Paying Attention to Mudrocks: Priceless!

173 Pages·2015·12.255 MB·English

by  Daniel Larsen, Sven O. Egenhoff, Neil S. Fishman

Introduction
Siliciclastic mudrocks, often termed shales, represent more than two thirds of all sedimentary rocks
on Earth (Potter et al., 2005), yet they are also the least understood. The topic of mudstone deposition
and diagenesis has only recently begun to emerge as important and widely recognized (e.g., Schieber et
al., 2007; Taylor and Macquaker, 2014), mostly because of increasing interest from the petroleum industry
(e.g., Kaminski, 2014). In spite of their fi ne grain size and low permeabilities, mudstones have contributed
signifi cantly to North American production of natural gas. The abundance of geological units in the United
States that contain economically recoverable amounts of shale gas led to an oversupply and subsequent drop
in the price of natural gas in 2008, from which prices have yet to appreciably recover (Gotham et al., 2013).
But it is not only the petroleum sector that has focused on the signifi cance of mudstones: clay minerals in
fi ne-grained sediments are sought after to provide raw materials incorporated into liners for landfi lls (e.g.,
Moran and Hettiarachchi, 2011), both at the base as well as sealing the tops of them. Clay minerals from
fi ne-grained rocks are also made into ceramics used in every household, they aid in the process of paper
making, are an essential component for producing cement, and are even used in medicine (e.g., Barua et al.,
2014). An understanding of shale sedimentology, the make-up of shales and muddy sediments from different
environments, and modifi cation of muddy sediments by post-depositional processes is therefore crucial for
a wide variety of practical uses of fi ne-grained sediments and sedimentary rocks. The desire to broaden our
knowledge of mudrocks and further research on them was the motivation for organizing a technical session
at the GSA Annual Meeting in 2011, and this volume is an outgrowth of that session.
The case studies included in this volume span two overall thematic blocks: the fi rst one focuses on mud-
stone deposition in several settings and the second focuses on the diagenetic processes that have affected
important mudstone units. The two papers opening the volume (McGlue et al. and Ellis et al.) describe
recent lake sedimentation from South America and Africa, respectively. Deocampo discusses the depo-
sitional and early diagenetic controls on clay mineral composition in modern and ancient lacustrine sedi-
ments. The paper by Gierlowski-Kordesch et al. focuses on sedimentology of Jurassic terrestrial mudstone
interlayers in basalt fl ows. Egenhoff et al. describe Cambrian marine mudstones from Scandinavia. The
last three papers (Larsen et al., Fishman et al., and Blood and Lash) describe the diagenesis of mudrocks in
ancient marine successions.
It is well known that lacustrine rocks have been the source for petroleum in a variety of sedimentary
units around the world (Katz and Lin, 2014). In Chapter 1, McGlue et al. describe deep-lake strata of modern
Laguna Mar Chiquita in central Argentina. This lake records deposition of potential source rocks during lake-
level highstands and shows only minimal lateral variation of organic-rich facies in the profundal part of this
lacustrine setting. The paper stresses the importance of lake basins in thick-skinned foreland basins where
they may represent valuable source rocks and unconventional reservoirs, whereas in thin-skinned settings
back-bulge and wetland strata are more important.
In contrast, a modern tropical rift basin provides the study area for Chapter 2 by Ellis et al. Their inves-
tigations of the organic geochemistry show that about one third of the organic material in the lake sediments
stems from primary productivity in Lake Malawi. Dilution of organic matter by inorganic sediment in Lake
Malawi plays a key role in determining the organic content of the sediment, and the degree of dilution varies
according to the amount of local terrestrial input. The subdivision of organic-rich and organic-poor sedi-
ments on the lake bottom is a result of physical processes: dense, non-organic terrestrial material is separated
from less dense organic material, the latter being preferentially transported offshore. Ellis et al. illustrate how
intermediate water depth areas of this tropical syn-rift lake with shallow bathymetric gradients are the most
conducive settings for source-rock potential, not the most distal deep-lake environments.
Egenhoff, S.O., Fishman, N.S., and Larsen, D., Introduction, in Larsen, D., Egenhoff, S.O., and Fishman, N.S., eds., Paying Atten-
tion to Mudrocks: Priceless!: Geological Society of America Special Paper 515, p. v–vii, doi:10.1130/2015.2515(00). For permission
to copy, contact [email protected]. © 2015 The Geological Society of America. All rights reserved.
v
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/980061/spe515-v.pdf
by guest
on 23 April 2020
vi Introduction
In Chapter 3, Deocampo provides a comprehensive review of clay mineral authigenesis in lacustrine
settings, particularly in modern and ancient African lake basins. Boundary conditions for forming authigenic
clays include pore or surface waters that are alkaline, silica input into these lakes is high, the lake water
contains dissolved magnesium, and overall they are sediment-starved. These conditions are generally more
likely to be met in underfi lled lakes (after Carroll and Bohacs, 1999) in evaporite basins with siliceous input
from either volcaniclastic or hydrothermal sources. Deocampo’s paper then provides a diagenetic framework
for clay mineral formation, especially in underfi lled lake basins.
Staying on the African continent in Chapter 4, Gierlowski-Kordesch et al. focus on an unusual setting
for siliciclastic mudstones where several meter-thick units are intercalated with Jurassic basalt fl ows. These
are the only mudstones reported in this volume that clearly originated in an entirely non-lacustrine terres-
trial setting as part of infi lling small tectonically formed depressions. Sheetfl ood and, less commonly, grain
fl ow processes delivered pedogenic mud aggregates and other siliciclastic sediment to the depressions, with
paleosol development and marsh deposition playing only minor roles.
Marine mudrock successions are being successfully exploited for hydrocarbons, especially in the United
States, with intense interest also focused on these types of rocks worldwide. One of the key units of inter-
est in Europe is the Cambrian Alum Shale that is present in the subsurface beneath the Baltic Sea and also
in surface exposures in parts of Sweden and Norway. In Chapter 5, the study by Egenhoff et al. shows how
careful examination from one critical core and incorporating hitherto unrecognized fecal strings allows for
reconstructing the sedimentary environment in which the Alum Shale was deposited. Importantly, the fecal
strings point to at least dysoxic conditions at the time of mudstone deposition in much of the Alum succes-
sion, which thereby serves to revise existing models that proposed mostly anoxic conditions during deposi-
tion of organic-rich black shales in the Alum.
In Chapter 6, Larsen et al. investigate the mineralogy of clay-rich units directly overlying the Cretaceous-
Paleogene boundary in southeastern Missouri. Through X-ray diffraction and petrographic study, Larsen et
al. evaluated the signifi cance of impact debris related to the Chicxulub impact event on sediment composi-
tion and subsequent diagenesis. Not only does the mineralogy of the clays change signifi cantly in mudstones
deposited post-impact, but sedimentological and paleontological data also suggest that no early Paleogene
sediments are preserved in the study site. The authors conclude that the absence of volcaniclastic grains and
associated heavy minerals in the early Tertiary sediments argues against altered volcanic detritus causing the
change in mineralogy. The presence of altered impact spherules in clasts and key clay mineral characteristics
favor altered impact-related material as a possible source for some of the clays and zeolites and diagenetic
modifi cation of detrital clays in a marine shelf environment subject to variably reducing chemical conditions.
The Bakken Formation in the Williston Basin of North Dakota and adjacent Montana has been the
prime focus of exploration activities of the past years. Nevertheless, information on the two organic-rich
siliciclastic mudstone units that bound the productive and mostly silt-rich middle Bakken member is scarce.
Fishman et al. present in Chapter 7 the fi rst comprehensive study focusing on the diagenesis of the upper
Bakken shale, and the relationship between depositional environment and diagenesis, including mudstone
fracturing. The succession of authigenic minerals refl ects an evolution of pore water from initially being oxic
to eventually anoxic. This study of the upper Bakken member clearly shows that diagenesis of siliciclastic
mudstones is directly linked to the depositional environment, as the depositional setting governs the original
composition of the sediment.
Blood and Lash document in Chapter 8 the diameter of pyrite framboids throughout the Devonian
Marcellus Shale in two cores from West Virginia. Results from this study, which focuses on the diagenesis
of one of the major shale gas plays in the United States, indicate that sedimentary conditions fl uctuated
between anoxic-euxinic and dysoxic during the basal two transgressive-regressive cycles in the Marcel-
lus. The diameters of pyrite framboids suggest that the more proximal portion of the Marcellus shelf
experienced overall improved and slightly more oxygen-rich bottom-water conditions during the second
transgressive-regressive cycles in comparison to the fi rst, whereas the distal part of the shelf remained in
an anoxic-euxinic realm.
This volume aims at presenting current research on mudstones and shales from different settings
focusing on several key aspects. Whereas some of the papers concentrate on units that are currently explo-
ration targets or used for applications of the mineral industry, others focus on mudstones of fundamen-
tal scientifi c interest. But all of the papers are to provide a deeper understanding of depositional and
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/980061/spe515-v.pdf
by guest
on 23 April 2020
Introduction vii
diagenetic processes associated with mudrocks in order to enhance our understanding of fi ne-grained
rocks in their multiple varieties.
Sven O. Egenhoff
Neil S. Fishman
Daniel Larsen
REFERENCES CITED
Barua, S., Dutta, N., Karmakar, S., Chattopadhyay, P., Aidew, L., Moran, A.R., and Hettiarachchi, H., 2011, Geotechnical
Buragohain, A.K., and Karak, N., 2014, Biocompatible high characterization of mined clay from Appalachian Ohio:
performance hyperbranched epoxy/clay nanocomposite as Challenges and implications for the clay mining indus-
an implantable material: Biomedical Materials (Bristol, Eng- try: International Journal of Environmental Research
land), v. 9, p. 025006, doi:10.1088/1748-6041/9/2/025006. and Public Health, v. 8, p. 2640–2655, doi:10.3390/
Carroll, A.R., and Bohacs, K.M., 1999, Stratigraphic classifi ca- ijerph8072640.
tion of ancient lakes—Balancing tectonic and climatic Potter, P.E., Maynard, J.B., and Depetris, P.J., 2005, Mud and
controls: Geology, v. 27, p. 99–102, doi:10.1130/0091 Mudstones: Introduction and Overview: Berlin, Heidel-
-7613(1999)027<0099:SCOALB>2.3.CO;2. berg, New York, Springer, 297 p.
Gotham, D.J., Nderitu, D.G., Giraldo, J.S., and Preckel, P.V., 2013, Schieber, J., Southard, J., and Thaisen, K., 2007, Accretion of
Natural Gas Market Study, http://www.in.gov/iurc/fi les/Natu- mudstone beds from migrating fl occule ripples: Science,
ral_Gas_Market_Study.pdf. v. 318, p. 1760–1763, doi:10.1126/science.1147001.
Kaminski, V., 2014, The microstructure of the North American oil Taylor, K.G., and Macquaker, J.H.S., 2014, Diagenetic altera-
market: Energy Economics, v. 46, p. S1–S10, doi:10.1016/j tions in a silt- and clay-rich mudstone succession: An
.eneco.2014.10.017. example from the Upper Cretaceous Mancos Shale of
Katz, B., and Lin, F., 2014, Lacustrine basin unconventional resource Utah, USA: Clay Minerals, v. 49, p. 213–227, doi:10.1180/
plays: Key differences: Marine and Petroleum Geology, v. 56, claymin.2014.049.2.05.
p. 255–265, doi:10.1016/j.marpetgeo.2014.02.013.
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/980061/spe515-v.pdf
by guest
on 23 April 2020
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/980061/spe515-v.pdf
by guest
on 23 April 2020
The Geological Society of America
Special Paper 515
2015
Modern muds of Laguna Mar Chiquita (Argentina):
Particle size and organic matter geochemical trends from
a large saline lake in the thick-skinned Andean foreland
Michael M. McGlue*
Central Energy Resources Science Center, U.S. Geological Survey,
Denver Federal Center, MS 977, P.O. Box 25046, Denver, Colorado 80225, USA, and
Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, USA
Geoffrey S. Ellis
Central Energy Resources Science Center, U.S. Geological Survey,
Denver Federal Center, MS 977, P.O. Box 25046, Denver, Colorado 80225, USA
Andrew S. Cohen
Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA
ABSTRACT
Laguna Mar Chiquita (central Argentina; ~latitude 31°S, longitude 63°W) pro-
vides an outstanding opportunity to examine organic facies development and petro-
leum source-rock potential in a modern thick-skinned foreland basin lake. In this case
study, we defi ne profundal, paleodelta, and lake-margin depositional environments
based on trends in bathymetry and lake-fl oor sediment particle size. Sedimentary
geochemical analyses indicate that organic carbon–rich muds accumulate in profun-
dal environments during the extant lake-level highstand. The lateral variability of
organic facies is minimal. The quality of organic facies is controlled by lake level
and depositional environment, both of which dictate patterns of algal productivity,
siliciclastic dilution, and early diagenesis. We present conceptual models of lacus-
trine source rocks in both thick-skinned and thin-skinned foreland basins based on
modern analog data from both Laguna Mar Chiquita and other lakes in the central
Andean foreland. Over relatively short time intervals (102–104 yr), climatically driven
water-level fl uctuations infl uence the source-rock potential of these basins. Over time
intervals >105 yr, contraction and lateral migration of the basin fl exural profi le con-
trol stratal stacking patterns and the potential for hydrocarbon play development.
*[email protected]
McGlue, M.M., Ellis, G.S., and Cohen, A.S., 2015, Modern muds of Laguna Mar Chiquita (Argentina): Particle size and organic matter geochemical trends from a
large saline lake in the thick-skinned Andean foreland, in Larsen, D., Egenhoff, S.O., and Fishman, N.S., eds., Paying Attention to Mudrocks: Priceless!: Geologi-
cal Society of America Special Paper 515, p. 1–18, doi:10.1130/2015.2515(01). For permission to copy, contact [email protected]. © 2015 The Geological
Society of America. All rights reserved.
1
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/979967/spe515-01.pdf
by guest
on 23 April 2020
2 McGlue et al.
INTRODUCTION geomorphology, hydrogeology, and paleoclimate (e.g., Piovano
et al., 2004; Mon and Gutiérrez, 2009; Troin et al., 2010). Com-
Ancient lake systems associated with thick-skinned foreland prehensive sedimentological and geochemical data sets from
basins (e.g., broken foreland basins of Jordan, 1995; Laramide LMC are scarce, and the research presented herein attempts to
sedimentary basins of Lawton, 2008) developed some of the most build upon initial work published earlier (Martínez, 1995). Kröh-
economically important sedimentary deposits in North America. ling and Iriondo (1999) developed a useful Quaternary geologi-
This is particularly true of the Eocene Green River Formation cal map of the LMC region through analysis of onshore land-
of the western United States (e.g., Eugster and Hardie, 1975; forms and stratigraphy; however, geological details from the
Roehler, 1993; Smith et al., 2008). Dubiel (2003), for example, offshore remain limited. Our work aims to provide the fi rst basin-
noted the importance of Green River Formation source rocks to wide characterization of detrital particle size and organic matter
both conventional oil (e.g., the Altamont-Bluebell fi eld reser- geochemistry for the modern muds of LMC. Huc et al. (1990)
voirs) and unconventional tar sands prospects in the Uinta Basin noted the importance of lateral variations in sediment fabric and
of Utah. Recent assessments of in-place oil shale for the Green organic richness to lacustrine petroleum source rocks. We inves-
River Formation of the Greater Green River, Uinta, and Piceance tigated the relationships among depositional environments and
Basins indicate a vast resource base of ~4.3 × 1012 barrels of oil organic facies in LMC, with a goal of improving hydrocarbon
in place (Johnson et al., 2011). The thick, cyclic strata of the exploration models in continental foreland settings.
Wilkins Peak Member, a saline-lake phase in the Green River
Formation, exclusively accounts for >700 × 109 barrels of oil in TECTONIC SETTING
the Greater Green River Basin (Johnson et al., 2011). In addi-
tion, lacustrine coal beds with the potential for unconventional Laguna Mar Chiquita shares a broadly similar tectonic heri-
methane accumulations are present throughout the Greater Green tage to some of the large lakes associated with Laramide defor-
River and Uinta Basins (Roberts, 2005). Saline- lacustrine depos- mation in the western United States (e.g., the Eocene Green River
its from the Green River Formation have also been exploited for Formation; Eugster and Hardie, 1975; Surdam and Stanley, 1979;
sodium carbonates (e.g., nahcolite and trona) that have numerous Jordan and Allmendinger, 1986). The lake is located in the Pam-
commercial uses, including soda ash production, air and water pean plains of central Argentina (~latitude 31°S, longitude 63°W;
pollution control, animal feed, detergents, cement production, Fig. 1A). Laguna Mar Chiquita occupies a topographically closed
glass manufacturing, and a host of medical applications (Dyni, basin in the eastern Sierras Pampeanas, an area well known in
1996; Santini et al., 2006; Brownfi eld et al., 2010). More gener- the southern Andes for its fl at-slab subduction (Jordan and All-
ally, the source of many base-metal and oil resources in China, mendinger, 1986; Cahill and Isacks, 1992; Ramos et al., 2002;
Australia, and North America are thought to have been saline- Gans et al., 2011). To the west of the lake, the basement-cored
lake environments (e.g., Smoot and Lowenstein, 1991). Sierras de Córdoba formed from westward-verging thrust faults
South America provides a rich natural laboratory for the (Fig. 1B). To the east, a buried westward-verging thrust fault,
study of modern foreland basin saline lakes and their muds the Tostado-Selva fault, forms a broad topographic swell known
(Cohen et al., 2014). The central Andean retroarc foreland basin as the San Guillermo High (Fig. 1B). This feature is believed to
(Horton and DeCelles, 1997) is a classic, four-part, thin-skinned have impounded the southward-fl owing Río Dulce and eastward-
system composed of wedgetop, foredeep, forebulge, and back- fl owing Ríos Primero and Segundo, thus helping to form LMC.
bulge depozones (DeCelles and Giles, 1996), as well as vast Mon and Gutiérrez (2009) speculated that hydrologic capture
regions of sediment accumulation in the high-altitude hinterland and lake formation occurred in the middle Pleistocene, but there
(Horton, 2011). Saline and brackish lakes are common in hinter- are no absolute geochronology data, and age constraints are lim-
land basins and in areas of the back-bulge (e.g., Wirrmann and ited to regional correlations. Pelletier (2007) suggested that LMC
Mourguiart, 1995; Valero-Garcés et al., 2000; Furquim et al., is within the wedgetop depozone of this broken foreland, as the
2008; McGlue et al., 2012). In the southern Andean foreland, western lake margin is <150 km from the thrust front. Dávila et
Laramide-style deformation in the Miocene and Pliocene (Jor- al. (2010) suggested that the lake occupies a position in the distal
dan and Allmendinger, 1986; Kay and Abbruzzi, 1996) led to foredeep of a thick-skinned foreland, produced by the interplay
the development of thick-skinned foreland basins adjacent to or of loading by the Sierras de Córdoba in the Miocene and loading
between basement-cored uplifts. A number of large saline lakes induced by a dynamic fl at slab.
or playas occupy these basins today, including Salina de Ambar-
gasta, Salinas Grandes, and Laguna Mar Chiquita (e.g., Dargám, HYDROLOGY AND LIMNOLOGY
1994; Piovano et al., 2002; Zanor et al., 2012).
In this case study we report on the lake-fl oor sediments of Laguna Mar Chiquita is a hydrologically closed, underfi lled
Laguna Mar Chiquita (LMC; also referred to by some as Mar de lake basin (rates of potential accommodation continually exceed
Ansenuza), one of the best modern analogs available for ancient the rate of sediment plus water infi ll; Carroll and Bohacs, 1999).
saline lakes in a thick-skinned foreland basin setting. Most of the The catchment of LMC is very large (>37,000 km2), and the
geological research on LMC to date has focused on basin-scale most important surface water and solute input to the lake is the
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/979967/spe515-01.pdf
by guest
on 23 April 2020
Modern muds of Laguna Mar Chiquita (Argentina) 3
Figure 1. (A) Laguna Mar Chiquita (LMC) and its watershed in central Argentina. Inset map shows the position of the catchment in
South America. (B) Shuttle Radar Topography Mission (90 m) digital elevation model of the thick-skinned foreland basin that contains
LMC and surrounding ranges and basins. Transect A-A′ illustrates the topography adjacent to the lake; numbers correspond to features
labeled in A. (C) Surface sediment sample map (dots) and calculated bathymetry for LMC. Contour interval is 1 m. Inset map illus-
trates recent highstand shoreline and the location of the sample grid. The northern end of the lake, which was only recently formed by
fl ooding of the Río Dulce, was not sampled. The lake basin reaches maximum depth of ~7 m. ZC—zero contour.
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/979967/spe515-01.pdf
by guest
on 23 April 2020
4 McGlue et al.
Río Dulce (Martínez, 1995; Troin et al., 2010). North of LMC, and station records from Córdoba (1963–1989), where complete,
the Río Dulce incorporates tributaries from portions of Salina show that mean annual rainfall is 814 mm (Vörösmarty et al.,
de Ambargasta, which can act as an open-hydrologic system 1998). Additional precipitation data from Santa Fe province
under certain climatic conditions (Zanor et al., 2012; Fig. 1A). (1981–1990; http://www.smn.gov.ar) indicate mean annual rain-
The Sierras de Córdoba forms the headwater region for the Ríos fall of ~942 mm/yr in the lowlands northeast of LMC. A number
Primero and Segundo, two key rivers that enter LMC from the of studies indicate that strong variability in total rainfall has char-
south (Fig. 1A); these rivers are also known as the Ríos Susquía acterized the historic period, and the annual average rainfall prior
and Xanaes, respectively. Another river channel, the Río Segundo to 1973 was ~653 mm (Genta et al., 1998; Piovano et al., 2004).
Viejo, interacts with the southern shoreline, but discharge to the Surface area data for LMC refl ect this variability and range from
lake is limited due to anthropogenic engineering (Kröhling and 2000 km2 to 6000 km2 (Troin et al., 2010). Values of potential
Iriondo, 1999; Fig. 1A). Although the extent of its contribution evapotranspiration in the region vary, but in all cases exceed total
is still unknown, groundwater is believed to play an important annual precipitation, indicating a negative hydrologic balance
role in maintaining the water balance of LMC (Reati et al., 1996; (Reati et al., 1996).
Troin et al., 2010).
The terminal Río Dulce wetlands, which currently form METHODS
the northern end of the lake, developed as a result of a dramatic
transgression in the 1970s. As a consequence, the surface area of Lake-fl oor sediment samples (n = 61) were collected from
LMC today is much larger than in the recent past. The lake surface southern LMC in a grid pattern using a Ponar-style dredge. Our
elevation is at ~70 m above sea level (a.s.l.) and is juxtaposed with survey focused on southern LMC because this area has most
the San Guillermo High, which is adjacent to the lake’s eastern likely been the main site of lacustrine deposition in the late Qua-
margin at 100 m a.s.l. Reati et al. (1996) reported a maximum ternary. The diffuse and shallow northern end of LMC is essen-
water depth (Z ) of 8.6 m and an average depth (Z ) of 3.7 m, tially a forest that drowned due to recent Río Dulce fl ooding.
max ave
whereas Piovano et al. (2002) reported a Z of 10 m. The Z This region is extremely diffi cult to navigate safely and there-
max max
we encountered in the fi eld was 7.0 m (Z = 5.9 m; Fig. 1C). fore we excluded it from the survey. The spacing between our
ave
Based on the bathymetric map of Reati et al. (1996), it is highly sample sites was typically ≤5 km (Fig. 1C). Water depth was
likely that our survey covered the deepest part of the basin, which measured at each site using a hand-held sonar device. For the
implies that annual-decadal bathymetric variations of as much as purpose of classifi cation, all samples <10 km from the shoreline
~3 m may characterize LMC, perhaps in response to prevailing (water depths <6 m) were defi ned as nearshore. Samples clas-
conditions of effective precipitation and river discharge. Multiple sifi ed as offshore were typically found >10 km from the shore-
measurements of pH taken from the southern lake shoreline dur- line in deeper water. The upper ~1 cm of each grab sample was
ing our 2008 survey averaged 8.2. Surface-water chemical assess- transferred into labeled storage jars in the fi eld and shipped to the
ments conducted by Martínez (1995) indicated a Na2+-Cl–-SO2– United States for analysis.
4
brine type and no evidence of thermal stratifi cation. Thus, the Upon arrival in the lab, samples were homogenized and split.
lake is believed to be polymictic. Reati et al. (1996) indicated that A representative subset of samples was analyzed by microscopic
persistent winds affect LMC and fair-weather waves as high as 0.4 examination of 125 µm screen-washed residues and smear slides
m were common offshore, potentially preventing water-column to obtain qualitative insights on sedimentary components. The
stratifi cation. Prior to the 1970s transgression, values of salin- mineralogy of a subset of samples (n = 25) was investigated by
ity for LMC ranged between 251 and 360 g/L (Martínez, 1995; bulk powder X-ray diffraction (XRD) following the analytical
Reati et al., 1996). Salinity values refl ecting the highstand lake- protocol of Eberl (2003). Clay mineralogy was verifi ed using the
level condition ranged from 29 to 35 g/L (Martínez et al., 1994). techniques outlined in Moore and Reynolds (1989). All samples
Vegetation associated with the lake is not well described in the were evaluated for particle size and sedimentary geochemistry
literature, but both Reati et al. (1996) and Varandas da Silva et using standard techniques, with a special focus on organic mat-
al. (2008) documented various species of the macrophyte Ruppia ter (OM) geochemistry. Sample splits for particle size analysis,
growing in sheltered lake-margin environments. total inorganic carbon (TIC), biogenic silica (BiSi), and Rock-
Eval pyrolysis were prepared following the procedures discussed
CLIMATE in McGlue et al. (2012). Sample preparation and analysis for
total nitrogen (TN) and bulk δ15N assays followed the proce-
Using the Köppen-Geiger climate system, LMC and its dures described in McGlue et al. (2011). Nitrogen isotope val-
catchment are within the temperate Cwa and Cfa classes (Peel ues reported herein use the conventional delta notation relative to
et al., 2007). Summers are generally warm and wet, with aver- atmospheric nitrogen. The precision of the nitrogen isotope analy-
age temperatures above 20 °C between November and March sis was typically better than ~0.2‰. Total organic carbon (TOC)
(Vörösmarty et al., 1998); winters are cool and dry with aver- was calculated as the weight difference between total carbon and
age temperatures of ~13 °C from April and October. The average TIC, determined using a UIC Inc. total carbon coulometer at the
annual temperature is 17–18 °C. Precipitation is strongly seasonal University of Minnesota. Analytical precision associated with the
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/979967/spe515-01.pdf
by guest
on 23 April 2020
Modern muds of Laguna Mar Chiquita (Argentina) 5
technique was ~0.2%. Carbon isotopes were measured on OM at and shallower (Fig. 3). Clay concentration is highest (silt:clay
the U.S. Geological Survey using an Optima isotope ratio mass < 0.7) in the west; we attribute this to sedimentation dominated
spectrometer coupled with an elemental analyzer. Carbon isotope by suspension-settling processes (Fig. 4C). We interpret the shoal
values reported herein use the conventional delta notation relative between the profundal EODs as a paleodelta deposit, and sand
to Vienna Peedee belemnite. The precision was ~0.07‰ based on with gastropod shell hash associated with this environment cov-
repeated analysis of internal standards. In order to remove carbon- ers at least 70 km2. We favor a deltaic interpretation because of
ates that could infl uence the δ13C results, samples were acidifi ed the wedge shape exhibited by percent sand contours and the loca-
OM
using 20% HCl for 24 h prior to analysis. Reported weight percent tion near the mouth of the Río Segundo Viejo (Figs. 3 and 4A).
total organic nitrogen (TON) and C/N values refl ect only OM, The Río Segundo Viejo no longer contains active channels, due to
as contributions from inorganic-bound N were estimated and cor- a canalization project that was completed in 1930 (Kröhling and
rected using the technique described by Talbot (2001). Iriondo, 1999). This suggests that the delta is a relict feature that
Contour maps of particle size and geochemistry were created is not actively aggrading or prograding. The bathymetric map of
using the radial basis gridding function of Surfer v. 8.05 (Golden Reati et al. (1996) suggests that both subaerial and subaqueous
Software Inc.; www.goldensoftware.com). We imposed a zero deltaic features are likely to be encompassed by our percent sand
contour that was defi ned by the position of the southern lake and percent silt contours. Areas adjacent to the northern end of
shoreline and the sample grid. This exercise closes contours and the shoal in both profundal EODs show elevated concentrations
improves the quality of the maps, but it also introduces error in of detrital silt (Fig. 4B). Given this particle size pattern, we can-
the form of spuriously low values along the margins of the sample not discount the possibility that this feature is a sand spit, gener-
grid. Ternary diagrams and cross plots were developed using Sig- ated by littoral drift. Anecdotal evidence suggests that a west-
maPlot v. 12 (Systat Software Inc.; www.sigmaplot.com). ward littoral current impacts the southern lake shore; however,
the results of an experiment designed to confi rm the presence of
RESULTS AND INTERPRETATIONS the current were inconclusive (Reati et al., 1996).
Sandy and silty sediments found along the western margin
Detrital Particle Size of LMC are likely associated with the dune fi eld described in
Kröhling and Iriondo (1999), according to whom defl ation of the
Results Río Primero and Río Segundo fl oodplains during the Little Ice
Using the nomenclature of Folk (1980), the majority of Age carpeted the western margin of LMC with fi ne eolian sands
LMC modern sediments, regardless of water depth, plot within (Campo Maare Formation). Where exposed onshore, the dunes
the mud texture class (Table 1; Fig. 2A). The ratio of silt to clay vary in height to as high as ~4 m (Kröhling and Iriondo, 1999).
of mud-class samples varied from 0.6 to 1.8, and a histogram of Our sampling grid was too coarse to fully resolve the bathymetric
silt:clay shows that clay sizes (<3.9 µm) predominate (Fig. 2B). expression of the submerged dune fi eld, and it is likely that these
XRD analyses indicate that clay minerals in the majority of sam- features would be heavily reworked by wave action. Neverthe-
ples consist of abundant illite, with lesser amounts of calcium- less, the high silt concentration along the western edge of the
rich smectite and minor to trace amounts of kaolinite (Fig. 2C). sample grid (Fig. 4B) hint at the presence of these eolian deposits.
Coarse grain sizes are uncommon (<3.0%) in mud-class samples;
however, terrigenous sand is abundant in samples trending off- TIC and BiSi
shore from the mouth of the Río Segundo Viejo (Figs. 3 and 4A).
The texture classes of these samples range from sand to sandy Results
silt, and D values (125–880 µm) indicate that maximum par- Values of TIC range from 0.13 to 1.74 wt%, with a mean of
90
ticle sizes are in the fi ne to coarse sand range (Wentworth, 1922; 0.97 wt% at nearshore sampling sites; offshore the range expands
Fig. 2A). X-ray diffraction indicates that the mineralogy of these from 0.01 to 2.33 wt% and the mean increases to 1.33 wt% (see
sands consists of quartz, plagioclase, volcanic glass, and micas. Table 2). Contours of TIC appear to show that concentration
Sandy and silty sediments were also encountered at nearshore increases with distance from the southern lake shore, reaching
(<6 m water depth; Fig. 4A) sites along the western boundary of maximum values in the western profundal EOD (Fig. 4D). How-
the sample grid. ever, TIC > 1.0 wt% spans all EODs where water depths exceed
6 m. X-ray diffraction data indicate that calcite is the dominant
Interpretations carbonate mineralogy, which is consistent with the fi ndings of
Coupled with bathymetric data, we interpret the spatial dis- earlier studies (Martínez, 1995).
tribution of sand, silt, and clay contours to signify the presence Analyses of screen-washed samples identifi ed carbonate ostra-
of two profundal environments of deposition (EOD) separated code carapaces, foraminiferan tests, and gastropod shells. The gas-
by a relatively narrow, north-south–oriented shoal (Figs. 3, 4A, tropods consist of the high-spired Littoridina parchappei, which are
4B, and 4C). The main profundal EOD, on the western side of abundant in samples from the paleodelta EOD. A single, minute
the sample grid, is large (~650 km2) and characterized by rela- gastropod (4–5 mm long; globose shell form) was also encountered,
tively deep water, whereas the eastern profundal EOD is smaller but taphonomic damage prevented identifi cation. The ostracodes
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/979967/spe515-01.pdf
by guest
on 23 April 2020