ORDOVICIAN BIOGENIC STRUCTURES and PALEOENVIRONMENTS 1333 1957; Osgood, 1970; Chamberlain, 1971). Only One Recognizable Specimen
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ORDOVICIAN BIOGENIC STRUCTURES AND PALEOENVIRONMENTS 1333 1957; Osgood, 1970; Chamberlain, 1971). Only one recognizable specimen was found on a bedding plane in sublithofacies C2; other burrows of the same size abundant in sub- lithofacies C2 may also be parts of Chondrites systems. ?Planolites.—These straight, unbranched horizontal cylindrical burrows or burrow fill- ings 5 to 10 mm in length and .75 to 1.75 mm wide (Fig. 6b) are rare in sublithofacies C2. Because of the close and near parallel align- ment of these structures, the possibility that they are part of a larger, perhaps three dimen- sional structure such as a chondritid can not be excluded; thus they are only tentatively placed in the genus Planolites. Planolites are inter- preted as the burrows of deposit feeding ani- mals (Osgood, 1970; Aipert, 1974). Horizontal ?burrow.—A meandering ridge 2 mm wide and 1 mm high, bounded on either side by a steep groove, occurs on a bedding surface (Fig. 7a). Superficially the horizontal burrow resembles those in the uScolicia group" in which Hantzschel (1962) places all the ridged and grooved, presumably mollus- can trails. When compared with previous illustrations (Lessertisseur, 1955, Fig. 23; Os- good, 1970, Text-fig. 27; Aipert, 1974, Text- FIG. 6.—A. Chondrites from lithofacies C (x 1.5) fig. 7E-L) the Silica Quarry specimen is seen UCLA Geology Dept. Cat. No. 38553. B. ? Plano- to have a broader ridge and narrower grooves lites from lithofacies C (x 2) UCLA Geology Dept. than others in the uScolicia group." In addi- Cat. No. 38552. tion, the material in the ridge is darker than FIG.7.—A. Horizontal Pburrow from lithofacies C (x 1) UCLA Geology Dept. Cat. No. 38554. B. Bioturbated texture of lithofacies C (X 1) UCLA Geology Dept. Cat. No. 38555. 1334 M. F. MILLER the surrounding matrix, suggesting that the may be either continuous or interrupted. Simi- "trail" is actually a feeding burrow. lar structures produced by bivalves moving up- Bioturbated Texture.—In both occurrences ward through the sediment to maintain con- of lithofacies C there has been abundant re- tact with the sediment-water interface have working by organisms (Fig. 7b); in rocks of been illustrated by Kranz (1974, Fig. 11) and sublithofacies C2 the bioturbated texture is Schafer (1972, Fig. 223). Schafer (1972, Fig. nearly pervasive. 165) also shows that sea anemones are capable of escape behavior. Several of the structures at Lithofacies D the Silica Quarry locality appear to have a cen- Bioturbated Texture.—This is the most tral tube, suggesting that tube dwelling ani- prevalent biogenic structure in both occur- mals may also move up in response to rences of lithofacies D. The laminations show sedimentation. The escape interpretation is all degrees of disruption from undisturbed to supported by the fact that several of the struc- intensely bioturbated (Fig. 3). tures terminate along the same horizon. Skolithos.—These occur on vertical sur- Similar structures in Cambrian rocks are faces as burrows 2 to 5 mm wide (Fig. 4b) or as con^dcred by Bruun-Petersen (1973) to be es- circular welts on bedding surfaces. As noted cape features. Eagar (1974) described occur- above, forms in the ichnogenus Skolithos are rences of the bivalve Carbonicola in considered to represent the dwelling burrows downwarped burrows in the Upper Car- of sedentary suspension feeding animals. boniferous of the British Pennines. He Because suspension feeders derive their food placed the burrows in the ichnogenu- from the water column, it might be expected Pelecypodichnus. The present structures at- that each individual would require a certain not included in this ichnogenus, however, a volume of water and that the dwelling bur- they may have been formed by organisms rows would be evenly spaced. Cursory exami- other than bivalves. nation of a bedding surface containing Escape structures may very roughly indicate Skolithos, however, suggested that the forms the rate of sedimentation, even though it is are randomly distributed. As a check, the dis- impossible to determine whether the upward tribution of the burrows was compared with movement took place over a lifetime or in re- that expected if they were randomly (Poisson) sponse to a sudden influx of sediment. Accord- distributed. An approximately 1-cm2 grid was ing to C. A. Hall (1975, personal communica- laid over the surface (approximately 100 cm2) tion) the average lifespan of many modern and the mean number and variance of bur- clams is about 6 years. Assuming that the es- rows per square were calculated. In the Pois- cape structures in lithofacies D were made by son distribution, variance/mean = 1; clams and that Ordovician bivalves had simi- variance/mean < 1 indicates even spacing, lar lifespans to those living now, a layer of representing avoidance, and variance/mean >1 sediment the thickness of the escape structure indicates clumping. For Skolithos variance/ represents six years (or less) of deposition. De- mean = 1.128. Using the (Poisson) Dispersal position of lithofacies D was, therefore, at Test (Kendall and Stuart, 1967, p. 579), the least at times very rapid, for the escape struc- null hypothesis that the burrows are randomly ture 20 cm long indicates a minimum average distributed was not rejected at the .05 signifi- depositional rate of ZVz cm per year. cance level. This suggests that the Skolithos- forming organisms were neither clumped to- DISCUSSION gether nor evenly spaced, and, by inference, that the volume of water available to each in- Depositional Environments dividual (as indicated by the spacing) was not Lithofacies A.—The lithology and sedimen- a critical factor in determining the distribution tary structures of lithofacies A suggest that it of the Skolithos-producing animals. was deposited by sedimentary processes simi- Escape Structures.—Downwarped "cone- lar to those active in modern shoreface envi- in-cone" structures 3 to 20 cm in length ex- ronments. The abundance of cross-bedding, posed on vertical surfaces (Fig. 4b) are inter- presence of ripple marks and rarity of biotur- preted to result from escape behavior of bation resembles the shoreface features de- infaunal organisms. The deformed lamellae scribed by Reinecke and Singh (1973, p. 312 ORDOVICIAN BIOGENIC STRUCTURES AND PALEOENVIRONMENTS 1335 and Fig. 462) from the Gulf of Gaeta Italy and Brunswick. The great diversity in bedding Howard and Reinecke (1972, p. 104) from the types probably results from fluctuations in Georgia coast. Trough cross-beds, rare in current and energy conditions. This interpre- lithofacies A, are also found in the Cretaceous tation is further supported by the patchy dis- shoreface sediments described by Howard tribution of bioturbated layers, suggesting a (1972) from the Book Cliffs of Utah. Biogenic temporal and spatial mosaic of the generally structures characteristic of energetic low inter- stable substrate conditions required by the tidal to high subtidal environments with un- bioturbating organisms. stable substrates are simple robust dwelling Herringbone cross-laminations and reacti- burrows (Seilacher, 1967; Campbell, 1971; vation surfaces in lithofacies D indicate tidal Howard, 1972), similar to those found in influence; variations in energy levels, there- lithofacies A. fore, may have resulted at least in part from Lithofacies B.—This thin, laterally con- fluctuation in tidal intensity. Lithofacies D is tinuous bed of highly bioturbated dolomitic thickest at the Silica Quarry and thins mark- sandstone reflects deposition in an environ- edly within Vi km in all directions, strongly ment less affected by currents and waves than suggesting that these depositional conditions by the activity of organisms. This suggests were local in extent. deeper water conditions, probably similar to Lithofacies E.—Deposition of these rocks those in a modern lower shoreface zone. Howard occurred in a quiet offshore environment, as (1972) has described a Cretaceous lower indicated by the presence of rugose corals in shoreface facies as intensely bioturbated; life orientation and by the paucity and fine- biogenic structures include predominantly grained texture of the terrigenous sediment in horizontal Ophimorpha, presumably the bur- the dolomite. row of a callianassid shrimp. Perhaps the fea- Summary of Environmental Processes.— tures interpreted as possible arthropod bur- The sequence from lithofacies A through E in- rows in lithofacies B are analogous structures. dicates a change in the dominant processes, Lithofacies C.—The finer grained sediment, from those characteristic of a modern abundance of biogenic structures and presence shoreface (lithofacies A) to those predominant of fossils indicate that lithofacies C was depos- in an offshore environment (lithofacies E), and ited by processes characteristic of more a corresponding decrease in energy. The trend offshore, quiet water conditions than toward more quiet water conditions upward lithofacies A or B. In sublithofacies Ci (Fig. 2), through the vertical sequence is interrupted parallel laminations generally are obscured by twice by the shoal lithofacies, lithofacies D. bioturbated texture; in sublithofacies C2 laminae are rare, indicating that burrowing Distribution of Biogenic Structures was even more intense. Similar features are Particular biogenic structures are charac- found in the modern offshore environments teristically associated with certain lithofacies along the coast of Italy (Reinecke and Singh, in the Silica Quarry section. Lebensspuren in 1973, p. 313) and