Jean-Jacques Macaire

Relationship of the presence of a non-calcareous clay–loam horizon to DEM attributes in a gently sloping area

Abstract The initial objective of this study was to identify relationships between terrain attributes and soil cover over a Loessic flat area covering a limestone plateau. The second objective was to determine those energy factors (flow path, solar radiation, wind intensity) which could improve the understanding of morphology and soil genesis. We describe 341 field observations taken over 1600 ha of an experimental area used for monitoring the water and nitrate supply in the Petite Beauce Region (100 km to the Southwest of Paris). For each observation, several soil variables were encoded. One was the presence of a non-calcareous clay–loam (NCCL) horizon. Relief is very smooth in this region (mean slope around 0.5%). The main terrain attributes were derived from a Digital Elevation Model (DEM) at 20×20 m and assigned to the pedological observations. A multiple logistic regression was used to analyse the relationship between the NCCL horizon and terrain attributes. Special statistics were used for aspect, because of the circular nature of this variable. The results show a strong relationship between the presence of NCCL horizon and slope gradient and slope aspect, while hydrological parameters are not correlated with this horizon. The mean angle of the aspect frequency of the NCCL horizon was calculated and compared to the mean angles of wind direction and solar radiation balance. There is a small difference between wind direction and aspect frequency of the NCCL horizon. This result confirms the role of wind in the spatial pattern of soils. Further data are required to better understand the combination of several factors (role of vegetation) and the age of the reshaping.

SEDIMENT YIELD DURING LATE GLACIAL AND HOLOCENE PERIODS IN THE LAC CHAMBON WATERSHED, MASSIF CENTRAL, FRANCE

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1± 6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ±1360, 1770± 960 and 380±100 m 3 km -3 a -1 , respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49 ±15, 120 ± 36 and 79± 24 m 3 km -2 a -1 , respectively, during the Bolling‐Allerod, the Younger Dryas and the Pre-Boreal‐Boreal periods. They were 56±17 and 166± 50 m 3 km -2 a -1 during the Sub-Atlantic period before and after 1360 a BP, respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation.

Quantification and regulation of organic and mineral sedimentation in a late-Holocene floodplain as a result of climatic and human impacts (Taligny marsh, Parisian Basin, France)

Quantification in grams per metres squared per year of the sediment accumulation in a flood plain (‘marsh’) located in the southwestern Parisian basin showed that there is no close relationship between the accumulation of organic matter (OM) and mineral matter (MM) during the late Holocene, and provided an accurate view of the distinct yield and storage conditions of both sediment components. Endogenic OM accumulation in peaty sediments is not related to the climate but to felling of the alder forest and its substitution by Cyperaceae and paludal taxa in the marsh (Iron Age and Middle Ages). MM accumulation expresses mainly the sediment yield on the slopes, determined by landuse. During an initial phase (from the late Neolithic to the early Middle Ages), land-use change from crop cultivation to pastureland, possibly related to climate deterioration, led to a decrease in the sediment yield. During a second phase, since the early Middle Ages, the considerable development of crop cultivation over pasturing, even during periods of climate deterioration (such as the ‘Little Ice Age’), led to a sharp increase in sediment yield. However, although sediment yield was high, the hydrodynamics in the fen did not favour particle retention. Thus, since the Neolithic, yield and storage of OM and MM sediment have been marked by human activities, initially with high climatic stress, but since the Middle Ages without significant climatic stress.

Quantitative analysis of climate versus human impact on sediment yield since the Lateglacial: The Sarliève palaeolake catchment (France)

Minimum rates of solid (SSY) and dissolved (DSY) sediment yield (SY) were evaluated in t/km 2 per yr from sediments stored in the Sarliève palaeolake (French Massif Central) for seven phases of the Lateglacial and Holocene up to the seventeenth century. The catchment (29 km2), mainly formed of limestones and marls, is located in an area rich in archaeological sites in the Massif Central. The respective impacts of human activities and climate on SY were compared by quantification of human settlements through archaeological survey and palynological data. During the Lateglacial and early Holocene up to about 7500 yr cal. BP, variations in SSY and DSY rates were mainly related to climate change with higher rates during colder periods (Younger Dryas and Preboreal) and lower rates during warmer periods (Bölling-Alleröd and Boreal). However, CF1 tephra fallout induced a sharp increase in SY during the Alleröd. During the middle and late Holocene after 7500 yr cal. BP, SSY and DSY greatly increased (by factors of 6.5 and 2.8, respectively), particularly during the Final Neolithic at about 5300 yr cal. BP when the climate became cooler and more humid. After this date, at least 75% of the SSY increase and more than 90% of the DSY increase resulted from human activities, but SSY rates showed little variation during Protohistoric and Historic Times up to the seventeenth century. SSY and DSY rates and DSY/SSY ratio indicate that catchment soils began to form during the Lateglacial and Preboreal, thickened considerably during the Boreal and Atlantic, finally thinning (rejuvenation) mainly as the result of human-induced erosion during the sub-Boreal and sub-Atlantic. Increased mechanical erosion during the late Holocene also induced an increase in chemical erosion.

Morpho-sedimentary evolution of the lower Moulouya (North Eastern Morocco) during middle and upper Holocene. Seismicity and neotectonic effects

Analysis of sediments and morpho-structural elements of the last 25 km of the lower Moulouya valley has shown that this river evolved during the middle and upper Holocene in a tectonic context of uplift, at the South-Eastern end of the Kebdana Mountains. This geodynamic context limited the sedimentation and inhibited the formation of a delta on the Mediterranean shore. A brief sedimentary episode allowed about 10 m of sediments to be deposited, forming a well-marked alluvial terrace. The sedimentation appeared before 3,750 years cal BC in connection with the Holocene sea transgression (“Mellahian” period) and a probably more humid period. It developed at an average rate of 0.6 mm.a–1, five kilometres from the shoreline, before stopping just after 395 years cal BC, at the same time as the post- Mellahian regression. The sediments show evidence of syn- and post-sedimentary seismic activity (seismites, diaclases, faults). They also show some traces of the “Mellahian” transgression (beds containing halite and gypsum) which indicate an average rise of 2.0 mm.a–1 since about 3,700 years cal BC, 22 km from the Moulouya mouth. The sediment composition indicates a subarid climate during the Holocene, but does not enable possible climatic variations to be detected.

Weathering of a quaternary glass-rich basalt in Bakrit, Middle Atlas Mountains, Morocco. Comparison with a glass-poor basalt

Abstract We compared the disintegration processes and mineralogic and chemical evolution pathways of two Quaternary basalts at Bakrit and Ifrane, weathered in the same physiographic and hydrologic conditions, but differing in texture according to the quantity of glass present. At Bakrit, quite abundant glass favoured the formation of a microfissure network throughout the rock and its disintegration without any distinct weathering front. As a result, basaltic sand with polymineral grains and a clayey-silty matrix were produced. At Ifrane, weathering of a glass-poor basalt produced only a clayey-silty saprolite. In glass-rich basalts, secondary minerals formed in microfissures and were 2/1 clay-mineral rich. In glass-poor basalts, secondary minerals formed mainly within primary minerals and were 1/1 clay-mineral rich. Because glass could be easily dissolved, it protected the minerals of close chemical composition, especially the plagioclases. The order of basalt-mineral weathering (olivine, labrador, augite, Fe-Ti oxides) was modified when glass was abundant (glass, olivine, augite, labrador, Fe-Ti oxides).

Petrography of alluvial sands as a past and present environmental indicator： Case of the Loire River （France）

Four sample sets of the Upper and Middle Loire river sands were analyzed in order to study the impact of natural and anthropogenic factors on their petrographic composition in space (on an 800 km stretch) and time. Composition was determined by modal analysis of three sand-size fractions using a polarizing optical microscope and calculated for each sample (“standard sand” = Sst). The watershed is composed mainly of endogenic (Massif Central) and sedimentary (southern Parisian Basin) rocks. B-set sands collected in channels for different water flows in 1996 show that Sst compositions vary by only 5 %. Present-day sands in the Upper Loire and Middle Loire have very high petrographic immaturity comparing to others worldwide fluvial sands, although bio-climatic conditions favor sand maturation by source-rock weathering in the watershed. This shows the strong impact of the Massif Central on sediment yield due to relief rejuvenation as a consequence of the formation of the Alps during the Quaternary. Fluvial sands stored during the Weichselian and the Holocene in the Middle Loire floodplain, although partly weathered since their deposition, show higher inputs from the endogenic rocks of the Massif Central than present-day deposits. This can be explained by Weichselian periglacial conditions and the development of crop farming since the Neolithic, which favored mechanical erosion, particularly in the Massif Central which is characterized by a cold, humid climate and steep slopes. The upstream-downstream change in the composition of presently deposited sand is low in the diked area. It shows however that basalt and some heavy mineral grains are vulnerable to abrasion during transport and indicates a marked sediment yield from ancient sediment stored in the floodplain. This is in line with the high incision of the river bed over the last 150 years partly due to dam construction and aggregate mining.

CHEMICAL CHANGES AND GENESIS OF SECONDARY MINERALS DURING THE ALTERATION OF BIOTITES FROM IGNIMBRITES IN THE TAZZEKA MOUNTAIN (MOROCCO)

The Tazzeka Mountain, located approximately 20 km south of Taza, eastern Morocco, is composed of a Westphalian volcano-sedimentary complex. It contains rhyolitic ignimbrites with the following minerals: quartz, potassium feldspar, oligoclase-andesine, and biotite. The ignimbrites are extensively altered because of a dense network of fractures in the massif. Alteration has resulted in the formation of spheroidal rocks and saprolite, the thickness of which depends on local topography. The evolution of the biotites in the ignimbrites was investigated by microprobe analysis of the mica crystals. This technique provides data that are not accessible through classical analytical methods. Biotites are transformed into secondary clay minerals, mainly chlorites and illites; intermediate stages are related to the degree of alteration of biotite, the latter being expressed by the K2O content which decreases progressively from 7.3 to 1.3%. Next come protochlorites and chlorites sensu stricto, in which the K2O content is 0.3%. Several processes including retrodiagenesis, hydrothermal activity, fumarolic activity, and geochemical weathering contributed to the transformation of the biotites at Tazzeka.

Comparison of labradorite evolution during weathering in basanite and pyroxenite. Effect of texture on plagioclase alteration

Summary The behaviour of labradorite plagioclases during weathering, was investigated by comparing two arenites the first one developed on a cretaceous pyroxenite at Mont Saint-Bruno (Quebec), and the second one on a Pliocene basanite from the French Massif Central. The evolution factors, climatic conditions, mineralogical relationship of the fresh rocks, and cristallochemical relationship of the labradorites are the same for the two rocks. The decreasing order of vulnerability of the minerals is, for the pyroxenite: olivine> labrador = augite, and in the basanite: olivine > augite > verre > labrador. Labradorite plagioclases in the pyroxenite are more or less strongly weathered to smectites. On the contrary, labradorites in the basanite, are slightly vulnerable and give little or no clay minerals. Size and shape of the crystals and a different textural environment (granular for the pyroxenites and microcrystalline within a vitreous phase for the basanite) influence the resistance of the plagioclases to weathering and modify the classical sequence of vulnerability of these silicates.