Hummocky cross-stratification, tropical hurricanes, and intense winter storms

Most previous workers have inferred a storm origin for hummocky cross-stratification, which typically occurs in shallow-marine deposits. On the modern Earth, the only storms capable of profoundly affecting shallow-marine depositional environments are severe tropical cyclones (hurricanes) and mid-latitude winter wave cyclones (intense winter storms). This paper examines the palaeogeographic distribution (including palaeolatitude and palaeogeographic setting) of 107 occurrences of hummocky cross-stratification, ranging in age from the Proterozoic to Recent. In each of these stratigraphic units, both palaeolatitude and palaeogeography are consistent with a direct storm influence (associated with the passage of hurricanes or winter storms directly over the site of deposition). This palaeogeographic evidence lends support to the inferred storm origin for hummocky cross-stratification; further, the distribution of the structure suggests that most occurrences (73%) were generated by tropical hurricanes, the remaining 27% being generated by intense mid-latitude winter storms. The preferential generation of hummocky cross-stratification by hurricanes is consistent with: (1) the known differences in the nature of the bottom flows generated by the two major storm types, and (2) the inferred nature of the flows which form hummocky cross-stratification. Hurricanes couple less effectively with the water column than do intense winter storms. Due to this ineffective coupling, hurricane-generated bottom flows tend to be oscillatory-or multidirectional-dominant, with only minor unidirectional components of motion. In contrast, intense winter storms generally do couple effectively with the water column, generating bottom flows which possess a dominant or significant unidirectional component. Most previous workers have suggested that hummocky cross-stratification forms under oscillatory- or multidirectional-dominant flow; thus, it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross-stratification were probably hurricane-generated, as suggested by the aforementioned palaeogeographic distribution. The Proterozoic, Palaeozoic, Neogene, and Quaternary were times when global climate was similar to that of today. The distribution of hummocky cross-stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts. The Mesozoic and Palaeogene were non-glacial times when global climate was much warmer than that of today. The distribution of hummocky cross-stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non-glacial times than they do at present. The possibility of a broadened hurricane belt during the Mesozoic and Palaeogene is consistent with climatic considerations. A limited number of Mesozoic and Palaeogene rock units containing hummocky cross-stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence; these examples were deposited in the middle latitudes, suggesting that intense winter storms continued to form hummocky cross-stratification in the middle latitudes during these much warmer times. Some previous workers have suggested that tsunamis may be capable of generating hummocky cross-stratification. The palaeogeographic distribution of the structure does not support an origin due to tsunamis. Lacustrine examples of hummocky cross-stratification reported herein are the first known non-marine occurrences; they suggest that storm effects strongly influence the sedimentary record of some lakes.     

Distribution of the Period Four Transition Elements among Olivine, Calcic Clinopyroxene and Mafic Silicate Liquid: Experimental Results

The distribution of Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Fe(total), Co²⁺ andNi²⁺ among synthetic olivine, calcic clinopyroxene and maficsilicate liquid has been studied between 1125 and 1250 ?C underanhydrous conditions at 1 bar total pressure. The distributionof iron and magnesium among the three phases was concluded tobe independent of temperature and may be described by the twoequations Titanium and vanadiumdid not enter olivine in significant amounts. The mean valueof the ratio (wt. per cent TiO₂ in Cpx)/(wt. per cent TiO₂ inL) was 0.29?0.04 for assemblages in which the liquid had botholivine and hypersthene in the norm but the ratio was greaterif the liquid was nepheline normative. Vanadium was concentratedin the pyroxene in some experiments and in the liquid in others,but it was not possible to conclude whether the change in distributionbehavior was due to varying temperature or changing liquid composition.Equilibrium partitioning of chromium was not achieved but theresults indicate that Cr₂O₃ was most strongly enriched in clinopyroxeneand showed a slight preference for olivine over the liquid.The divalent transition elements were each enriched in olivinerelative to clinopyroxene and the degree of enrichment increasedin the order predicted by crystal field theory. The mean (wt.per cent oxide in 01)/(wt. per cent oxide in Cpx) ratios were2.0 for MnO, 2.4 for FeO, 3.9 for CoO and 5.6 for NiO. Manganesewas enriched in olivine relative to the liquid and in the liquidrelative to the clinopyroxene. Cobalt and nickel were more concentratedin the crystalline phases than in the liquid but the degreeof enrichment was markedly less in the experiments in whichthe liquids were more mafic.     

Hummocky cross-stratification, tropical hurricanes, and intense winter storms

Duke (1985b) argues that ‘most examples (of hummocky cross-stratification) were formed by tropical hurricanes.’ His statement is based on the assumption that ‘hurricane-generated surface gravity waves form powerful oscillating or multidirectional flows at the water-sediment interface which do not possess a significant unidirectional component.’ It is true, as one of us has previously stated, that hurricanes are rapidly-moving, short-lived, localized, and infrequent systems as compared with mid-latitude storms; midlatitude storms are consequently more efficient in coupling with the shelf water-column than are hurricanes. However, Duke's argument that hummocky cross-stratification may be the result of purely oscillatory flow is untenable. His reasoning contradicts established theory about oscillatory bedforms, and his numerous examples of hummocky cross-stratification come largely from continental shelf settings where the storms (tropical or otherwise) would have created concurrent alongshelf undirectional flow as well as wave oscillatory motion. There is no theoretical or observational basis for the belief that water movement on the sea-floor during hurricanes is qualitatively different from water movement during mid-latitude storms. Consequently, hummocks are no more liable to form beneath hurricanes than they are beneath mid-latitude storms.