Brian R. Pratt

Syneresis cracks: subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering

Abstract Syneresis cracks, often confused with subaerial desiccation phenomena, are traditionally ascribed to subaqueous shrinkage whereby salinity changes caused deflocculation of clay. This and other previously proposed mechanisms fail to account for their occurrence in low-energy, typically non-evaporitic facies, stratigraphically sporadic distribution, intrastratal formation under shallow burial depths, variation in morphology, degree of contraction, generation of sedimentary dikelets as crack fills, and deformation of dikelets and enclosing layers. Instead, it is suggested that ground motion from strong synsedimentary earthquakes caused argillaceous sediments to dewater, interbedded sands and silts to be almost simultaneously liquefied and injected into the resulting fissures, and then these dikelets to be distorted. Comparative rarity of syneresis cracks in Phanerozoic versus Precambrian marine strata is considered to be primarily an evolutionary consequence of theological changes caused by increased organic binding of clay flocs in the water column, greater input of organic matter into the sediment, and the diversification of sediment-dwelling bacteria and meiofauna in Phanerozoic deposits.

Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic

Abstract Predation as an important driver of evolutionary change long has been assumed, despite difficulties to substantiate it with specific examples of predatory interaction, especially for the early Paleozoic diversification of animal life. This study corroborates the existence of shell-drilling predation in the uppermost Neoproterozoic of China. Nearly one-fifth of almost one hundred tubular shells of one of the earliest mineralized animals, Cloudina, are perforated by undoubted predatory borings 15–85 µm wide. By contrast, no specimens of co-occurring shells belonging to Sinotubulites were affected. The identity of the predator remains elusive, but variation in size of the borings suggests a predatory lifestyle throughout its growth, after it reached a minimum size. The relatively uniform distance of the borings from the shell apertures points to either control by the life orientation of the shells, such as the position of the sediment surface, or, more likely, an avoidance response by the predator to protective measures located near the aperture. Assuming Sinotubulites had similar life habits and was potential prey, the absence of borings in this taxon is evidence that these tubes may have been protected by organic material or toxins that fended off shell-drilling predators. Hence, this earliest example of predation in the fossil record already shows prey selectivity and site-specific behavior, pointing to a level of Precambrian predator-prey interaction that approaches the complexity seen in younger Paleozoic benthic animal communities. This is consistent with the suggestion that predation was indeed an active contributor to the Cambrian radiations.

Stromatolitic Framework of Carbonate Mud-Mounds

ABSTRACT Mud-mounds are an important Phanerozoic reef type generally formed in deeper water on carbonate shelf-to-basin slopes. They commonly contain radiaxial calcite spar-filled depositional cavities (stromatactis) but lack abundant metazoan framebuilders. Mud-mounds are often flanked by steeply dipping bioclastic grainstones and sometimes attained widths and depositional reliefs of over a hundred meters. They occasionally occur as basal cores on which developed shallow-water metazoan-constructed reefs. Stromatactoid mud-mounds are composed of a framework of submarine-cemented, crudely reticulate masses or a succession of laminar crusts, surrounded by cement-filled cavities. These cavities developed when unconsolidated sediment around the framework was removed by winnowing. Mud-mound frameworks of all ages show strong geometric and petrographic similarities to shallow-water cryptalgal structures, especially thrombolites (unlaminated stromatolites) which occur in many Phanerozoic shallow-water reefs. Both thrombolites and mud-mound frameworks are largely composed of lime mudstone, cemented on the sea floor, and contain fenestral fabric and diagnostic cryptalgal microstructure. Their depositional surfaces were irregular, not bored or burrowed, and rarely encrusted. Stromatolitic lamina ion, although commonly not well developed in the subtidal zone anyway, occasionally occurs in mud-mounds. It is proposed that mud-mounds accumulated from organic (probably blue-green algal) binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension. Patchy distribution of algal mats resulted in areas of both bound and unbound sediment. Sporadic floodlike episodes of rapid sedimentation, however, temporarily smothered the living algal mat and also left layers of unbound sediment. Periods of winnowing washed out unconsolidated unbound sediment and promoted synsedimentary cementation of resulting stromatactoid cavities and algal-bound framework.

Stromatolite decline—A reconsideration

The prevailing concept of Phanerozoic stromatolite decline is considered too simplistic. Changes in stromatolite character and proportion to other lithologies after the Precambrian can be explained as responses to substrate competition and changing sedimentological conditions caused by the evolution of skeletal metazoans. Grazing pressure was likely insignificant as a limiting factor.

Calcification of cyanobacterial filaments: Girvanella and the origin of lower Paleozoic lime mud

The origin of lime mud in ancient shallow-marine carbonate platforms is one of the most vexing mysteries in sedimentary geology. Possible sources previously suggested include the physical and biological disintegration of animal and algal bioclasts, mineralizing pelagic organisms, and spontaneous whitings from either abiotic precipitation or calcification of suspended picoplankton and organic matter. The styles of occurrence of micrite- walled tubes referred to Girvanella in lower Paleozoic limestones argue that occasional permineralization of mucilaginous sheaths of the benthic filamentous cyanobacteria precursor could have produced lime mud when the tubes fell apart. Estimated volumes are comparable to those calculated for the modern Bahama platform. The copious growth of cyanobacteria typical in shallow-marine settings indicates that this mechanism could account for a considerable amount of lime mud, especially in the early Paleozoic.

Molar-tooth structure in Proterozoic carbonate rocks: Origin from synsedimentary earthquakes, and implications for the nature and evolution of basins and marine sediment

Proterozoic argillaceous lime mudstones commonly contain molar-tooth structure, a curious synsedimentary feature consisting of crumpled, generally vertically oriented veins filled with calcite microspar. Long enigmatic, these veins are interpreted here as earthquake-induced deformation structures. A model is proposed whereby violent ground motion caused shrinkage, dewatering, and fissuring of the colloidal sea-floor sediment of clay and lime mud. A slurry of equant particles of lime mud, segregated from the clay platelets trapped in the matrix, was expelled into these voids. The host bed underwent as much as 80% compaction as well as shear, causing the lime mud-filled fissures to be ptygmatically folded and shingled both vertically and horizontally, and in places pulled apart. This lime mud lithified rapidly by high-Mg calcite grain growth, such that subsequent earthquakes shattered the folded veins. In the Belt basin of western North America, tsunamis from different events created currents that transported ooids and coarse sand from shoals into deeper water, and scoured deeply into the now-consolidated sediment, leaving behind a lag of vein fragments. Molar-tooth structure is a signature of basin tectonism recorded specifically in argillaceous lime mudstones deposited in low-energy settings above the thermocline in supersaturated marine water. Syneresis cracks are the counterpart in terrigenous facies, in that silt and sand were injected into the shrinkage fissures. This process points to the importance of seismically induced compaction under negligible burial. Smectite might have made up a significant proportion of the sediment, making it more conducive to shrinkage. The absence of molar-tooth structure in Phanerozoic strata is ascribed mainly to changes in sediment rheology brought about by increased organic binding due to diversification of the microbiota.

Middle Cambrian Arthropod Embryos with Blastomeres

A phosphatized Middle Cambrian (∼510 million years ago) fauna from Duyun, southern China, has yielded fossil embryos that may be of arthropod affinity and could belong to the co-occurring eodiscid trilobite Pagetia sp. The shell was most likely flexible and possessed at least two thin layers. Four embryos reveal blastomeres, and two embryonic stages are represented. These embryos demonstrate that the basic paradigm for the growth of the invertebrate embryo has not changed in more than half a billion years.

Storms versus tsunamis: Dynamic interplay of sedimentary, diagenetic, and tectonic processes in the Cambrian of Montana

The Upper Cambrian Deadwood Formation of north-central Montana is composed mainly of fine-grained calcareous sandstone and intercalated silty shale, and contains scattered intraclastic flat- pebble conglomerate. These rocks, typical of many mixed carbonate- siliciclastic, subtidal shelf successions of Proterozoic through Early Ordovician age, are distal storm deposits in which the conglomerates are conventionally thought to record occasional, very high energy events that scoured incipiently cemented layers. In the Deadwood Formation, most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay. The cracks exhibit a consistent northwest-southeast trend, and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in Idaho. Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. “Tsunamites” are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.

Seismites in the Mesoproterozoic Altyn Formation (Belt Supergroup), Montana: A test for tectonic control of peritidal carbonate cyclicity

The Mesoproterozoic Altyn Formation of western Montana contains abundant seismites that record the physical state of the sediment at the time of earthquake-induced ground motion. Earthquakes were likely generated by normal faulting during subsidence. Because part of the Altyn is composed of cyclic, upward-shallowing peritidal carbonates, it may afford an opportunity to test whether spasmodic subsidence provides a viable mechanism for the episodic generation of the requisite accommodation space at cycle tops. No stratigraphic relation between the seismites and peritidal cycles is perceived, however, so it appears that this process may not be a control of cyclicity.

Nutrient-triggered bioerosion on a giant carbonate platform masking the postextinction Famennian benthic community

The Palliser Formation of western Canada constitutes a giant, tropical carbonate platform of Famennian age. Although now mostly composed of peloids, aggregates, intraclasts, and cortoids, a major proportion of these micritic particles originally consisted of bioclasts, primarily crinoidal, that were obliterated by bioerosion. Analogous to modern tropical environments, microendoliths may have proliferated on this platform during widespread mesotrophic conditions owing to excess nutrients derived from the developing Ellesmerian orogen in the Canadian Arctic. The orogeny was coincident with profound changes in the middle Paleozoic biosphere due to increased pedogenesis accompanying the spread of deep-rooting gymnosperms. This evolutionary event may have resulted in the disturbance of the ecological balance in epicontinental seas by causing enhanced nutrient mobilization and riverine nutrient flux. This precursor to the observed Mesozoic increase in bioerosion hides a bountiful, although low-diversity, skeleton- secreting benthos on the Famennian platform, thereby concealing the extent of the Late Devonian faunal crisis and its recovery.