Stromatoporoid shape and burial ratio changes during growth history and their methodological consequences in morphometrical analyses

Abstract

LETHAIAŁuczyń ski, P. 2006 12 01: Stromatoporoid shape and burial ratio changes during growth history and their methodological consequences in morphometrical analyses. Lethaia , Vol. 39, pp. 339 Á358. Oslo. ISSN 0024-1164. Eight Devonian stromatoporoids with clearly exposed arrangements of latilaminae were subjected to detailed morphometrical analyses. Distinctive sets of latilaminae were marked on photographs taken from polished specimens, which allowed reconstruction of their individual growth histories by presenting consecutive stages of their growth. The growth forms measured above the sediment surface and the overall shapes of the skeleton at a given time have proved to differ distinctly between each other throughout the growth history of these stromatoporoids. The morphometrical features of individual specimens (both of their growth forms and of their whole skeletons) distinctly changed throughout their development. Changing environmental factors directly influenced the growth form above the sea floor, and each specimen shows its own specific history of growth form changes. Instead, overall shapes of the skeleton of most of the studied specimens changed in a similar general manner resulting from growth during sediment accumulation. Basing the palaeoenvironmental considerations only on the stromatoporoids' final shapes may thus be very misleading. It is therefore suggested that the ascription of a specimen to a particular morphometrical category should be based on the mode of its growth history represented by a curve recording the V /B changes (vertical height versus basal length) during consecutive growth stages. Certain sedimentary processes have their direct reflection in the mode of stromatoporoid growth, and are recorded by the attributes of the shape profile (V /B ), and burial ratio (BR ) curves, which allows deciphering such features as, for example, periodicity of sediment supply, substrate consistency and tempo of sediment cementation. This is particularly valuable when the deposits are recrystallized and the sedimentary structures are not visible. The paper also tests the applicability of the new definitions of the parameters describing the stromatoporoid shape introduced recently by the author. I Burial ratio, measurements, ontogenesis, shape changes, stromatoporoids.

Stromatoporoid Beds and Flat-Pebble Conglomerates Interpreted as Tsunami Deposits in the Upper Silurian of Podolia, Ukraine

“…Unsorted stromatoporoids of various dimensions are evenly distributed throughout the thickness of the beds, forming clast-supported textures. No individuals show continuation of growth after redeposition, which is a common feature of stromatoporoids (Kershaw and Brunton 1999;Łuczyński 2006). This indicates that they were totally buried by sediment after deposition in the parabiostromal bed.…”

Tsunamites versus tempestites: Various types of redeposited stromatoporoid beds in the Devonian of the Holy Cross Mountains (Poland), a case study from the Ołowianka Quarry

“…In order to systematically describe the observed variety of stromatoporoid shapes, Kershaw and Riding [ 67 ] introduced a stromatoporoid morphology parameterization, which has been widely adopted, improved, and supplemented [ 18 , 24 , 52 , 61 , 68 , 69 ]. The shape of massive basal stromatoporoid skeletons ranges from laminar , through domical (divided into low-, high-, extended-, and highly extended domical), to bulbous (divided into low- and high bulbous) ( Fig 3A ).…”

“…Other than the shape itself, the most important macroscopic morphometric features interpretable in terms of environmental conditions are: upper surface character, which can be smooth or ragged ; the arrangement of latilaminae (major growth bands), which can be enveloping and non-enveloping (with intermediate variants); and the type of initial surface, which is referred to as flat , initial elevation , anchor , and encrusting ( Fig 3B ). An analysis of latilaminae arrangement enables workers to discern skeletal shape from the living surface profile (i.e., growth form), which represents the part of the specimen protruding over the sediment surface ( Fig 3C ); the relation between the two is described by the burial ratio ( BR ) parameter [ 69 ]. More sophisticated measurements, designed to quantify the shape of the upper surface (for instance, upper surface curvature and convexity) and the shape and inclination of the skeletal sides, have been recently introduced [ 61 ].…”

“…Stromatoporoids grew as sediments accumulated around them; thus, usually only parts of their upper surfaces remained unburied and were covered by soft living tissue from where growth could continue. Therefore, the final skeletal shape and the changing growth form protruding from the seafloor in successive growth stages ( Fig 3C ) are two distinct features with different implications for paleoenvironmental analyses [ 69 ]. Enveloping latilaminae arrangements (with subsequent latilaminae completely covering those preceding and protruding to the bottom of specimens; Fig 3B ), in conjunction with smooth upper surfaces, indicate low deposition rates and, usually, calm sedimentary settings.…”

“…The hazard of potential burial via sedimentation, and thus the necessity to keep pace with rapid deposition, was probably the most important factor influencing stromatoporoid growth. Initial elevations, high profiles, and ragged margins (and in particular, the co-occurrence of these characters) are the clearest indications of such conditions, in addition to internal sediment increments interrupting skeletal growth [ 52 , 65 , 69 , 71 ].…”