Fluvial Dominated Deltas
Rio Grande - TST 1B and TST 2B
Based on the seismic reflection configuration the unit was interpreted as two separate seismic units. Therefore, the unit will be treated as two parts until the depositional sub-environments are explained.
Tst 1B is interpreted as a delta based on the prograding clinoform seismic pattern. The unit provides an excellent example of an oblique tangential reflection configuration that is interpreted as delta front deposits. Seismic line 331 displays the sub-parallel reflectors that are produced due to loading of the delta front from the overlying Tst 2B.
Due to the lobate fan-mound geometry and lithologic data, Tst 1B seemed to be a lobate fluvially-dominated delta. However, it was difficult to determine the character of the proximal part of the unit because it appeared to be heavily eroded by the overlying Tste surface. The interpretation of the overlying Tst 2B as a bar-finger sand led to the conclusion that the Tste surface reflected the deformation of Tst 1B due to loading by the overlying Tst 2B.
core data
The geometry of the Tste surface, the Tst 2B isochron pattern, and the chaotic and shingled clinoform reflection patterns lead to the interpretation of Tst 2B as part of an elongate fluvially-dominated delta. The chaotic seismic reflection configuration that fills the topographic lows in the Tste surface is interpreted as representing bar-finger sand deposits. The deformation of Tste occurred as mouth bars prograded down major distributaries and caused flowage in the underlying sediments (Seismic lines 331, 16ab and 11b). A shingled reflection configuration overlies and is laterally equivalent to the chaotic reflection configuration of the bar-finger sands. A shingled reflection configuration is commonly interpreted as depositional units prograding into shallow water. Therefore, the upper portions of the shingled reflection configuration in Tst 2B are interpreted as representing the marshes and bays of the interdistributary troughs in the delta plain. The lower portions of the shingled clinoforms are interpreted as distal bar and prodelta deposits.
In Tst 2B, the bar-finger sands are approximately 15 m thick. This value lies within a range of thicknesses from other area, which span from approximately 4 m in the Holocene Guadalupe delta to 75 m in the Mississippi. As the thickness of bar finger sands is a function of the displacement of the underlying delta-front sediments due to flowage as the bearing capacity of the sediments is exceeded and secondarily due to compaction of the underlying sediments, it cannot be used as a proxy for the paleodepth of the distributary channel. The sand thickness does reveal information about the strength of the underlying sediments.
The width of bar-finger sands in Tst 2B at five to seven km fits into the range of widths from the Guadalupe system at one to two km and in the Mississippi system at ten km. The length of the elongate bar-finger sands in the Mississippi ranges from 24 to 32 km, in the Guadalupe from 16 to 32 km, and in Tst B form 25 to 35 km. Therefore, the dimensions of the bar-finger sands in the Rio Grande are within the range of similar deposits in the Guadalupe and Mississippi systems.
As no lithologic information exists for Tst 2B, lithologic description s from the Mississippi and Guadalupe are suggested as probably analogs for the deposits. Bar-finger sands of the Mississippi River are composed of well sorted, unfossiliferous fine and very fine sands and silts with thin localized clay layers. The sand grades laterally and downward into massive delta-front clayey silts. In the Guadalupe system distributary-mouth bars are represented by sands and silts intermixed with clay and prodelta deposits by clay. Therefore, a reasonable suggestion for the composition of the bar-finger sands of Tst 2B is fine to very fine sands and silts with some clay layers and clayey silt composition for the surrounding prodelta deposits.
TST 1B Overview
Main Characteristics
Width (km)
x
Length (km) |
Thickness (ms) |
Shape |
Reflection
Configuration |
Bounding
Surfaces |
Lithology |
Subbottom
Depth (m) |
| 50 x 30 |
~ 30 |
fan
mound |
reflection-free to
sub-parallel, oblique
tangential, chaotic |
top - Tste
base - 2sb-ts |
SP-3, SP-4, SP-5 |
30-70 |
Depositional Interpretation
| Interpretation |
Sediment
Volume (km3) |
Timing (ybp) |
Deposition Rate
per 1000 yrs |
Sea Level |
Eustatic vs
Climatic |
elongate
fluvial-dominated
delta |
18.6 |
11,000
to
>5,000 |
6.86 km3 |
rising |
climatic/
sediment supply |
Sediment Yields
Volume = (4/3)*(Each isochron is divided into a series of spheroids whose volume is then summed (a * b* c, where a,b,c are radii).
| total volume (km3) |
deposition timing |
sediment yield (km3)
per 10000 yrs |
| 18.59 |
|
|
TST 2B Overview
Main Characteristics
Width (km)
x
Length (km) |
Thickness (ms) |
Shape |
Reflection
Configuration |
Bounding
Surfaces |
Lithology |
Subbottom
Depth (m) |
| 30 x 25 |
30 |
digitate |
chaotic
chaotic shingled |
top - depositional surface
either top of TST 3 or 4
base - Tste or TST 1 |
none |
35-70 |
Depositional Interpretation
| Interpretation |
Sediment
Volume (km3) |
Timing (ybp) |
Deposition Rate
per 1000 yrs |
Sea Level |
Eustatic vs
Climatic |
elongate
fluvial-dominated
delta |
9.05 |
11,000
to
5,000 |
6.86 km3 |
rising |
climatic/
sediment supply |
Sediment Yields
Volume = (4/3)*(Each isochron is divided into a series of spheroids whose volume is then summed (a * b* c, where a,b,c are radii).
| total volume (km3) |
deposition timing |
sediment yield (km3)
per 10000 yrs |
| 9.05 |
|
|
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