Volumen 1 (1994). Nº 1: 35-78
Características
texturales de
flujos de detritos subácueos:
implicaiones genéticas
Oscar R. LÓPEZ GAMUNDÍ
Dos características importantes para diferenciar flujos de detritos cohesivos de no cohesivos son: (1) concentración de clastos, la cual define el predominio de fuerzas cohesivas o no cohesivas (friccionales) en el mecanismo de sostén de granos y (2) la relación entre espesor de banco (Eb) y tamaño máximo de clasto (TMC). La mayoría de los depósitos de flujos de detritos muestra geometrías de banco bien definidas, fábricas clasto-soportadas y ausencia de clastos intraformacionales y deformación sinsedimentaria. Ambos tipos de flujos de detritos muestran alta correlación positiva entre Eb y TMC interpretada como balance entre la capacidad y la competencia de flujo. A diferencia de los depósitos originados a partir de flujos de detritos no cohesivos, aquellos derivados de flujos de detritos cohesivos muestran en valor positivo de TMC para Eb = 0, denominado factor cohesivo.
Se propone en este trebajo una nueva clasificación de depósitos derivados de flujos de detritos cohesivos subácueos basada en la geometría del banco, contactos, concentración de clastos y gradación, origen (intraformacional o extraformacional) de los clastos de tamaño grava-arena y presencia y tipo de deformación sinsedimentaria. Dos factores genéticos son importantes en esta clasificación: origen de la fracción gránulo-grava y estado de desagregación de los clastos de arenisca y fangolita.
Los tipos definidos son:
Ia. Fangolita arenosa guijosa-guijarrosa heterogénea.
Ib. Fangolita guijosa-guijarrosa heterogénea.
IIa. Fangolita arenosa homogénea.
Iib. Fangolita guijosa-guijarrosa homogénea.
Las variedades IIa. y IIb. corresponden a las denominadas “pebbly mudstones” de Crowell (1957). Los depósitos Ia. y Ib., caracterizados por el bajo grado de desagregación de los clastos intraformacionales, representan tipos genéticamente cercanos “slumps” con geometrías de banco comunmente irregulares. En estos tipos de flujos de detritos la mayoría de la carga de sedimento es transportada pasivamente en un tapón (“plug”) por encima de una zona cizallada desarrollada en la base del flujo.
Los tipos texturales descriptos son extremos dentro de un continuo que se origina en un deslizamiento subácueo y que, caracterizado por la destrucción creciente de la resistencia de matriz, termina en la generación de un flujo de detrito cohesivo totalmente cizallado (“fully sheared debris flow”). Las variedades heterogéneas Ia. y Ib. Preservan rasgos heredados como deformación sinsedimentaria y una importante población intraformacional de clastos; los flujos de detritos generadores son relativamente lentos y no turbulentos. Dado que los clastos son transportados pasivamente en un tapón, el tamaño máximi de clasto no puede ser usado como estimación de la competencia del flujo y, por lo tanto los diagramas Eb y TMC no son aplicables para estas variedades de flujos de detritos.
La transformación de variedades heterogéneas a homogéneas resulta de procesos de dilución por incorporación de agua y/o aumentos de pendiente (y velocidad). La transición de variedades heterogéneas a homogéneas está caracterizada por la parcial desagregación de clastos intraformacionales (evidenciada por una disminución de tamaño máximi de clasto), tendencia a la organización externa (geometría) e interna (gradación) de los bancos y valores crecientes de Eb/MCS.
Probably the most fundamental distinction is whether the clast in a debris flow are mostly supported by either non-cohesive (frictional) forces or cohesive forces. Clast concentration and bed thickness (Bth) versus maximum clast size (MCS) plots are the main two characteristics that enbable to discern between non-cohesive (grainflows of Lowae, 1979; cohesionless debris flows, Nemec and Steel, 1984) and cohesive debris flow (mudflows of Lowe, 1979). Most non-cohesive debris flow beds have well-defined bed geometries, are clast-supported and lack intraformational clasts and soft-sediemt deformation. Slthough both fully sheared cohesive and non-cohesive debris flows show high linear correlation in the Bth vs. MCS plots, indicative of balance between capacity and competence of the flow, cohesive ones can be distinguished by the presence of a cohesive strength factor, mathematically expresed as a positive value of MCS for Bth = 0.
A new classification of subaqueous cohesive debris flow beds in proposed in this contribution. The most diagnostic field criteria for the identification of the types are: (1) bed geometry, (2) bed contacts, (3) clast concentration and size grading, (4) source or provenance (intraformational vs. extraformational) of gravel - or sand-sized material, and, (5) presence and type (ductile or brittle) soft-sediment deformation. Ultimately, the distinction of textural types is principally based on the following genetic factors: (i) source of granule- and gravel-size material (intra- versus extraformational), and, (ii) degree of disaggregation of intabasinal mudstone and ssandstone clasts.
The end members of this classification are:
Ia. Heterogeneous debris flows with clasts exclusively derived from intraformational, partially consolidated mudstone and sandstone clasts with low degree of disaggregation.
IIa. Homogeneous fine debris flow (fine-grained debris flows of Hampton, 1975) derived from intraformational sand- to granule-sized material with high degree of disaggregation. Maximum grain size does not exceed that of pebbles (64mm).
Ib. Heterogeneous coarse debris flows (patchy pebbly mudstones of López-Gamundí, 1993), texturally characterized by the abundance of intraformational, gravel-sized mudstone and sandstone clast with soft sediment deformation. Due to their early disaggregation, intraformational clasts of conglomerate are exceptionally present only in this textural type. Extraformational cobbles and boulders may be common.
IIb. Homogeneous coarse debris flows, consisting of a uniform mixture of mud, sand and extraformational gravel.
Both IIa and IIb deposits correspond to the well known textural category of pebbly mudstones (Crowell, 1957). Sediments od Ia and Ib types represent end members closer to the slump stage. They are characterized by irregular to (less frequently) regular bed geometries, soft sediment deformation and MCS in the intraformational population. Most of the sediment load passively trasported in a rigid plug by an underlying sheared zone developed at the base of the flow. The resulting deposits commonly exhibits an inverse coarse-tail grading at the base of the beds.
Availability of grain size, rather than flow behavior, distinguishes type I from type II deposits. Disaggregation of partially lithified intraformational clast becomes significant as shear-strain progresses and a fully remolded cohesive debris flow develops. The disaggregation of the intraformational clasts becomes pervasive with the incorporation of the mud and sand clasts into the matrix (fluid) phase. The products of this process, homogeneous varieties IIa and IIb, are characterized by tabular beds with non erosive, almost flat contacts, MCS in extraformational sand/granule - (in type Ia) or gravel-sized (in type IIa) fraction and scarce to absent soft-sediment deformation. Although some small, abraded intraformational clasts may be preserved in some homogeneous beds, most type IIb debris flow beds are devoid of intraformational clasts. This process completes the transition from the heterogeneous type I varieties to the more homogeneous tupe II beds. The latter represent the fully remolded, more diluted end member of the slump - debris flow continuum. The resultant deposits of these fully sheared debris flows show values of MCS which can be used as estimates of their competence.
Furher water intake as the sediment load flows downslope leads to flow bipartition consisting of a turbulent layer that develops on top of the main mass of the largely laminar debris flows, a typical flow transformation extensively documented in environments denominated by sediment gravity flows.
Due to the lower density of poorly consolidated intraformational clasts as opposed to completely lithifield extraformational clasts, maximum particle sizes will always lie in the intraformational population. Therefore, Bth vs. MCS plots for the proximal heterogeneous varieties, with abundant intraformational clasts and plug flow conditions, can provide erroneous estimates of flow competence. In contrast, Bth vs. MCS plots for beds deposited from fully developed debris flows can provide semiquantitative information on rheological characteristics such as emplacement conditions, flow behavior (competence and capacity) and relative importence of cohesive and non-cohesive (frictional) forces in clast-support mechanisms.
The characteristics above described can be used to better understand the transitional stages of the continuum from slumps to fully developed debris flows.