in ocean surface waters.

Pitfalls of paleobathymetric interpretations based on trace fossil assemblages

Trace fossils have been used widely as indicators of original water depth in paleoenvironmental studies, but most paleobathymetric interpretations in ichnology have been based on only a small number of standard ichnofacies. In addition to bathymetry, it is important to recognize that salinity, oxygen concentration, and substrate character also play a major role in controlling the distribution of trace fossils. Several pitfalls of paleobathymetric interpretation based on trace fossils can lead to erroneous paleoenvironmental reconstructions. The most common of these pitfalls, of which historical geologists should be wary, include the following: 1) occurrence of a particular ichnogenus does not necessarily indicate the presence of the ichnofacies of the same name, and a standard ichnofacies may be identified without the presence of the namesake trace fossil; 2) environmental shifts of certain trace fossils have occurred through time, so the paleoenvironmental significance of a particular ichnotaxon may have changed during its histoty; 3) the nine standard ichnofacies that have enjoyed wide use by ichnologists have broad environmental significance that may include, but certainly extends beyond, bathymetry; 4) not all environmental situations are represented in the nine standard ichnofacies, so many trace fossil associations in the real world cannot be easily fit into those categories.

Bioerosional innovation for living in carbonate hardgrounds in the Early Ordovician of Sweden

Some of the worlds oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Oland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance.

Trace fossils in Cretaceous-Tertiary (KT) boundary beds in northeastern Mexico; implications for sedimentation during the KT boundary event

Cretaceous-Tertiary (KT) sequences in Nuevo Leon and Tamaulipas, Mexico, contain trace fossils that shed important light on the nature and duration of deposition of the KT boundary strata in that region. The KT clastic sequence in northeastern Mexico typically is divided into three distinct sedimentary units which represent distinctly different depositional events. Unit I is an unbioturbated, laminated deposit of alternating smectite grains and calcite spherules. Unit II is a sandstone that is mostly unbioturbated, but a few spherule-filled burrows occur near the base of the unit. Several burrows were truncated by overlying sand layers within Unit II, indicating that they were excavated following deposition of the first sand layers and then filled with spherules, scoured, and overlain by more Unit II sand. Unit III consists of alternating sandstone, siltstone, and shale that contain abundant trace fossils, including Chondrites, Ophiomorpha, Planolites, and Zoophycos. The nature of the trace fossil occurrences attest to at least three successive colonization episodes of the accreting substrate. The sandstone beds of Unit III were deposited episodically, and burrowing occurred during the period of deposition, not after deposition had ceased. The burrows were filled with late Cretaceous sediment. Trace fossil evidence indicates, therefore, that the entire KT clastic sequence must have been deposited over a long period of time. If the spherules in Unit I are material derived from an extraterrestrial impact, that impact must have predated the extinction of Cretaceous plankton by a significant time interval, which is represented by the periods of deposition of Units II and III. The ichnologic information indicates episodic deposition of Units II and III over an extended time period. Thus, the event that produced the calcite spherules in Unit I is not directly related to the Cretaceous plankton extinctions at the KT boundary, which occur at the top of Unit III.

Paleoethologic interpretation of complex Thalassinoides in shallow-marine limestones, Lower Ordovician, southern Sweden

Abstract Lower Ordovician Thalassinoides (T. bacae isp. nov.) from Oland, Sweden, is characterized by irregularly anastomosing, horizontal tunnel mazes with highly variable branching angles, accompanied by numerous closely spaced, short, vertical shafts that must have provided a large number of burrow openings to the sea floor. These complex trace fossils appear to be agrichnial burrow systems that represent chemosymbiotic feeding behavior by infaunal animals. They occur profusely in thin-bedded fossiliferous, lime mudstone and wackestone that were deposited on a shallow-marine, carbonate platform.

A day and a night in the life of a cleft-foot clam : Protovirgularia-Lockeia-Lophoctenium

A remarkable specimen of a compound trace fossil in Pennsylvanian sandstone comprises three very different ichnotaxa in conjunction: Protovirgularia dichotoma, Lockeia siliquaria and Lophoctenium isp. The combined activities represented by these ichnotaxa reflect the locomotion, resting and feeding behavior of a cleft-foot, protobranch clam (bivalve) that burrowed through the sediment, paused five times to deposit-feed, and then burrowed on to a new location, possibly as a reaction to a depositional event. It is estimated that the complete trace fossil was made in 24 hours or less. The three ichnotaxa also provide morphologic details of the bivalves shell and soft parts (foot and labial palps).

Paleoethologic significance of bioglyphs: Fingerprints of the subterraneans

Abstract Bioglyphs are features in burrow or boring walls produced by such animal activity as scratching, drilling, plucking, gnawing, poking, and etching. Bioglyphs are important aspects to consider when making paleoethologic interpretations of trace fossils, because they can offer direct clues to understanding the mechanism of excavation of the trace fossil, the identity of the tracemaker, the purpose of the burrow or boring, and the character of the sediment in which the trace fossil has been produced.

Ichnofacies of an Ancient Erg: A Climatically Influenced Trace Fossil Association in the Jurassic Navajo Sandstone, Southern Utah, USA

SUMMARY: Arid eolian environments usually exhibit a paucity of organism traces, but some eolianite facies in the geologic record contain a great abundance of trace fossils, which characterize a distinctive ichnofacies, herein termed the Entradichnus Ichnofacies. This arid landscape ichnofacies is exemplified by a locally dense and diverse invertebrate trace fossil assemblage, which is preserved in the Navajo Sandstone, a Jurassic eolianite exposed in the Paria Canyons Primitive Area in southern Utah. The trace fossils (Planolites, Palaeophycus, Skolithos, Arenicolites, Entradichnus, Taenidium and Digitichnus) all appear to be the products of shallow burrowing by desert-dwelling arthropods, such as beetles and other insects, that kept pace with the dune migration. The paleoclimate was monsoonal, characterized by rainy summers and windy (but relatively dry) winters. The burrowed beds were produced during long-lived pluvial intervals that brought higher than usual amounts of moisture to the Navajo dune fields. Most of the sand in the Navajo at the study site was deposited as dry grain flows during the winter months, and the only possibility of rainfall or dew precipitation came during the summer months. Nevertheless, the burrowers apparently were active year-round and exploited resources within both dry and damp sand.